Merge b647-twincat: ADS sum-read (opt-in) + TwinCAT diagnostics on /hosts (#645 #310/#314, #612 docs)

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

CLAUDE.md

@@ -1130,6 +1130,13 @@ host logger factory (wired in `DriverFactoryBootstrap`; the FOCAS and TwinCAT fa
   `DriverHealthChanged.Diagnostics` / proto `has_diagnostics`+`diagnostics`; its digest is in the publish fingerprint).
 - **TwinCAT (#637):** TIME/TOD publish Float64 ms, DATE/DT a UTC DateTime; a tag authored `UDInt` keeps the raw counter
   (and keeps historizing — DateTime is not historized).
+- **TwinCAT (#645, #310/#314):** opt-in ADS sum-read (`useSumRead`, `sumReadMaxItems` default 200 / ceiling 500, 32 KiB payload
+  budget). It reads raw bytes and decodes as the declared type, so only a scalar passing the strict width+kind check
+  (`CheckNotification`) rides a batch; value-converted pairs (DInt over INT, Real over LREAL/DINT) read singly. Only overall
+  ADS 1793/1794 latch the device to single reads; any other non-comms overall code (1792/1795/1797/1798...) reads that chunk singly for the poll without latching, with a once-per-device Warning; a comms failure neither.
+  Opt-in `diagnosticsEnabled` (default off) puts ADS state/app/online-change/cycle/exec/`HeadroomMs` on the `/hosts`
+  `IDriverDiagnostics` bag (up to 4 extra reads per device per probe tick). Unit-tested only; live gate pending a
+  user-approved route (the presses' routes name the Mac's old VPN IP). Sum-write not ported.
 - **AbLegacy (#636):** `AbLegacyAddress` (in `AbLegacy.Contracts`) is family-aware: PLC-5 `I:`/`O:` indices are OCTAL
   and are emitted to libplctag as decimal (libplctag's `pccc.c` parses decimal only). MicroLogix function files parse
   but read `BadNotSupported`. Indirect `N7:[N7:0]` (#646) is resolved by the driver at read/write time (index re-read

docs/drivers/TwinCAT.md

@@ -185,9 +185,56 @@ notifications; `DInt` over a `STRING(80)` → `BadTypeMismatch` in both paths wi
 the rest of the subscription Good. No PLC value held a genuine non-ASCII string
 to exercise Windows-1252 live — that rule is unit-tested only.
 
+## Sum-read and diagnostics (Gitea #645)
+
+Two items ported from the stranded `auto/driver-gaps` branch (#310, #314), re-implemented against the
+current raw-byte read path rather than cherry-picked (the branch predates the type guard, the string codec
+and the array codec). Unit-tested against the fake client only; the live gate is read-only and pending a
+route (see `docs/v3/twincat-backlog.md`).
+
+### Sum-read (opt-in)
+
+`useSumRead: true` (default **off**) batches a poll's reads for one device into ADS sum-read commands
+(`IndexGroup 0xF080`) of at most `sumReadMaxItems` symbols (default 200, ceiling 500) and at most 32 KiB of
+payload (read lengths + 4 bytes each) instead of one round-trip per tag. TC2 CE 2.11 support for the sum
+command is unverified, hence opt-in.
+
+- **Eligibility is strict.** The sum path reads raw bytes at the symbol's index group / offset and decodes them
+  as the DECLARED type (the notification rules), so a scalar rides the batch only when it passes
+  `TwinCATTypeGuard.CheckNotification` — same width, same float/integer kind. A tag the lenient single read
+  accepts by value conversion (`DInt` over a PLC `INT`, `Real` over `LREAL` or `DINT`) is read **individually**
+  inside the call, keeping the Beckhoff conversion; the batch never publishes reinterpreted bytes at Good.
+  Arrays use the same raw-bytes rule as on the single path. A type-mismatched item gets `BadTypeMismatch`
+  without joining the batch; a bit-indexed BOOL and a handle-addressed symbol (`0xF005`) are read singly.
+- Per-item ADS error codes map to that tag only (`MapAdsError`); a response shorter than asked is a size error
+  for that item, not a failed chunk. The overall code is the command's.
+- **Latch vs read-singly.** Only overall ADS **1793 / 1794** (service / index group not supported) throws
+  `TwinCATSumReadRefusedException`; the driver logs once, marks the device `SumReadUnsupported` and reads
+  singly until re-initialise. **Any other non-comms overall code** (1792 general error, 1795 offset, 1797 size, 1798 data, ...) means
+  *this request* was rejected — typically too big — so that chunk is read singly for this poll, nothing is
+  latched, and a Warning is logged once per device so the live gate can see sum-read is not working there. A
+  comms-level failure (router / port / timeout) is neither: that chunk goes `BadCommunicationError` and the next poll tries again.
+- A one-item chunk is just a single read. Sum-**write** is not implemented (writes stay per-tag).
+
+### `/hosts` diagnostics (`IDriverDiagnostics`, opt-in)
+
+Behind `diagnosticsEnabled` (default **off** — not yet live-gated). The driver implements the optional
+`IDriverDiagnostics` bag (the S7 `NegotiatedPduSize` precedent, #640), deliberately not synthetic raw tags.
+When enabled, after each healthy probe tick **and after the host-state transition** it reads up to four
+`TwinCAT_SystemInfoVarList` symbols per device — `_AppInfo.AppName`, `_AppInfo.OnlineChangeCnt`,
+`_TaskInfo[1].CycleTime`, `_TaskInfo[1].LastExecTime` (100 ns ticks) — so up to **4 extra read requests per
+device per probe interval** (default 5 s). The ADS state is the one `ProbeAsync` already read; no second
+ReadState. One device: flat keys `TwinCAT.AdsState`, `.AppName`, `.OnlineChangeCnt`, `.CycleTimeMs`,
+`.LastExecTimeMs`, `.HeadroomMs` (cycle − last exec; negative = the task overran; this is spare time, not
+jitter). Several devices: one line per device, `TwinCAT.<DeviceName or host>` =
+`Run; Plc30 App; online-changes 7; cycle 10.0 ms; exec 2.5 ms; headroom 7.5 ms`, so the host's 16-entry cap
+covers 16 devices. Times round to 0.1 ms (the bag's digest is in the publish fingerprint). Each symbol is
+best-effort: a runtime that does not export one (older TC2) keeps the others, and a symbol answering "not
+found" is not re-asked until a new client is installed (reconnect).
+
 ## Capability surface
 
-`TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver, IRediscoverable`
+`TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver, IRediscoverable, ISkippedTagReporter, IDriverDiagnostics`
 (`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs`).
 
 | Capability | Path | Notes |

docs/v3/twincat-backlog.md

@@ -5,27 +5,61 @@ The v2 TwinCAT driver is considered solid: 28 integration tests (14 `[TwinCATFac
 three latent driver bugs shaken out (notification cycle units, `STRING(N)` mapper,
 bit-indexed BOOL path). Further work is deferred.
 
-Archived from `docs/drivers/TwinCAT-Test-Fixture.md` § Follow-up candidates.
+Archived from `docs/drivers/TwinCAT-Test-Fixture.md` § Follow-up candidates. Refreshed 2026-10-08
+(Gitea #612 / #645); live-validation results live in `docs/drivers/TwinCAT.md` § "Live validation".
 
-## Deferred items
+## Hardware available (as of 2026-10-08)
 
-1. **TC2 coverage** — spin up a TC2 runtime (Windows CE IPC or legacy XAR)
+The Elma cleaner fleet (25 presses, 23 live) is the only real TwinCAT hardware in reach, and it is
-   and run the same suite; any delta surfaces. Blocked on hardware.
+**production — strictly read-only**:
 
-2. **Notification coalescing under load** — run the subscribe test while
+| Press | Runtime | AMS | Notes |
-   the PLC cycle is saturated (bump `lineSim` complexity, watch for
+|---|---|---|---|
-   dropped notifications). Doable on current rig; deferred as lower
+| 22 presses (e.g. Z33856 `10.201.45.236.1.1`, Z33733 `5.25.53.46.1.1`) | **TC2 2.11 on WinCE 6.0** | port **801** | read-validated 2026-10-07 |
-   priority than v3 feature work.
+| **Z35474** (`5.118.56.93.1.1`, 10.201.23.227) | **TC3 3.1.4024** (Win NT 10) | port **851** | the **only TC3 press**; "Plc30 App" |
+| TCBSD fixture `10.100.0.128` (`41.169.163.43.1.1:851`) | TC3 | 851 | not a press; unreachable from the Mac |
 
-3. **Multi-hop AMS route** — add a test behind an IPC gateway with a
+Access facts: ADS is TCP 48898. The driver sends no AddRoute (routes are provisioned out of band) and has
-   chained route entry. Blocked on hardware (gateway IPC).
+no IP field, so a host needs a local AMS router — TwinCAT XAR, or `Beckhoff.TwinCAT.Ads.TcpRouter` hosted
+in-process (the driver csproj does not reference TcpRouter; the 2026-10-07 run used a scratch harness).
+The Mac reaches the presses directly over the VPN. **The PLC routes left by earlier tooling are STALE:**
+they name the Mac's old VPN IP (10.181.198.189), and the Mac is now 10.181.207.140, so a source NetId of
+`10.181.207.140.1.1` has no route on any press. We must not AddRoute on production, so the **live gate for
+this backlog is deferred pending a user-approved route** (owner refreshes the route, or the user approves
+reusing the old source NetId). Nothing below that says "live" has been run against TC3.
 
-4. **License-rotation automation** — XAR's 7-day trial expires on
+## Status of the original items
-   schedule. Either automate `TcActivate.exe /reactivate` via a scheduled
-   task on the VM (not officially supported; reportedly works for some
-   TC3 builds), or buy a paid runtime license (~$1k one-time per runtime
-   per CPU) to kill the rotation. Ops item, not code.
 
-5. **Lab rig** — cheapest IPC (CX7000 / CX9020) on a dedicated network;
+1. **TC2 coverage — DONE (read side).** No longer "blocked on hardware": the 2026-10-07 Elma run
-   the only route that covers TC2 + real EtherCAT I/O timing + cycle
+   (TwinCAT 2.11) verified browse, scalar/array/string reads and native notifications on TC2. What is
-   jitter under CPU load. Blocked on hardware + budget.
+   still unverified there: sum-read support (see item 6) and anything that writes.
+
+2. **Notification coalescing under load** — run the subscribe test while the PLC cycle is saturated.
+   Needs the TCBSD fixture (loading a production press is out of the question) and the `GVL_Perf` /
+   `FB_PerfChurn` fixture project from the stranded `auto/driver-gaps` branch. Fixture-gated.
+
+3. **Multi-hop AMS route** — a test behind an IPC gateway with a chained route entry. No hardware.
+
+4. **License-rotation automation** — XAR's 7-day trial expires on schedule. Ops item, not code.
+
+5. **Lab rig** — cheapest IPC on a dedicated network; the only route that covers real EtherCAT I/O
+   timing and cycle jitter under CPU load. Blocked on hardware + budget.
+
+## Added by #612 / #645
+
+6. **ADS sum-read (#310) — code DONE, live gate pending.** Opt-in `useSumRead` / `sumReadMaxItems`
+   (strict width+kind eligibility; latch only on ADS 1793/1794; see `docs/drivers/TwinCAT.md` § "Sum-read"). Unit-tested against the fake client only. Live gate
+   (read-only, valid on a press): enable on Z33856 / Z33733 (TC2) and Z35474 (TC3), compare a batched poll
+   to individual reads value-for-value, and record whether TC2 CE 2.11 answers or refuses the sum command
+   (either outcome is a result — a refusal must fall back cleanly). Needs the route above.
+
+7. **Driver diagnostics (#314) — code DONE (opt-in `diagnosticsEnabled`, default off), live gate pending.** ADS state, app name, online-change
+   count, task cycle time, last exec time and headroom on the `/hosts` diagnostics bag. Live gate: confirm
+   `TwinCAT_SystemInfoVarList._TaskInfo[1]` resolves on TC2 2.11 (the driver tolerates its absence).
+
+8. **Flat vs VirtualTree browse on TC3 (#612)** — open. Only Z35474 is TC3; read-only symbol-table
+   upload in both modes, compare leaf sets. Add a `SymbolsLoadMode` option only if Flat drops struct
+   members. Needs the route. (Flat on TC2 already proved fine: 45,083 / 42,666 variables.)
+
+9. **Discovered struct/FB member writes (#612)** — deferred: writes, never on a press; fixture-only.
+   Likewise **sum-write** was deliberately NOT ported from the stranded branch.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Contracts/TwinCATDriverOptions.cs

@@ -61,6 +61,32 @@ public sealed class TwinCATDriverOptions
     /// </summary>
     public int NotificationMaxDelayMs { get; init; }
 
+    /// <summary>
+    ///     When <c>true</c>, a poll's reads for one device are batched into ADS sum-read commands
+    ///     (IndexGroup <c>0xF080</c>) of up to <see cref="SumReadMaxItems"/> symbols each instead of one
+    ///     round-trip per tag (Gitea #645 / #310). Default <c>false</c> (opt-in): a TwinCAT 2 CE runtime's
+    ///     sum-command support is unverified. A target that refuses the sum command is detected at runtime
+    ///     and that device falls back to individual reads for the rest of the driver's life.
+    /// </summary>
+    public bool UseSumRead { get; init; }
+
+    /// <summary>
+    ///     Maximum symbols per ADS sum-read command; larger polls are split into chunks of this size.
+    ///     Bounds the request/response size against the target's ADS buffer. Values below 1 are treated as 1.
+    ///     Values above 500 (the ADS sub-command ceiling) are treated as 500. Only consulted when
+    ///     <see cref="UseSumRead"/> is <c>true</c>.
+    /// </summary>
+    public int SumReadMaxItems { get; init; } = 200;
+
+    /// <summary>
+    ///     When <c>true</c>, each healthy probe tick also reads up to four <c>TwinCAT_SystemInfoVarList</c>
+    ///     symbols (app name, online-change count, task cycle and last-exec time) and the ADS state the probe
+    ///     already fetched, and publishes them on the <c>/hosts</c> diagnostics bag (Gitea #645 / #314).
+    ///     Default <c>false</c>: that is up to four extra read requests per device per probe interval against
+    ///     the PLC, not yet live-gated. Pending the read-only live gate on a TC2 and a TC3 target.
+    /// </summary>
+    public bool DiagnosticsEnabled { get; init; }
+
     /// <summary>
     ///     Timeout for the AdminUI Test Connect probe, in seconds. The AdminUI clamps to a
     ///     60s server-side maximum; this default is what the form pre-fills for new instances.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/AdsTwinCATClient.cs

@@ -3,6 +3,7 @@ using System.Runtime.CompilerServices;
 using System.Threading.Channels;
 using TwinCAT;
 using TwinCAT.Ads;
+using TwinCAT.Ads.SumCommand;
 using TwinCAT.Ads.TypeSystem;
 using TwinCAT.TypeSystem;
 
@@ -166,6 +167,184 @@ internal sealed class AdsTwinCATClient : ITwinCATClient
         }
     }
 
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<(object? value, uint status)>> ReadValuesAsync(
+        IReadOnlyList<TwinCATBulkReadItem> reads, CancellationToken cancellationToken)
+    {
+        var results = new (object? value, uint status)[reads.Count];
+        var planned = new List<PlannedSumRead>(reads.Count);
+
+        Task<(object? value, uint status)> ReadSingly(int index) =>
+            ReadValueAsync(reads[index].SymbolPath, reads[index].Type, reads[index].BitIndex,
+                reads[index].ArrayCount, cancellationToken);
+
+        // Plan every item exactly as the single read would (symbol resolved from the Beckhoff symbol cache,
+        // declared type checked against it), but stop short of the wire read: eligible items become sum
+        // commands. Anything the sum command cannot carry exactly is read on its own right here.
+        for (var i = 0; i < reads.Count; i++)
+        {
+            var r = reads[i];
+            try
+            {
+                if (r.BitIndex is not null && r.Type == TwinCATDataType.Bool)
+                {
+                    results[i] = await ReadSingly(i).ConfigureAwait(false);
+                    continue;
+                }
+
+                var (shape, array, symbol, symbolStatus) = await DescribeSymbolAsync(r.SymbolPath, cancellationToken)
+                    .ConfigureAwait(false);
+                if (symbolStatus != TwinCATStatusMapper.Good) { results[i] = (null, symbolStatus); continue; }
+
+                TwinCATArrayLayout layout;
+                if (r.ArrayCount is int count)
+                {
+                    // Arrays are raw bytes on the single path too, with the same strict element rule.
+                    if (TwinCATTypeGuard.CheckArray(r.Type, count, shape, array, out layout) is not null)
+                    {
+                        results[i] = (null, TwinCATStatusMapper.BadTypeMismatch);
+                        continue;
+                    }
+                }
+                else
+                {
+                    if (TwinCATTypeGuard.CheckRead(r.Type, shape) is not null)
+                    {
+                        results[i] = (null, TwinCATStatusMapper.BadTypeMismatch);
+                        continue;
+                    }
+                    // The single read value-converts a numeric pair (DInt over a PLC INT is fine there); raw
+                    // bytes decoded as the declared type would not. Only an exact width + kind match rides the
+                    // sum command — the rest keeps the Beckhoff conversion by reading singly.
+                    if (!TwinCATSumReadPlanner.ScalarEligible(r.Type, shape))
+                    {
+                        results[i] = await ReadSingly(i).ConfigureAwait(false);
+                        continue;
+                    }
+                    // A scalar is a one-element "array" at the type's own width (a STRING at the PLC's declared
+                    // width), so scalar and array share one raw decode.
+                    layout = new TwinCATArrayLayout(1, r.Type is TwinCATDataType.String or TwinCATDataType.WString
+                        ? shape.ByteSize
+                        : TwinCATTypeGuard.DeclaredByteSize(r.Type));
+                }
+
+                // A handle-addressed symbol (0xF005) needs a fresh variable handle per read — not expressible
+                // as an index-group/offset sum entry. Read it individually.
+                if (symbol!.IndexGroup == SymbolValueByHandle)
+                {
+                    results[i] = await ReadSingly(i).ConfigureAwait(false);
+                    continue;
+                }
+
+                planned.Add(new PlannedSumRead(i, symbol.IndexGroup, symbol.IndexOffset, layout, r.ArrayCount is not null));
+            }
+            catch (AdsErrorException ex)
+            {
+                results[i] = (null, MapAndSignal((uint)ex.ErrorCode));
+            }
+        }
+
+        if (planned.Count == 0) return results;
+
+        // Split to the byte budget; a lone item (or an oversized one) is just a read.
+        var groups = TwinCATSumReadPlanner.Partition(
+            [.. planned.Select(p => p.Layout.ByteLength)], TwinCATSumReadPlanner.MaxBatchBytes);
+        foreach (var group in groups)
+        {
+            var items = group.Select(g => planned[g]).ToList();
+            if (items.Count == 1)
+            {
+                results[items[0].Index] = await ReadSingly(items[0].Index).ConfigureAwait(false);
+                continue;
+            }
+            await ReadGroupAsync(items, reads, results, ReadSingly, cancellationToken).ConfigureAwait(false);
+        }
+        return results;
+    }
+
+    private async Task ReadGroupAsync(
+        List<PlannedSumRead> items, IReadOnlyList<TwinCATBulkReadItem> reads,
+        (object? value, uint status)[] results,
+        Func<int, Task<(object? value, uint status)>> readSingly, CancellationToken cancellationToken)
+    {
+        async Task FallBackAsync()
+        {
+            foreach (var p in items) results[p.Index] = await readSingly(p.Index).ConfigureAwait(false);
+        }
+
+        ResultSumReadRaw sum;
+        try
+        {
+            var entities = new List<IgIoSumReadEntity>(items.Count);
+            foreach (var p in items)
+                entities.Add(new IgIoSumReadEntity(p.IndexGroup, p.IndexOffset, p.Layout.ByteLength));
+            sum = await new SumRead(_client, entities, SumCommandErrorStrategy.Relaxed)
+                .ReadRawAsync(cancellationToken).ConfigureAwait(false);
+        }
+        catch (AdsErrorException ex) when (TwinCATSumReadPlanner.IsRefusal((uint)ex.ErrorCode))
+        {
+            throw new TwinCATSumReadRefusedException((uint)ex.ErrorCode, ex);
+        }
+        catch (ArgumentException)
+        {
+            // The library slices the response by the REQUESTED lengths and throws when a target answers with
+            // less. That is this target mangling this response, not a comms fault: read the group singly.
+            NoteSumFallback(0);
+            await FallBackAsync().ConfigureAwait(false);
+            return;
+        }
+
+        // On an overall failure the library RETURNS the code (SubErrors all set, ReadBlocks null).
+        if (sum.ErrorCode != AdsErrorCode.NoError)
+        {
+            var overall = (uint)sum.ErrorCode;
+            if (TwinCATSumReadPlanner.IsRefusal(overall))
+                throw new TwinCATSumReadRefusedException(overall);
+            if (TwinCATSumReadPlanner.IsRequestRejected(overall))
+            {
+                NoteSumFallback(overall);
+                await FallBackAsync().ConfigureAwait(false); // too big / bad size for THIS request: no latch
+                return;
+            }
+            var mapped = MapAndSignal(overall);
+            foreach (var p in items) results[p.Index] = (null, mapped);
+            return;
+        }
+
+        var utf8 = StringsAreUtf8();
+        for (var k = 0; k < items.Count; k++)
+        {
+            var p = items[k];
+            var subError = sum.SubErrors is { } errs && k < errs.Length ? errs[k] : AdsErrorCode.NoError;
+            ReadOnlyMemory<byte>? block = sum.ReadBlocks is { } blocks && k < blocks.Count ? blocks[k] : null;
+            results[p.Index] = TwinCATSumReadPlanner.DecodeItem(
+                reads[p.Index].Type, subError, block, p.Layout, p.IsArray, utf8, MapAndSignal);
+        }
+    }
+
+    private const uint SymbolValueByHandle = 0xF005;
+
+    private readonly record struct PlannedSumRead(int Index, uint IndexGroup, uint IndexOffset, TwinCATArrayLayout Layout, bool IsArray);
+
+    private string? _lastAdsState;
+    private long _sumFallbackCount;
+    private long _lastSumFallbackCode;
+
+    private void NoteSumFallback(uint code)
+    {
+        Interlocked.Exchange(ref _lastSumFallbackCode, code);
+        Interlocked.Increment(ref _sumFallbackCount);
+    }
+
+    /// <inheritdoc />
+    public long SumFallbackCount => Interlocked.Read(ref _sumFallbackCount);
+
+    /// <inheritdoc />
+    public uint LastSumFallbackCode => (uint)Interlocked.Read(ref _lastSumFallbackCode);
+
+    /// <inheritdoc />
+    public string? LastAdsState => Volatile.Read(ref _lastAdsState);
+
     /// <summary>
     ///     Resolves the PLC symbol behind <paramref name="symbolPath"/> into the facts
     ///     <see cref="TwinCATTypeGuard"/> compares a declared type against. Uses the Beckhoff symbol
@@ -289,7 +468,9 @@ internal sealed class AdsTwinCATClient : ITwinCATClient
         try
         {
             var state = await _client.ReadStateAsync(cancellationToken).ConfigureAwait(false);
-            return state.ErrorCode == AdsErrorCode.NoError;
+            var ok = state.ErrorCode == AdsErrorCode.NoError;
+            Volatile.Write(ref _lastAdsState, ok ? state.State.AdsState.ToString() : null);
+            return ok;
         }
         catch
         {

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/ITwinCATClient.cs

@@ -55,6 +55,53 @@ public interface ITwinCATClient : IDisposable
         int? arrayCount,
         CancellationToken cancellationToken);
 
+    /// <summary>
+    ///     Reads several symbols at once — the ADS sum-read path (Gitea #645 / #310). The result is a parallel
+    ///     list: element <c>i</c> is the outcome of <paramref name="reads"/>[<c>i</c>], each decoded and
+    ///     status-mapped exactly as <see cref="ReadValueAsync"/> would (an item the sum command cannot carry —
+    ///     a bit-indexed BOOL, a handle-addressed symbol — is read individually inside the call).
+    /// </summary>
+    /// <remarks>
+    ///     The default implementation reads one item at a time, so a client without a sum path (or a test
+    ///     fake) is correct if not faster. An implementation that issues a real sum command throws
+    ///     <see cref="TwinCATSumReadRefusedException"/> when the TARGET refuses the command as a whole
+    ///     (service / index group not supported); the caller then falls back to individual reads. A
+    ///     comms-level failure propagates as the ordinary exception.
+    /// </remarks>
+    /// <param name="reads">The symbols to read, at most the caller's chunk size.</param>
+    /// <param name="cancellationToken">Cancellation token for the read.</param>
+    /// <returns>One <c>(value, status)</c> per item, in request order.</returns>
+    /// <exception cref="TwinCATSumReadRefusedException">The target refused the sum command itself.</exception>
+    async Task<IReadOnlyList<(object? value, uint status)>> ReadValuesAsync(
+        IReadOnlyList<TwinCATBulkReadItem> reads, CancellationToken cancellationToken)
+    {
+        var results = new (object? value, uint status)[reads.Count];
+        for (var i = 0; i < reads.Count; i++)
+        {
+            var r = reads[i];
+            results[i] = await ReadValueAsync(r.SymbolPath, r.Type, r.BitIndex, r.ArrayCount, cancellationToken)
+                .ConfigureAwait(false);
+        }
+        return results;
+    }
+
+    /// <summary>
+    ///     The ADS device state name (<c>Run</c>, <c>Stop</c>, <c>Config</c>, …) the most recent
+    ///     <see cref="ProbeAsync"/> read, or <c>null</c> when unknown. Reuses the probe's own ReadState so
+    ///     diagnostics add no request (Gitea #645 / #314). Default <c>null</c>.
+    /// </summary>
+    string? LastAdsState => null;
+
+    /// <summary>
+    ///     How many sum-read chunks this client has read singly because the target rejected the sum request
+    ///     without refusing the service (non-latching). The driver warns once per device when it grows, so the
+    ///     live gate can see sum-read is not working on that target. Default 0.
+    /// </summary>
+    long SumFallbackCount => 0;
+
+    /// <summary>The overall ADS code of the most recent such fallback (0 = a malformed response).</summary>
+    uint LastSumFallbackCode => 0;
+
     /// <summary>
     ///     Write a symbolic value. Returns the mapped OPC UA status for the operation
     ///     (0 = Good, non-zero = error mapped via <see cref="TwinCATStatusMapper"/>).
@@ -132,6 +179,18 @@ public interface ITwinCATClient : IDisposable
     IAsyncEnumerable<TwinCATDiscoveredSymbol> BrowseSymbolsAsync(CancellationToken cancellationToken);
 }
 
+/// <summary>One element of an <see cref="ITwinCATClient.ReadValuesAsync"/> request — the same four facts
+/// <see cref="ITwinCATClient.ReadValueAsync"/> takes.</summary>
+/// <param name="SymbolPath">The ADS symbol path (a bit-indexed BOOL keeps its <c>.N</c> suffix, as for a single read).</param>
+/// <param name="Type">The declared data type.</param>
+/// <param name="BitIndex">The bit to extract for a BOOL-within-word tag; null otherwise.</param>
+/// <param name="ArrayCount">The authored array length for an array tag; null for a scalar.</param>
+public sealed record TwinCATBulkReadItem(
+    string SymbolPath,
+    TwinCATDataType Type,
+    int? BitIndex = null,
+    int? ArrayCount = null);
+
 /// <summary>Opaque handle for a registered ADS notification. <see cref="IDisposable.Dispose"/> tears it down.</summary>
 public interface ITwinCATNotificationHandle : IDisposable { }
 

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDeviceDiagnostics.cs

@@ -0,0 +1,26 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     One device's probe-sampled PLC facts (Gitea #645 / #314), surfaced through the driver's
+///     <see cref="Core.Abstractions.IDriverDiagnostics"/> bag to the <c>/hosts</c> page. Every field is
+///     best-effort: a runtime that does not export a system symbol leaves that field at its last known value
+///     (null if never seen).
+/// </summary>
+/// <param name="AdsState">The ADS device state name (<c>Run</c>, <c>Stop</c>, <c>Config</c>, …).</param>
+/// <param name="AppName">The running PLC application's name (<c>_AppInfo.AppName</c>).</param>
+/// <param name="OnlineChangeCnt">How many online changes the application has taken (<c>_AppInfo.OnlineChangeCnt</c>).</param>
+/// <param name="CycleTimeMs">The first PLC task's configured cycle time in milliseconds.</param>
+/// <param name="LastExecTimeMs">How long the task's last cycle took to execute, in milliseconds.</param>
+/// <param name="UpdatedUtc">When the snapshot was taken.</param>
+internal sealed record TwinCATDeviceDiagnostics(
+    string? AdsState,
+    string? AppName,
+    uint? OnlineChangeCnt,
+    double? CycleTimeMs,
+    double? LastExecTimeMs,
+    DateTime UpdatedUtc)
+{
+    /// <summary>Gets cycle time minus last execution time: how much of the cycle was spare. Negative means the
+    ///     task overran its cycle. (Not jitter — that needs the variation across cycles.)</summary>
+    public double? HeadroomMs => CycleTimeMs is { } c && LastExecTimeMs is { } x ? c - x : null;
+}

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs

@@ -12,7 +12,8 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
 ///     resolver land in PRs 2 and 3.
 /// </summary>
 public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery, ISubscribable,
-    IHostConnectivityProbe, IPerCallHostResolver, IRediscoverable, ISkippedTagReporter, IDisposable, IAsyncDisposable
+    IHostConnectivityProbe, IPerCallHostResolver, IRediscoverable, ISkippedTagReporter, IDriverDiagnostics,
+    IDisposable, IAsyncDisposable
 {
     /// <summary>Authored tags this driver could not build into its poll table (Gitea #541).</summary>
     private readonly SkippedTagLedger _skippedTags = new();
@@ -271,6 +272,10 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
         var now = DateTime.UtcNow;
         var results = new DataValueSnapshot[fullReferences.Count];
 
+        // #645/#310: with UseSumRead the reads for one device are gathered and issued as ADS sum-reads
+        // after this pass; everything else (and a device that refused the sum command) reads one by one.
+        Dictionary<DeviceState, List<SumReadEntry>>? buckets = null;
+
         for (var i = 0; i < fullReferences.Count; i++)
         {
             var reference = fullReferences[i];
@@ -285,47 +290,145 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
                 continue;
             }
 
+            if (_options.UseSumRead && !device.SumReadUnsupported)
+            {
+                var parsed = TwinCATSymbolPath.TryParse(def.SymbolPath);
+                var symbolName = parsed?.ToAdsSymbolName() ?? def.SymbolPath;
+                buckets ??= [];
+                if (!buckets.TryGetValue(device, out var bucket))
+                    buckets[device] = bucket = [];
+                bucket.Add(new SumReadEntry(i, reference, def, symbolName, parsed?.BitIndex));
+                continue;
+            }
+
+            results[i] = await ReadOneAsync(reference, def, device, now, cancellationToken).ConfigureAwait(false);
+        }
+
+        if (buckets is not null)
+            foreach (var (device, bucket) in buckets)
+                await ReadBucketAsync(device, bucket, results, now, cancellationToken).ConfigureAwait(false);
+
+        return results;
+    }
+
+    private readonly record struct SumReadEntry(
+        int Index, string Reference, TwinCATTagDefinition Def, string SymbolName, int? BitIndex);
+
+    /// <summary>One tag read on its own — the pre-sum-read path, also the fallback for a refused sum command.</summary>
+    private async Task<DataValueSnapshot> ReadOneAsync(
+        string reference, TwinCATTagDefinition def, DeviceState device, DateTime now, CancellationToken cancellationToken)
+    {
+        try
+        {
+            var client = await EnsureConnectedAsync(device, cancellationToken).ConfigureAwait(false);
+            var parsed = TwinCATSymbolPath.TryParse(def.SymbolPath);
+            var symbolName = parsed?.ToAdsSymbolName() ?? def.SymbolPath;
+            // An array-typed tag (def.ArrayLength != null) reads the array as raw bytes and decodes
+            // it with TwinCATArrayCodec — the same decode a native array notification runs — into an
+            // element-typed CLR array. Scalar tags pass null (scalar path) — bit-indexed BOOLs are
+            // inherently scalar so the client ignores arrayCount when a bitIndex is present.
+            var (value, status) = await client.ReadValueAsync(
+                symbolName, def.DataType, parsed?.BitIndex, def.ArrayLength, cancellationToken)
+                .ConfigureAwait(false);
+            return CompleteRead(reference, def, symbolName, value, status, now);
+        }
+        catch (OperationCanceledException) { throw; }
+        catch (Exception ex)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+            return new DataValueSnapshot(null, TwinCATStatusMapper.BadCommunicationError, null, now);
+        }
+    }
+
+    /// <summary>Maps one read outcome to a snapshot and applies the health / log side effects.</summary>
+    private DataValueSnapshot CompleteRead(
+        string reference, TwinCATTagDefinition def, string symbolName, object? value, uint status, DateTime now)
+    {
+        if (status == TwinCATStatusMapper.Good)
+        {
+            _health = new DriverHealth(DriverState.Healthy, now, null);
+            return new DataValueSnapshot(value, status, now, now);
+        }
+
+        if (status == TwinCATStatusMapper.BadTypeMismatch)
+            _logger.LogWarning(
+                "TwinCAT driver '{DriverInstanceId}' refused '{Reference}': declared {DataType} " +
+                "cannot decode the PLC symbol '{Symbol}' (string/numeric kind mismatch, array element type " +
+                "or length mismatch, or a reference/container)",
+                _driverInstanceId, reference, def.DataType, symbolName);
+        else
+            _logger.LogWarning(
+                "TwinCAT driver '{DriverInstanceId}' ADS read error 0x{Status:X8} for '{Reference}'",
+                _driverInstanceId, status, reference);
+        _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead,
+            $"ADS status {status:X8} reading {reference}");
+        return new DataValueSnapshot(value, status, now, now);
+    }
+
+    /// <summary>
+    ///     Reads one device's gathered tags as ADS sum-reads of at most <c>SumReadMaxItems</c> symbols each.
+    ///     A target that refuses the sum command flips <see cref="DeviceState.SumReadUnsupported"/> and the
+    ///     rest of the bucket is read individually; any other failure marks only the affected chunk Bad.
+    /// </summary>
+    private async Task ReadBucketAsync(
+        DeviceState device, List<SumReadEntry> bucket, DataValueSnapshot[] results, DateTime now,
+        CancellationToken cancellationToken)
+    {
+        var chunkSize = Math.Clamp(_options.SumReadMaxItems, 1, TwinCATSumReadPlanner.MaxItemsCeiling);
+        for (var start = 0; start < bucket.Count; start += chunkSize)
+        {
+            var chunk = bucket.GetRange(start, Math.Min(chunkSize, bucket.Count - start));
+
+            if (device.SumReadUnsupported || chunk.Count == 1)
+            {
+                foreach (var e in chunk)
+                    results[e.Index] = await ReadOneAsync(e.Reference, e.Def, device, now, cancellationToken).ConfigureAwait(false);
+                continue;
+            }
+
             try
             {
                 var client = await EnsureConnectedAsync(device, cancellationToken).ConfigureAwait(false);
-                var parsed = TwinCATSymbolPath.TryParse(def.SymbolPath);
+                var items = new TwinCATBulkReadItem[chunk.Count];
-                var symbolName = parsed?.ToAdsSymbolName() ?? def.SymbolPath;
+                for (var k = 0; k < chunk.Count; k++)
-                // An array-typed tag (def.ArrayLength != null) reads the array as raw bytes and decodes
+                    items[k] = new TwinCATBulkReadItem(chunk[k].SymbolName, chunk[k].Def.DataType, chunk[k].BitIndex, chunk[k].Def.ArrayLength);
-                // it with TwinCATArrayCodec — the same decode a native array notification runs — into an
-                // element-typed CLR array. Scalar tags pass null (scalar path) — bit-indexed BOOLs are
-                // inherently scalar so the client ignores arrayCount when a bitIndex is present.
-                var (value, status) = await client.ReadValueAsync(
-                    symbolName, def.DataType, parsed?.BitIndex, def.ArrayLength, cancellationToken)
-                    .ConfigureAwait(false);
 
-                results[i] = new DataValueSnapshot(value, status, now, now);
+                var fallbacksBefore = client.SumFallbackCount;
-                if (status == TwinCATStatusMapper.Good)
+                var bulk = await client.ReadValuesAsync(items, cancellationToken).ConfigureAwait(false);
-                    _health = new DriverHealth(DriverState.Healthy, now, null);
+                if (client.SumFallbackCount > fallbacksBefore && !device.SumFallbackWarned)
-                else
                 {
-                    if (status == TwinCATStatusMapper.BadTypeMismatch)
+                    device.SumFallbackWarned = true;
-                        _logger.LogWarning(
+                    _logger.LogWarning(
-                            "TwinCAT driver '{DriverInstanceId}' refused '{Reference}': declared {DataType} " +
+                        "TwinCAT driver '{DriverInstanceId}' device '{Device}': the target rejected an ADS sum-read " +
-                            "cannot decode the PLC symbol '{Symbol}' (string/numeric kind mismatch, array element type " +
+                        "request (ADS error {AdsError}; 0 = malformed response) and the chunk was read singly. Sum-read " +
-                            "or length mismatch, or a reference/container)",
+                        "is not working on this target; this warning is logged once per device",
-                            _driverInstanceId, reference, def.DataType, symbolName);
+                        _driverInstanceId, device.Options.HostAddress, client.LastSumFallbackCode);
-                    else
+                }
-                        _logger.LogWarning(
+                for (var k = 0; k < chunk.Count; k++)
-                            "TwinCAT driver '{DriverInstanceId}' ADS read error 0x{Status:X8} for '{Reference}'",
+                {
-                            _driverInstanceId, status, reference);
+                    var e = chunk[k];
-                    _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead,
+                    var (value, status) = k < bulk.Count ? bulk[k] : (null, TwinCATStatusMapper.BadCommunicationError);
-                        $"ADS status {status:X8} reading {reference}");
+                    results[e.Index] = CompleteRead(e.Reference, e.Def, e.SymbolName, value, status, now);
                 }
             }
             catch (OperationCanceledException) { throw; }
+            catch (TwinCATSumReadRefusedException refused)
+            {
+                device.SumReadUnsupported = true;
+                _logger.LogWarning(
+                    "TwinCAT driver '{DriverInstanceId}' device '{Device}' refused the ADS sum-read command " +
+                    "(ADS error {AdsError}); falling back to individual reads for this device",
+                    _driverInstanceId, device.Options.HostAddress, refused.AdsErrorCode);
+                foreach (var e in chunk)
+                    results[e.Index] = await ReadOneAsync(e.Reference, e.Def, device, now, cancellationToken).ConfigureAwait(false);
+            }
             catch (Exception ex)
             {
-                results[i] = new DataValueSnapshot(null, TwinCATStatusMapper.BadCommunicationError, null, now);
+                foreach (var e in chunk)
+                    results[e.Index] = new DataValueSnapshot(null, TwinCATStatusMapper.BadCommunicationError, null, now);
                 _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
             }
         }
-
-        return results;
     }
 
     // ---- IWritable ----
@@ -774,6 +877,7 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
         while (!ct.IsCancellationRequested)
         {
             var success = false;
+            ITwinCATClient? probed = null;
             try
             {
                 // Probe-initiated connects honor TwinCATProbeOptions.Timeout — distinct from
@@ -791,9 +895,12 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
                 if (!success)
                     await RecycleClientAsync(state, ct).ConfigureAwait(false);
                 else
+                {
                     // 05/STAB-16: on a healthy tick, re-register any notifications whose replay
                     // failed (a transient AddNotification blip) so they recover without a client swap.
                     await RetryDeadRegistrationsAsync(state, ct).ConfigureAwait(false);
+                    probed = client;
+                }
             }
             catch (OperationCanceledException) when (ct.IsCancellationRequested) { break; }
             catch
@@ -808,11 +915,139 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
 
             TransitionDeviceState(state, success ? HostState.Running : HostState.Stopped);
 
+            // #645/#314: opt-in PLC state / task-cycle sampling for /hosts. After the host-state transition so a
+            // slow target cannot delay it; never throws (except cancellation).
+            if (_options.DiagnosticsEnabled && probed is not null)
+            {
+                try { await SampleDeviceDiagnosticsAsync(state, probed, ct).ConfigureAwait(false); }
+                catch (OperationCanceledException) when (ct.IsCancellationRequested) { break; }
+            }
+
             try { await Task.Delay(_options.Probe.Interval, ct).ConfigureAwait(false); }
             catch (OperationCanceledException) { break; }
         }
     }
 
+    // ---- IDriverDiagnostics (#645 / #314) ----
+
+    // Well-known system symbols, exported by TC3 (and most TC2) PLC runtimes under TwinCAT_SystemInfoVarList.
+    // _TaskInfo[1] is the first PLC task; CycleTime / LastExecTime are UDINT counts of 100 ns.
+    internal const string SymOnlineChangeCnt = "TwinCAT_SystemInfoVarList._AppInfo.OnlineChangeCnt";
+    internal const string SymAppName = "TwinCAT_SystemInfoVarList._AppInfo.AppName";
+    internal const string SymCycleTime = "TwinCAT_SystemInfoVarList._TaskInfo[1].CycleTime";
+    internal const string SymLastExecTime = "TwinCAT_SystemInfoVarList._TaskInfo[1].LastExecTime";
+
+    /// <summary>Converts a TwinCAT 100 ns tick count to milliseconds.</summary>
+    internal static double TicksToMilliseconds(uint ticks100ns) => ticks100ns / 10_000.0;
+
+    /// <summary>
+    ///     Most recent <see cref="TwinCATDeviceDiagnostics"/> for a device, or null before its first
+    ///     successful probe sample (or for an unknown host address).
+    /// </summary>
+    /// <param name="hostAddress">The device's ADS host address.</param>
+    /// <returns>The snapshot, or null.</returns>
+    internal TwinCATDeviceDiagnostics? GetDeviceDiagnostics(string hostAddress) =>
+        _devices.TryGetValue(hostAddress, out var s) ? s.LastDiagnostics : null;
+
+    /// <summary>
+    ///     Samples the ADS device state and the four system symbols once per healthy probe tick and stores the
+    ///     result on <see cref="DeviceState.LastDiagnostics"/>. Every read is individually best-effort: a
+    ///     runtime that does not export a symbol (older TwinCAT 2 builds) keeps the previous value, and a
+    ///     symbol that answers "not found" is not asked again until the client is rebuilt. Never throws
+    ///     (except cancellation) — diagnostics must not disturb the probe.
+    /// </summary>
+    internal async Task SampleDeviceDiagnosticsAsync(DeviceState state, ITwinCATClient client, CancellationToken ct)
+    {
+        try
+        {
+            var prev = state.LastDiagnostics;
+            // The probe's own ReadState (ProbeAsync) already fetched this; no second request.
+            var adsState = client.LastAdsState ?? prev?.AdsState;
+
+            var appName = await ReadDiagSymbolAsync(state, client, SymAppName, TwinCATDataType.String, ct).ConfigureAwait(false) as string
+                ?? prev?.AppName;
+            var onlineChange = ToUInt(await ReadDiagSymbolAsync(state, client, SymOnlineChangeCnt, TwinCATDataType.UDInt, ct).ConfigureAwait(false))
+                ?? prev?.OnlineChangeCnt;
+            var cycleTicks = ToUInt(await ReadDiagSymbolAsync(state, client, SymCycleTime, TwinCATDataType.UDInt, ct).ConfigureAwait(false));
+            var execTicks = ToUInt(await ReadDiagSymbolAsync(state, client, SymLastExecTime, TwinCATDataType.UDInt, ct).ConfigureAwait(false));
+            var cycleMs = cycleTicks is { } c ? TicksToMilliseconds(c) : prev?.CycleTimeMs;
+            var execMs = execTicks is { } x ? TicksToMilliseconds(x) : prev?.LastExecTimeMs;
+
+            state.LastDiagnostics = new TwinCATDeviceDiagnostics(adsState, appName, onlineChange, cycleMs, execMs, DateTime.UtcNow);
+        }
+        catch (OperationCanceledException) when (ct.IsCancellationRequested) { throw; }
+        catch { /* best-effort */ }
+    }
+
+    private static uint? ToUInt(object? v) => v switch
+    {
+        uint u => u,
+        null => null,
+        _ => Convert.ToUInt32(v, System.Globalization.CultureInfo.InvariantCulture),
+    };
+
+    private async Task<object?> ReadDiagSymbolAsync(
+        DeviceState state, ITwinCATClient client, string symbol, TwinCATDataType type, CancellationToken ct)
+    {
+        if (state.UnavailableDiagSymbols.ContainsKey(symbol)) return null;
+        try
+        {
+            var (value, status) = await client.ReadValueAsync(symbol, type, null, null, ct).ConfigureAwait(false);
+            if (status == TwinCATStatusMapper.Good) return value;
+            if (status == TwinCATStatusMapper.BadNodeIdUnknown)
+                state.UnavailableDiagSymbols[symbol] = 0;
+            return null;
+        }
+        catch (OperationCanceledException) when (ct.IsCancellationRequested) { throw; }
+        catch { return null; }
+    }
+
+    /// <summary>
+    ///     The <c>/hosts</c> diagnostics bag (Gitea #640 channel): per device, ADS state, PLC app name, online
+    ///     change count, task cycle time, last execution time and headroom (cycle − last exec). A single-device
+    ///     driver uses flat <c>TwinCAT.*</c> keys; several devices prefix the device name (or host address).
+    ///     A pure in-memory read — the probe loop does the sampling. The host bounds the bag
+    ///     (<see cref="DriverDiagnostics.MaxEntries"/>), so a many-device driver keeps the first keys in
+    ///     ordinal order.
+    /// </summary>
+    /// <returns>The key/value pairs; empty before the first sample.</returns>
+    IReadOnlyDictionary<string, string> IDriverDiagnostics.GetDiagnostics()
+    {
+        var bag = new Dictionary<string, string>(StringComparer.Ordinal);
+        var multi = _devices.Count > 1;
+        foreach (var state in _devices.Values)
+        {
+            if (state.LastDiagnostics is not { } d) continue;
+            var name = state.Options.DeviceName ?? state.Options.HostAddress;
+            if (multi)
+            {
+                // One compact line per device so the host's 16-entry cap shows up to 16 devices, not 2.
+                var parts = new List<string>(6);
+                if (d.AdsState is not null) parts.Add(d.AdsState);
+                if (!string.IsNullOrEmpty(d.AppName)) parts.Add(d.AppName);
+                if (d.OnlineChangeCnt is { } n) parts.Add("online-changes " + n.ToString(System.Globalization.CultureInfo.InvariantCulture));
+                if (d.CycleTimeMs is { } cy) parts.Add("cycle " + FormatMs(cy) + " ms");
+                if (d.LastExecTimeMs is { } ex) parts.Add("exec " + FormatMs(ex) + " ms");
+                if (d.HeadroomMs is { } h) parts.Add("headroom " + FormatMs(h) + " ms");
+                bag[$"TwinCAT.{name}"] = string.Join("; ", parts);
+                continue;
+            }
+            const string prefix = "TwinCAT.";
+            if (d.AdsState is not null) bag[prefix + "AdsState"] = d.AdsState;
+            if (!string.IsNullOrEmpty(d.AppName)) bag[prefix + "AppName"] = d.AppName;
+            if (d.OnlineChangeCnt is { } oc) bag[prefix + "OnlineChangeCnt"] = oc.ToString(System.Globalization.CultureInfo.InvariantCulture);
+            if (d.CycleTimeMs is { } cy2) bag[prefix + "CycleTimeMs"] = FormatMs(cy2);
+            if (d.LastExecTimeMs is { } ex2) bag[prefix + "LastExecTimeMs"] = FormatMs(ex2);
+            if (d.HeadroomMs is { } h2) bag[prefix + "HeadroomMs"] = FormatMs(h2);
+        }
+        return bag;
+    }
+
+    // 0.1 ms resolution: the bag is published on a heartbeat and its digest is in the publish fingerprint,
+    // so sub-0.1 ms wobble should not read as a change.
+    private static string FormatMs(double ms) =>
+        Math.Round(ms, 1, MidpointRounding.AwayFromZero).ToString("0.0", System.Globalization.CultureInfo.InvariantCulture);
+
     private void TransitionDeviceState(DeviceState state, HostState newState)
     {
         HostState old;
@@ -976,6 +1211,7 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
             throw;
         }
         device.Client = client;
+        device.UnavailableDiagSymbols.Clear(); // a rebuilt client may face a PLC that now exports them
         device.Backoff.RecordSuccess(); // 05/STAB-8: recovery resets the window immediately
 
         // Re-register every native notification onto the fresh client. Native ADS is push, so
@@ -1233,11 +1469,27 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
         ///     so the loop cannot touch a disposed client.</summary>
         public Task? ProbeTask { get; set; }
 
+        /// <summary>Set when the target answered the ADS sum-read command with "service / index group not
+        ///     supported" (#645); the device reads one tag at a time from then on. Cleared only by a fresh
+        ///     <see cref="DeviceState"/> (driver re-initialise).</summary>
+        public volatile bool SumReadUnsupported;
+
+        /// <summary>True once the "sum request rejected, chunk read singly" warning was logged for this device.</summary>
+        public volatile bool SumFallbackWarned;
+
+        /// <summary>Latest probe-sampled PLC diagnostics (#645 / #314); null before the first sample.</summary>
+        public TwinCATDeviceDiagnostics? LastDiagnostics { get; set; }
+
+        /// <summary>Diagnostic system symbols the target answered "not found" for — not re-asked until the client is rebuilt.</summary>
+        public ConcurrentDictionary<string, byte> UnavailableDiagSymbols { get; } = new(StringComparer.Ordinal);
+
         /// <summary>Disposes the active ADS client if any.</summary>
         public void DisposeClient()
         {
             Client?.Dispose();
             Client = null;
+            // A rebuilt client may face a PLC that now exports the symbols (e.g. after a program download).
+            UnavailableDiagSymbols.Clear();
         }
 
         /// <summary>Disposes the connection gate semaphore.</summary>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriverFactoryExtensions.cs

@@ -78,6 +78,9 @@ public static class TwinCATDriverFactoryExtensions
             UseNativeNotifications = dto.UseNativeNotifications ?? true,
             EnableControllerBrowse = dto.EnableControllerBrowse ?? false,
             NotificationMaxDelayMs = dto.NotificationMaxDelayMs ?? 0,
+            UseSumRead = dto.UseSumRead ?? false,
+            SumReadMaxItems = dto.SumReadMaxItems ?? 200,
+            DiagnosticsEnabled = dto.DiagnosticsEnabled ?? false,
         };
     }
 
@@ -114,6 +117,15 @@ public static class TwinCATDriverFactoryExtensions
         /// <summary>Gets or sets the maximum notification delay in milliseconds.</summary>
         public int? NotificationMaxDelayMs { get; init; }
 
+        /// <summary>Gets or sets a value indicating whether a poll's reads are batched into ADS sum-reads.</summary>
+        public bool? UseSumRead { get; init; }
+
+        /// <summary>Gets or sets a value indicating whether probe-tick PLC diagnostics are sampled.</summary>
+        public bool? DiagnosticsEnabled { get; init; }
+
+        /// <summary>Gets or sets the maximum symbols per sum-read command.</summary>
+        public int? SumReadMaxItems { get; init; }
+
         /// <summary>Gets or sets the list of configured devices.</summary>
         public List<TwinCATDeviceDto>? Devices { get; init; }
 

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATSumReadPlanner.cs

@@ -0,0 +1,113 @@
+using Beckhoff = TwinCAT.Ads;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     The pure decisions behind an ADS sum-read (Gitea #645 / #310), split out of
+///     <see cref="AdsTwinCATClient"/> so they are unit-testable without an ADS connection: which items may
+///     ride a sum command, how a poll is split to fit a request budget, which overall ADS codes mean what,
+///     and how one item's raw block becomes a value.
+/// </summary>
+internal static class TwinCATSumReadPlanner
+{
+    /// <summary>Hard ceiling on items per sum command (ADS documents roughly 500 sub-commands).</summary>
+    public const int MaxItemsCeiling = 500;
+
+    /// <summary>Per-command byte budget: the sum of the items' read lengths plus <see cref="PerItemOverhead"/> each.
+    /// Keeps a poll that includes large arrays inside a modest router / CE buffer.</summary>
+    public const int MaxBatchBytes = 32 * 1024;
+
+    /// <summary>Bytes of per-item ADS error code carried in a sum response.</summary>
+    public const int PerItemOverhead = 4;
+
+    /// <summary>
+    ///     True when a SCALAR may be read as raw bytes in a sum command. The sum path decodes the bytes as the
+    ///     DECLARED type, so it needs the strict width + float/integer-kind rule (the notification rule); the
+    ///     lenient <see cref="TwinCATTypeGuard.CheckRead"/> relies on the Beckhoff value conversion a raw read
+    ///     does not do. An item failing this but passing CheckRead must be read individually.
+    /// </summary>
+    /// <param name="declared">The tag's declared type.</param>
+    /// <param name="plc">The PLC symbol's shape.</param>
+    /// <returns>True when the raw-bytes decode is exact.</returns>
+    public static bool ScalarEligible(TwinCATDataType declared, TwinCATPlcSymbolShape plc) =>
+        TwinCATTypeGuard.CheckNotification(declared, plc) is null;
+
+    /// <summary>The service or index group itself is unsupported: the device never does sum commands. Latches.</summary>
+    /// <param name="adsError">The overall ADS code.</param>
+    /// <returns>True for 1793 (service not supported) and 1794 (invalid index group).</returns>
+    public static bool IsRefusal(uint adsError) => adsError is 1793 or 1794;
+
+    /// <summary>A transport-level failure (router / port / timeout), not an answer from the sum service.</summary>
+    /// <param name="adsError">The overall ADS code.</param>
+    /// <returns>True when the code maps to a communication error or timeout.</returns>
+    public static bool IsCommsFailure(uint adsError) =>
+        TwinCATStatusMapper.MapAdsError(adsError) is var s
+        && (s == TwinCATStatusMapper.BadCommunicationError || s == TwinCATStatusMapper.BadTimeout);
+
+    /// <summary>
+    ///     The sum service answered with an overall error that is neither "unsupported" nor a comms failure
+    ///     (general error, bad size / data / offset, ...): it rejected THIS request. Read the chunk singly for this
+    ///     poll, do not latch.
+    /// </summary>
+    /// <param name="adsError">The overall ADS code.</param>
+    /// <returns>True for any such code.</returns>
+    public static bool IsRequestRejected(uint adsError) => !IsRefusal(adsError) && !IsCommsFailure(adsError);
+
+    /// <summary>
+    ///     Splits items (in order) into groups whose summed <c>length + PerItemOverhead</c> stays within
+    ///     <paramref name="budget"/>. An item that alone exceeds the budget gets its own group (the caller reads a
+    ///     one-item group individually).
+    /// </summary>
+    /// <param name="byteLengths">Each item's read length.</param>
+    /// <param name="budget">The byte budget per command.</param>
+    /// <returns>Groups of positions into <paramref name="byteLengths"/>.</returns>
+    public static List<List<int>> Partition(IReadOnlyList<int> byteLengths, int budget)
+    {
+        var groups = new List<List<int>>();
+        List<int>? current = null;
+        var used = 0;
+        for (var i = 0; i < byteLengths.Count; i++)
+        {
+            var cost = byteLengths[i] + PerItemOverhead;
+            if (current is null || used + cost > budget)
+            {
+                current = [];
+                groups.Add(current);
+                used = 0;
+            }
+            current.Add(i);
+            used += cost;
+        }
+        return groups;
+    }
+
+    /// <summary>
+    ///     Turns one item's sum-response slice into a value. A per-item ADS error maps to its status; a missing or
+    ///     short block (a target answering with less than was asked for) is a size error for that item only.
+    /// </summary>
+    /// <param name="type">The declared type.</param>
+    /// <param name="subError">The item's ADS error code (0 = none).</param>
+    /// <param name="block">The item's bytes, or null when the response had none.</param>
+    /// <param name="layout">The planned layout (a scalar is one element).</param>
+    /// <param name="isArray">True to return the whole array, false for element 0.</param>
+    /// <param name="utf8">True when the target's STRINGs are UTF-8.</param>
+    /// <param name="mapError">Maps an ADS code to an OPC UA status (also raises symbol-version signals).</param>
+    /// <returns>The value and status.</returns>
+    public static (object? value, uint status) DecodeItem(
+        TwinCATDataType type, Beckhoff.AdsErrorCode subError, ReadOnlyMemory<byte>? block,
+        TwinCATArrayLayout layout, bool isArray, bool utf8, Func<uint, uint> mapError)
+    {
+        if (subError != Beckhoff.AdsErrorCode.NoError) return (null, mapError((uint)subError));
+        if (block is not { } b || b.Length < layout.ByteLength)
+            return (null, TwinCATStatusMapper.MapAdsError(1797));
+        try
+        {
+            var decoded = TwinCATArrayCodec.Decode(type, b.Span, layout, utf8);
+            return (isArray ? decoded : decoded.GetValue(0), TwinCATStatusMapper.Good);
+        }
+        catch (ArgumentException)
+        {
+            return (null, TwinCATStatusMapper.MapAdsError(1797));
+        }
+    }
+}

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATSumReadRefusedException.cs

@@ -0,0 +1,21 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     Thrown by <see cref="ITwinCATClient.ReadValuesAsync"/> when the TARGET refuses the ADS sum-read
+///     command as a whole (service or index group not supported). It is not a comms fault: the driver marks
+///     the device "no sum-read" and re-reads the batch with individual reads, so the poll still completes.
+/// </summary>
+internal sealed class TwinCATSumReadRefusedException : Exception
+{
+    /// <summary>Initializes a new instance of the <see cref="TwinCATSumReadRefusedException"/> class.</summary>
+    /// <param name="adsErrorCode">The overall ADS error code the target answered with.</param>
+    /// <param name="inner">The underlying exception, if any.</param>
+    public TwinCATSumReadRefusedException(uint adsErrorCode, Exception? inner = null)
+        : base($"ADS sum-read refused by the target (ADS error {adsErrorCode})", inner)
+    {
+        AdsErrorCode = adsErrorCode;
+    }
+
+    /// <summary>Gets the overall ADS error code.</summary>
+    public uint AdsErrorCode { get; }
+}

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Drivers/Forms/TwinCATDriverForm.razor

@@ -43,6 +43,28 @@
                              class="form-control form-control-sm" />
                 <div class="form-text">0 = push immediately. Increase to coalesce high-churn signals.</div>
             </div>
+            <div class="col-md-4 mb-3">
+                <div class="form-check form-switch mt-4">
+                    <InputCheckbox id="tcSumRead" @bind-Value="_form.UseSumRead" @bind-Value:after="EmitAsync"
+                                   class="form-check-input" />
+                    <label class="form-check-label" for="tcSumRead">Batch polled reads (ADS sum-read)</label>
+                </div>
+                <div class="form-text">One round-trip per chunk instead of per tag. Falls back to single reads if the target refuses it. Default off.</div>
+            </div>
+            <div class="col-md-4 mb-3">
+                <label class="form-label" for="tcSumReadMax">Sum-read max items</label>
+                <InputNumber id="tcSumReadMax" @bind-Value="_form.SumReadMaxItems" @bind-Value:after="EmitAsync"
+                             min="1" max="500" class="form-control form-control-sm" />
+                <div class="form-text">Symbols per sum-read command (1-500, default 200).</div>
+            </div>
+            <div class="col-md-4 mb-3">
+                <div class="form-check form-switch mt-4">
+                    <InputCheckbox id="tcDiagnostics" @bind-Value="_form.DiagnosticsEnabled" @bind-Value:after="EmitAsync"
+                                   class="form-check-input" />
+                    <label class="form-check-label" for="tcDiagnostics">PLC diagnostics on /hosts</label>
+                </div>
+                <div class="form-text">Up to 4 extra reads per device per probe tick. Default off (not yet live-gated).</div>
+            </div>
         </div>
     </div>
 </section>
@@ -151,6 +173,9 @@
         public bool UseNativeNotifications { get; set; } = true;
         public bool EnableControllerBrowse { get; set; } = false;
         public int NotificationMaxDelayMs { get; set; } = 0;
+        public bool UseSumRead { get; set; } = false;
+        public int SumReadMaxItems { get; set; } = 200;
+        public bool DiagnosticsEnabled { get; set; } = false;
 
         // Probe
         public bool ProbeEnabled { get; set; } = true;
@@ -168,6 +193,9 @@
             UseNativeNotifications = o.UseNativeNotifications,
             EnableControllerBrowse = o.EnableControllerBrowse,
             NotificationMaxDelayMs = o.NotificationMaxDelayMs,
+            UseSumRead = o.UseSumRead,
+            SumReadMaxItems = o.SumReadMaxItems,
+            DiagnosticsEnabled = o.DiagnosticsEnabled,
             ProbeEnabled = o.Probe.Enabled,
             ProbeIntervalSeconds = (int)o.Probe.Interval.TotalSeconds,
             ProbeTimeoutSeconds = (int)o.Probe.Timeout.TotalSeconds,
@@ -181,6 +209,9 @@
             UseNativeNotifications = UseNativeNotifications,
             EnableControllerBrowse = EnableControllerBrowse,
             NotificationMaxDelayMs = NotificationMaxDelayMs,
+            UseSumRead = UseSumRead,
+            SumReadMaxItems = SumReadMaxItems,
+            DiagnosticsEnabled = DiagnosticsEnabled,
             Probe = new TwinCATProbeOptions
             {
                 Enabled = ProbeEnabled,

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/FakeTwinCATClient.cs

@@ -71,12 +71,54 @@ internal class FakeTwinCATClient : ITwinCATClient
         string symbolPath, TwinCATDataType type, int? bitIndex, int? arrayCount, CancellationToken ct)
     {
         if (ThrowOnRead) throw Exception ?? new InvalidOperationException();
+        SingleReadCount++;
         LastReadArrayCount = arrayCount;
         var status = ReadStatuses.TryGetValue(symbolPath, out var s) ? s : TwinCATStatusMapper.Good;
         var value = Values.TryGetValue(symbolPath, out var v) ? v : null;
         return Task.FromResult((value, status));
     }
 
+    /// <summary>Gets the number of individual <see cref="ReadValueAsync"/> calls served.</summary>
+    public int SingleReadCount { get; private set; }
+    /// <summary>Gets the size of each <see cref="ReadValuesAsync"/> batch the driver issued, in order (#645).</summary>
+    public List<int> SumReadBatches { get; } = new();
+    /// <summary>Gets or sets an exception <see cref="ReadValuesAsync"/> throws instead of answering — a
+    ///     <see cref="TwinCATSumReadRefusedException"/> models a target without sum commands.</summary>
+    public Exception? SumReadException { get; set; }
+    /// <summary>Gets or sets the ADS state text <see cref="LastAdsState"/> returns.</summary>
+    public string? AdsStateText { get; set; }
+
+    /// <summary>Simulates an ADS sum-read: records the batch size, then answers each item from the same
+    ///     tables the single read uses.</summary>
+    /// <param name="reads">The batch.</param>
+    /// <param name="ct">The cancellation token.</param>
+    /// <returns>One outcome per item.</returns>
+    public virtual Task<IReadOnlyList<(object? value, uint status)>> ReadValuesAsync(
+        IReadOnlyList<TwinCATBulkReadItem> reads, CancellationToken ct)
+    {
+        if (ThrowOnRead) throw Exception ?? new InvalidOperationException();
+        SumReadBatches.Add(reads.Count);
+        if (SumReadException is { } ex) throw ex;
+        if (SimulateSumFallback) SumFallbackCount++;
+        var results = new (object? value, uint status)[reads.Count];
+        for (var i = 0; i < reads.Count; i++)
+        {
+            var path = reads[i].SymbolPath;
+            results[i] = (Values.TryGetValue(path, out var v) ? v : null,
+                ReadStatuses.TryGetValue(path, out var s) ? s : TwinCATStatusMapper.Good);
+        }
+        return Task.FromResult<IReadOnlyList<(object? value, uint status)>>(results);
+    }
+
+    /// <summary>Gets or sets whether each batch counts as a non-latching sum fallback.</summary>
+    public bool SimulateSumFallback { get; set; }
+    /// <summary>Gets the simulated count of non-latching sum fallbacks.</summary>
+    public long SumFallbackCount { get; private set; }
+    /// <summary>Gets the simulated overall code of the last fallback.</summary>
+    public uint LastSumFallbackCode => 1795;
+    /// <summary>Gets the ADS state the "probe" last read.</summary>
+    public string? LastAdsState => AdsStateText;
+
     /// <summary>Simulates writing a value to a symbol.</summary>
     /// <param name="symbolPath">The path to the symbol to write.</param>
     /// <param name="type">The data type of the symbol.</param>

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATDiagnosticsTests.cs

@@ -0,0 +1,209 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests;
+
+/// <summary>Gitea #645 / #314 — PLC state / task-cycle facts on the optional IDriverDiagnostics bag.</summary>
+[Trait("Category", "Unit")]
+public sealed class TwinCATDiagnosticsTests
+{
+    private const string Host1 = "ads://5.23.91.23.1.1:851";
+    private const string Host2 = "ads://5.23.91.24.1.1:851";
+
+    private static TwinCATDriver NewDriver(FakeTwinCATClientFactory factory, bool probe, bool diagnostics, params (string host, string? name)[] devices) =>
+        new(new TwinCATDriverOptions
+        {
+            Devices = [.. devices.Select(d => new TwinCATDeviceOptions(d.host, d.name))],
+            Probe = new TwinCATProbeOptions { Enabled = probe, Interval = TimeSpan.FromMilliseconds(20) },
+            DiagnosticsEnabled = diagnostics,
+        }, "drv-diag", factory);
+
+    private static FakeTwinCATClient PlcClient() => new()
+    {
+        AdsStateText = "Run",
+        Values =
+        {
+            [TwinCATDriver.SymAppName] = "Plc30 App",
+            [TwinCATDriver.SymOnlineChangeCnt] = 7u,
+            [TwinCATDriver.SymCycleTime] = 100_000u,   // 10.0 ms
+            [TwinCATDriver.SymLastExecTime] = 25_000u, // 2.5 ms
+        },
+    };
+
+    private static async Task Sample(TwinCATDriver drv, FakeTwinCATClient client, string host)
+    {
+        var state = drv.GetDeviceState(host)!;
+        await drv.SampleDeviceDiagnosticsAsync(state, client, CancellationToken.None);
+    }
+
+    /// <summary>Verifies the driver advertises the optional capability and is empty before any sample.</summary>
+    [Fact]
+    public async Task Bag_is_empty_before_the_first_sample()
+    {
+        var drv = NewDriver(new FakeTwinCATClientFactory(), false, true, (Host1, null));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var diag = drv.ShouldBeAssignableTo<IDriverDiagnostics>()!;
+        diag.GetDiagnostics().ShouldBeEmpty();
+    }
+
+    /// <summary>Verifies a single-device driver publishes flat keys with state, app, cycle, exec and headroom.</summary>
+    [Fact]
+    public async Task Single_device_publishes_flat_keys()
+    {
+        var drv = NewDriver(new FakeTwinCATClientFactory(), false, true, (Host1, null));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await Sample(drv, PlcClient(), Host1);
+
+        var bag = ((IDriverDiagnostics)drv).GetDiagnostics();
+
+        bag["TwinCAT.AdsState"].ShouldBe("Run");
+        bag["TwinCAT.AppName"].ShouldBe("Plc30 App");
+        bag["TwinCAT.OnlineChangeCnt"].ShouldBe("7");
+        bag["TwinCAT.CycleTimeMs"].ShouldBe("10.0");
+        bag["TwinCAT.LastExecTimeMs"].ShouldBe("2.5");
+        bag["TwinCAT.HeadroomMs"].ShouldBe("7.5");
+        DriverDiagnostics.Bound(bag).Count.ShouldBe(bag.Count); // within the host's 16-entry cap
+    }
+
+    /// <summary>Verifies several devices are told apart by device name, falling back to host address.</summary>
+    [Fact]
+    public async Task Multiple_devices_are_prefixed()
+    {
+        var drv = NewDriver(new FakeTwinCATClientFactory(), false, true, (Host1, "Elma01"), (Host2, null));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await Sample(drv, PlcClient(), Host1);
+        await Sample(drv, PlcClient(), Host2);
+
+        var bag = ((IDriverDiagnostics)drv).GetDiagnostics();
+
+        bag["TwinCAT.Elma01"].ShouldBe("Run; Plc30 App; online-changes 7; cycle 10.0 ms; exec 2.5 ms; headroom 7.5 ms");
+        bag[$"TwinCAT.{Host2}"].ShouldContain("cycle 10.0 ms");
+        bag.ContainsKey("TwinCAT.AdsState").ShouldBeFalse();
+    }
+
+    /// <summary>Verifies a runtime without _TaskInfo (older TwinCAT 2) still yields the facts it has, and
+    /// the missing symbols are asked for once, not on every probe tick.</summary>
+    [Fact]
+    public async Task Missing_task_info_gives_a_partial_bag_and_is_not_re_asked()
+    {
+        var drv = NewDriver(new FakeTwinCATClientFactory(), false, true, (Host1, null));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        var client = new FakeTwinCATClient { AdsStateText = "Run" };
+        client.Values[TwinCATDriver.SymAppName] = "Plc1";
+        client.ReadStatuses[TwinCATDriver.SymCycleTime] = TwinCATStatusMapper.BadNodeIdUnknown;
+        client.ReadStatuses[TwinCATDriver.SymLastExecTime] = TwinCATStatusMapper.BadNodeIdUnknown;
+        client.ReadStatuses[TwinCATDriver.SymOnlineChangeCnt] = TwinCATStatusMapper.BadNodeIdUnknown;
+
+        await Sample(drv, client, Host1);
+        var readsAfterFirst = client.SingleReadCount;
+        await Sample(drv, client, Host1);
+
+        var bag = ((IDriverDiagnostics)drv).GetDiagnostics();
+        bag["TwinCAT.AdsState"].ShouldBe("Run");
+        bag["TwinCAT.AppName"].ShouldBe("Plc1");
+        bag.Keys.ShouldNotContain("TwinCAT.CycleTimeMs");
+        bag.Keys.ShouldNotContain("TwinCAT.HeadroomMs");
+        client.SingleReadCount.ShouldBe(readsAfterFirst + 1); // only AppName is read again
+    }
+
+    /// <summary>Verifies a failed sample keeps the previous snapshot rather than blanking the bag.</summary>
+    [Fact]
+    public async Task Failed_sample_keeps_the_last_values()
+    {
+        var drv = NewDriver(new FakeTwinCATClientFactory(), false, true, (Host1, null));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        var client = PlcClient();
+        await Sample(drv, client, Host1);
+
+        client.ThrowOnRead = true;
+        client.AdsStateText = null;
+        await Sample(drv, client, Host1);
+
+        var bag = ((IDriverDiagnostics)drv).GetDiagnostics();
+        bag["TwinCAT.CycleTimeMs"].ShouldBe("10.0");
+        bag["TwinCAT.AdsState"].ShouldBe("Run");
+    }
+
+    /// <summary>Verifies the probe loop itself fills the bag — no manual sampling.</summary>
+    [Fact]
+    public async Task Probe_loop_populates_the_bag()
+    {
+        var factory = new FakeTwinCATClientFactory { Customise = PlcClient };
+        var drv = NewDriver(factory, true, true, (Host1, null));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        try
+        {
+            IReadOnlyDictionary<string, string> bag = new Dictionary<string, string>();
+            for (var i = 0; i < 100 && bag.Count == 0; i++)
+            {
+                await Task.Delay(50);
+                bag = ((IDriverDiagnostics)drv).GetDiagnostics();
+            }
+            bag["TwinCAT.OnlineChangeCnt"].ShouldBe("7");
+        }
+        finally { await drv.ShutdownAsync(CancellationToken.None); }
+    }
+
+    /// <summary>Verifies a bag for the maximum number of devices fits the host's cap (one line per device).</summary>
+    [Fact]
+    public async Task Many_devices_all_fit_the_bag_cap()
+    {
+        var hosts = Enumerable.Range(1, 12).Select(i => ($"ads://5.23.91.{i}.1.1:851", (string?)$"D{i}")).ToArray();
+        var drv = NewDriver(new FakeTwinCATClientFactory(), false, true, hosts);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        foreach (var (host, _) in hosts) await Sample(drv, PlcClient(), host);
+
+        var bag = DriverDiagnostics.Bound(((IDriverDiagnostics)drv).GetDiagnostics());
+
+        bag.Count.ShouldBe(12);
+    }
+
+    /// <summary>Verifies sampling is opt-in: with it off the probe loop adds no reads and the bag stays empty.</summary>
+    [Fact]
+    public async Task Disabled_by_default_adds_no_reads()
+    {
+        var factory = new FakeTwinCATClientFactory { Customise = PlcClient };
+        var drv = NewDriver(factory, true, false, (Host1, null));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        try
+        {
+            for (var i = 0; i < 100 && drv.GetHostStatuses().All(h => h.State != HostState.Running); i++)
+                await Task.Delay(20);
+            await Task.Delay(100); // several more probe ticks
+        }
+        finally { await drv.ShutdownAsync(CancellationToken.None); }
+
+        factory.Clients[0].SingleReadCount.ShouldBe(0);
+        ((IDriverDiagnostics)drv).GetDiagnostics().ShouldBeEmpty();
+    }
+
+    /// <summary>Verifies a symbol the PLC did not export is asked for again once the client is rebuilt.</summary>
+    [Fact]
+    public async Task Unavailable_symbols_are_cleared_when_a_new_client_is_installed()
+    {
+        var factory = new FakeTwinCATClientFactory();
+        var drv = new TwinCATDriver(new TwinCATDriverOptions
+        {
+            Devices = [new TwinCATDeviceOptions(Host1)],
+            RawTags = TwinCATRawTags.Entries([new TwinCATTagDefinition("A", Host1, "MAIN.A", TwinCATDataType.DInt)]),
+            Probe = new TwinCATProbeOptions { Enabled = false },
+            DiagnosticsEnabled = true,
+        }, "drv-diag", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        var first = new FakeTwinCATClient();
+        first.ReadStatuses[TwinCATDriver.SymCycleTime] = TwinCATStatusMapper.BadNodeIdUnknown;
+        factory.Customise = () => first;
+        await drv.ReadAsync(["A"], CancellationToken.None); // installs the first client
+        await Sample(drv, first, Host1);
+        drv.GetDeviceState(Host1)!.UnavailableDiagSymbols.ContainsKey(TwinCATDriver.SymCycleTime).ShouldBeTrue();
+
+        first.SimulateWireDrop();
+        factory.Customise = () => new FakeTwinCATClient();
+        await drv.ReadAsync(["A"], CancellationToken.None); // reconnect installs a new client
+
+        drv.GetDeviceState(Host1)!.UnavailableDiagSymbols.ShouldBeEmpty();
+    }
+}

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATSumReadPlannerTests.cs

@@ -0,0 +1,109 @@
+using Shouldly;
+using TwinCAT.Ads;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests;
+
+/// <summary>The pure sum-read decisions (Gitea #645) — the part of the wire path a fake client cannot reach.</summary>
+[Trait("Category", "Unit")]
+public sealed class TwinCATSumReadPlannerTests
+{
+    private static TwinCATPlcSymbolShape Plc(string name, int size, bool isFloat = false) =>
+        new(TwinCATPlcKind.Scalar, size, isFloat, name);
+
+    /// <summary>Verifies the strict width + kind rule: only an exact match rides a sum command, the lenient
+    /// CheckRead pairs (value-converted by Beckhoff on a single read) are read singly instead.</summary>
+    [Theory]
+    [InlineData(TwinCATDataType.DInt, "DINT", 4, false, true)]
+    [InlineData(TwinCATDataType.DInt, "UDINT", 4, false, true)]    // same width + kind: tolerated
+    [InlineData(TwinCATDataType.Real, "REAL", 4, true, true)]
+    [InlineData(TwinCATDataType.Bool, "BOOL", 1, false, true)]
+    [InlineData(TwinCATDataType.DInt, "INT", 2, false, false)]     // would over-read into the neighbour
+    [InlineData(TwinCATDataType.Real, "LREAL", 8, true, false)]    // half a double
+    [InlineData(TwinCATDataType.Real, "DINT", 4, false, false)]    // bit pattern reinterpreted
+    [InlineData(TwinCATDataType.DInt, "REAL", 4, true, false)]
+    [InlineData(TwinCATDataType.Int, "DINT", 4, false, false)]     // truncation
+    public void Scalar_eligibility_needs_exact_width_and_kind(
+        TwinCATDataType declared, string plcType, int plcSize, bool plcFloat, bool eligible)
+    {
+        var shape = Plc(plcType, plcSize, plcFloat);
+
+        TwinCATSumReadPlanner.ScalarEligible(declared, shape).ShouldBe(eligible);
+        // The mismatching pairs are exactly the ones the lenient single-read check lets through.
+        if (!eligible) TwinCATTypeGuard.CheckRead(declared, shape).ShouldBeNull();
+    }
+
+    /// <summary>Verifies a string tag over a numeric symbol is not eligible.</summary>
+    [Fact]
+    public void A_string_tag_over_a_numeric_symbol_is_not_eligible() =>
+        TwinCATSumReadPlanner.ScalarEligible(TwinCATDataType.String, Plc("DINT", 4)).ShouldBeFalse();
+
+    /// <summary>Verifies only service / index-group errors latch; request-size errors split without latching.</summary>
+    [Theory]
+    [InlineData(1793u, true, false)]
+    [InlineData(1794u, true, false)]
+    [InlineData(1792u, false, true)]
+    [InlineData(1797u, false, true)]
+    [InlineData(1798u, false, true)]
+    [InlineData(1795u, false, true)]   // invalid offset: this request rejected, read singly
+    [InlineData(1796u, false, true)]
+    [InlineData(1808u, false, true)]
+    [InlineData(6u, false, false)]     // comms: every item goes Bad
+    [InlineData(7u, false, false)]
+    [InlineData(1861u, false, false)]
+    public void Overall_ads_codes_classify(uint code, bool refusal, bool rejected)
+    {
+        TwinCATSumReadPlanner.IsRefusal(code).ShouldBe(refusal);
+        TwinCATSumReadPlanner.IsRequestRejected(code).ShouldBe(rejected);
+    }
+
+    /// <summary>Verifies the byte budget splits a poll and gives an oversized item a group of its own.</summary>
+    [Fact]
+    public void Partition_respects_the_byte_budget()
+    {
+        // cost = length + 4:   104, 104, 104 | 1004 (own group) | 14
+        var groups = TwinCATSumReadPlanner.Partition([100, 100, 100, 1000, 10], 330);
+
+        groups.Select(g => g.ToArray()).ToArray().ShouldBe([[0, 1, 2], [3], [4]]);
+    }
+
+    /// <summary>Verifies an empty poll partitions to nothing.</summary>
+    [Fact]
+    public void Partition_of_nothing_is_empty() =>
+        TwinCATSumReadPlanner.Partition([], 1000).ShouldBeEmpty();
+
+    /// <summary>Verifies a Good block decodes to a scalar / an array.</summary>
+    [Fact]
+    public void Decode_returns_scalar_and_array_values()
+    {
+        byte[] four = [0x2A, 0, 0, 0];
+        var scalar = TwinCATSumReadPlanner.DecodeItem(TwinCATDataType.DInt, AdsErrorCode.NoError, four,
+            new TwinCATArrayLayout(1, 4), false, false, TwinCATStatusMapper.MapAdsError);
+        scalar.ShouldBe((42, TwinCATStatusMapper.Good));
+
+        byte[] arr = [1, 0, 2, 0];
+        var array = TwinCATSumReadPlanner.DecodeItem(TwinCATDataType.Int, AdsErrorCode.NoError, arr,
+            new TwinCATArrayLayout(2, 2), true, false, TwinCATStatusMapper.MapAdsError);
+        array.status.ShouldBe(TwinCATStatusMapper.Good);
+        array.value.ShouldBe(new short[] { 1, 2 });
+    }
+
+    /// <summary>Verifies a per-item ADS error maps to that item.</summary>
+    [Fact]
+    public void Decode_maps_a_sub_error() =>
+        TwinCATSumReadPlanner.DecodeItem(TwinCATDataType.DInt, (AdsErrorCode)1808, null,
+            new TwinCATArrayLayout(1, 4), false, false, TwinCATStatusMapper.MapAdsError)
+            .ShouldBe((null, TwinCATStatusMapper.BadNodeIdUnknown));
+
+    /// <summary>Verifies a short or missing block is a size error for that item, never an uncaught throw.</summary>
+    [Fact]
+    public void Decode_of_a_short_or_missing_block_is_a_per_item_size_error()
+    {
+        var expected = (default(object), TwinCATStatusMapper.MapAdsError(1797));
+        TwinCATSumReadPlanner.DecodeItem(TwinCATDataType.DInt, AdsErrorCode.NoError, new byte[] { 1, 2 },
+            new TwinCATArrayLayout(1, 4), false, false, TwinCATStatusMapper.MapAdsError).ShouldBe(expected);
+        TwinCATSumReadPlanner.DecodeItem(TwinCATDataType.DInt, AdsErrorCode.NoError, null,
+            new TwinCATArrayLayout(1, 4), false, false, TwinCATStatusMapper.MapAdsError).ShouldBe(expected);
+    }
+}

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATSumReadTests.cs

@@ -0,0 +1,239 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests;
+
+/// <summary>Gitea #645 / #310 — a poll's reads batched into ADS sum-read commands, with per-item status,
+/// a chunk cap and a fallback to individual reads when the target refuses the command.</summary>
+[Trait("Category", "Unit")]
+public sealed class TwinCATSumReadTests
+{
+    private const string Host = "ads://5.23.91.23.1.1:851";
+
+    private static TwinCATTagDefinition Tag(string name, TwinCATDataType type = TwinCATDataType.DInt) =>
+        new(name, Host, "MAIN." + name, type);
+
+    private static (TwinCATDriver drv, FakeTwinCATClientFactory factory) NewDriver(
+        bool useSumRead, int maxItems, params TwinCATTagDefinition[] tags)
+    {
+        var factory = new FakeTwinCATClientFactory();
+        var drv = new TwinCATDriver(new TwinCATDriverOptions
+        {
+            Devices = [new TwinCATDeviceOptions(Host)],
+            RawTags = TwinCATRawTags.Entries(tags),
+            Probe = new TwinCATProbeOptions { Enabled = false },
+            UseSumRead = useSumRead,
+            SumReadMaxItems = maxItems,
+        }, "drv-sum", factory);
+        return (drv, factory);
+    }
+
+    private static FakeTwinCATClient Seeded(params (string symbol, object value)[] values)
+    {
+        var c = new FakeTwinCATClient();
+        foreach (var (symbol, value) in values) c.Values[symbol] = value;
+        return c;
+    }
+
+    /// <summary>Verifies that sum-read is opt-in: off, every tag is read on its own.</summary>
+    [Fact]
+    public async Task Disabled_reads_each_tag_individually()
+    {
+        var (drv, factory) = NewDriver(false, 200, Tag("A"), Tag("B"), Tag("C"));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => Seeded(("MAIN.A", 1), ("MAIN.B", 2), ("MAIN.C", 3));
+
+        var snaps = await drv.ReadAsync(["A", "B", "C"], CancellationToken.None);
+
+        snaps.Select(s => s.Value).ShouldBe([1, 2, 3]);
+        factory.Clients[0].SumReadBatches.ShouldBeEmpty();
+        factory.Clients[0].SingleReadCount.ShouldBe(3);
+    }
+
+    /// <summary>Verifies that one poll's tags go out as a single sum command, values in request order.</summary>
+    [Fact]
+    public async Task Enabled_batches_a_poll_into_one_sum_command()
+    {
+        var (drv, factory) = NewDriver(true, 200, Tag("A"), Tag("B"), Tag("C"));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => Seeded(("MAIN.A", 1), ("MAIN.B", 2), ("MAIN.C", 3));
+
+        var snaps = await drv.ReadAsync(["C", "A", "B"], CancellationToken.None);
+
+        snaps.Select(s => s.Value).ShouldBe([3, 1, 2]);
+        snaps.ShouldAllBe(s => s.StatusCode == TwinCATStatusMapper.Good);
+        factory.Clients[0].SumReadBatches.ShouldBe([3]);
+        factory.Clients[0].SingleReadCount.ShouldBe(0);
+    }
+
+    /// <summary>Verifies that one item's error code lands on that tag only.</summary>
+    [Fact]
+    public async Task Per_item_error_code_is_mapped_to_that_tag_only()
+    {
+        var (drv, factory) = NewDriver(true, 200, Tag("A"), Tag("B"), Tag("C"));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () =>
+        {
+            var c = Seeded(("MAIN.A", 1), ("MAIN.C", 3));
+            c.ReadStatuses["MAIN.B"] = TwinCATStatusMapper.BadNodeIdUnknown; // ADS 1808 DeviceSymbolNotFound
+            return c;
+        };
+
+        var snaps = await drv.ReadAsync(["A", "B", "C"], CancellationToken.None);
+
+        snaps[0].StatusCode.ShouldBe(TwinCATStatusMapper.Good);
+        snaps[1].StatusCode.ShouldBe(TwinCATStatusMapper.BadNodeIdUnknown);
+        snaps[1].Value.ShouldBeNull();
+        snaps[2].StatusCode.ShouldBe(TwinCATStatusMapper.Good);
+        snaps[2].Value.ShouldBe(3);
+    }
+
+    /// <summary>Verifies the size cap: a bigger poll splits into chunks and a lone remainder reads singly.</summary>
+    [Fact]
+    public async Task Poll_larger_than_the_cap_is_split_into_chunks()
+    {
+        var tags = Enumerable.Range(0, 5).Select(i => Tag("T" + i)).ToArray();
+        var (drv, factory) = NewDriver(true, 2, tags);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () =>
+        {
+            var c = new FakeTwinCATClient();
+            for (var i = 0; i < 5; i++) c.Values["MAIN.T" + i] = i * 10;
+            return c;
+        };
+
+        var snaps = await drv.ReadAsync([.. tags.Select(t => t.Name)], CancellationToken.None);
+
+        snaps.Select(s => s.Value).ShouldBe([0, 10, 20, 30, 40]);
+        factory.Clients[0].SumReadBatches.ShouldBe([2, 2]);
+        factory.Clients[0].SingleReadCount.ShouldBe(1); // the 5th tag: a one-item "batch" is just a read
+    }
+
+    /// <summary>Verifies the refusal fallback: the poll still completes and the device stops asking.</summary>
+    [Fact]
+    public async Task Refused_sum_command_falls_back_to_individual_reads_and_sticks()
+    {
+        var (drv, factory) = NewDriver(true, 200, Tag("A"), Tag("B"), Tag("C"));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () =>
+        {
+            var c = Seeded(("MAIN.A", 1), ("MAIN.B", 2), ("MAIN.C", 3));
+            c.SumReadException = new TwinCATSumReadRefusedException(1793);
+            return c;
+        };
+
+        var first = await drv.ReadAsync(["A", "B", "C"], CancellationToken.None);
+        var second = await drv.ReadAsync(["A", "B", "C"], CancellationToken.None);
+
+        first.Select(s => s.Value).ShouldBe([1, 2, 3]);
+        first.ShouldAllBe(s => s.StatusCode == TwinCATStatusMapper.Good);
+        second.Select(s => s.Value).ShouldBe([1, 2, 3]);
+        factory.Clients[0].SumReadBatches.ShouldBe([3]);   // tried once, never again
+        factory.Clients[0].SingleReadCount.ShouldBe(6);
+        drv.GetHealth().State.ShouldBe(Core.Abstractions.DriverState.Healthy);
+    }
+
+    /// <summary>Verifies a comms failure is not mistaken for a refusal: the chunk goes Bad, sum stays on.</summary>
+    [Fact]
+    public async Task Comms_failure_marks_the_chunk_bad_and_does_not_disable_sum_read()
+    {
+        var (drv, factory) = NewDriver(true, 200, Tag("A"), Tag("B"));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () =>
+        {
+            var c = Seeded(("MAIN.A", 1), ("MAIN.B", 2));
+            c.SumReadException = new InvalidOperationException("router gone");
+            return c;
+        };
+
+        var failed = await drv.ReadAsync(["A", "B"], CancellationToken.None);
+        factory.Clients[0].SumReadException = null;
+        var recovered = await drv.ReadAsync(["A", "B"], CancellationToken.None);
+
+        failed.ShouldAllBe(s => s.StatusCode == TwinCATStatusMapper.BadCommunicationError);
+        recovered.Select(s => s.Value).ShouldBe([1, 2]);
+        factory.Clients[0].SumReadBatches.ShouldBe([2, 2]);
+    }
+
+    /// <summary>Verifies a non-latching fallback is surfaced once per device, not once per poll.</summary>
+    [Fact]
+    public async Task Non_latching_fallback_warns_once_per_device()
+    {
+        var (drv, factory) = NewDriver(true, 200, Tag("A"), Tag("B"));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => new FakeTwinCATClient { Values = { ["MAIN.A"] = 1, ["MAIN.B"] = 2 }, SimulateSumFallback = true };
+
+        await drv.ReadAsync(["A", "B"], CancellationToken.None);
+        var state = drv.GetDeviceState(Host)!;
+        state.SumFallbackWarned.ShouldBeTrue();
+        state.SumReadUnsupported.ShouldBeFalse(); // not latched: the next poll tries the sum command again
+        await drv.ReadAsync(["A", "B"], CancellationToken.None);
+        factory.Clients[0].SumReadBatches.ShouldBe([2, 2]);
+    }
+
+    /// <summary>Verifies unknown references do not disturb result order.</summary>
+    [Fact]
+    public async Task Unknown_reference_keeps_its_slot_in_a_batched_poll()
+    {
+        var (drv, factory) = NewDriver(true, 200, Tag("A"), Tag("B"));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => Seeded(("MAIN.A", 1), ("MAIN.B", 2));
+
+        var snaps = await drv.ReadAsync(["A", "nope", "B"], CancellationToken.None);
+
+        snaps[0].Value.ShouldBe(1);
+        snaps[1].StatusCode.ShouldBe(TwinCATStatusMapper.BadNodeIdUnknown);
+        snaps[2].Value.ShouldBe(2);
+        factory.Clients[0].SumReadBatches.ShouldBe([2]);
+    }
+
+    /// <summary>Verifies the batch carries bit index and array length so the client decodes like a single read.</summary>
+    [Fact]
+    public async Task Bulk_items_carry_array_length_and_bit_index()
+    {
+        var captured = new List<TwinCATBulkReadItem>();
+        var factory = new FakeTwinCATClientFactory { Customise = () => new CapturingClient(captured) };
+        var drv = new TwinCATDriver(new TwinCATDriverOptions
+        {
+            Devices = [new TwinCATDeviceOptions(Host)],
+            RawTags = TwinCATRawTags.Entries([
+                new TwinCATTagDefinition("Arr", Host, "MAIN.Arr", TwinCATDataType.Int, ArrayLength: 4),
+                new TwinCATTagDefinition("Bit", Host, "MAIN.Word.3", TwinCATDataType.Bool),
+            ]),
+            Probe = new TwinCATProbeOptions { Enabled = false },
+            UseSumRead = true,
+        }, "drv-sum", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.ReadAsync(["Arr", "Bit"], CancellationToken.None);
+
+        captured.Count.ShouldBe(2);
+        captured[0].ArrayCount.ShouldBe(4);
+        captured[1].BitIndex.ShouldBe(3);
+    }
+
+    /// <summary>Verifies the JSON keys reach the options.</summary>
+    [Fact]
+    public void Options_parse_from_driver_config_json()
+    {
+        var on = TwinCATDriverFactoryExtensions.ParseOptionsForTests(
+            """{"useSumRead": true, "sumReadMaxItems": 50}""", "d");
+        on.UseSumRead.ShouldBeTrue();
+        on.SumReadMaxItems.ShouldBe(50);
+
+        var off = TwinCATDriverFactoryExtensions.ParseOptionsForTests("{}", "d");
+        off.UseSumRead.ShouldBeFalse();
+        off.SumReadMaxItems.ShouldBe(200);
+    }
+
+    private sealed class CapturingClient(List<TwinCATBulkReadItem> sink) : FakeTwinCATClient
+    {
+        public override Task<IReadOnlyList<(object? value, uint status)>> ReadValuesAsync(
+            IReadOnlyList<TwinCATBulkReadItem> reads, CancellationToken ct)
+        {
+            sink.AddRange(reads);
+            return base.ReadValuesAsync(reads, ct);
+        }
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/TwinCATDriverPageFormSerializationTests.cs

@@ -72,6 +72,9 @@ public sealed class TwinCATDriverPageFormSerializationTests
             UseNativeNotifications = true,
             EnableControllerBrowse = false,
             NotificationMaxDelayMs = 100,
+            UseSumRead = true,
+            SumReadMaxItems = 64,
+            DiagnosticsEnabled = true,
             Probe = new TwinCATProbeOptions
             {
                 Enabled = true,
@@ -88,6 +91,9 @@ public sealed class TwinCATDriverPageFormSerializationTests
         roundTripped.UseNativeNotifications.ShouldBeTrue();
         roundTripped.EnableControllerBrowse.ShouldBeFalse();
         roundTripped.NotificationMaxDelayMs.ShouldBe(100);
+        roundTripped.UseSumRead.ShouldBeTrue();
+        roundTripped.SumReadMaxItems.ShouldBe(64);
+        roundTripped.DiagnosticsEnabled.ShouldBeTrue();
         roundTripped.Probe.Enabled.ShouldBeTrue();
         roundTripped.Probe.Interval.ShouldBe(TimeSpan.FromSeconds(6));
         roundTripped.Probe.Timeout.ShouldBe(TimeSpan.FromSeconds(2));