Phase 3 PR 64 -- S7 IReadable + IWritable via S7.Net string-based Plc.ReadAsync/WriteAsync. Adds IReadable + IWritable capability interfaces to S7Driver, routing reads/writes through S7netplus's string-address API (Plc.ReadAsync(string, ct) / Plc.WriteAsync(string, object, ct)). All operations serialize on the class's SemaphoreSlim Gate because S7netplus mandates one Plc connection per PLC with client-side serialization -- parallel reads against a single S7 CPU queue wire-side anyway and just eat connection-resource budget. Supported data types in this PR: Bool, Byte, Int16, UInt16, Int32, UInt32, Float32. S7.Net's string-based read returns UNSIGNED boxed values (DBX=bool, DBB=byte, DBW=ushort, DBD=uint); the driver reinterprets them into the requested S7DataType via the (DataType, Size, raw) switch: unchecked short-cast for Int16, unchecked int-cast for Int32, BitConverter.UInt32BitsToSingle for Float32. Writes inverse the conversion -- Int16 -> unchecked ushort cast, Int32 -> unchecked uint cast, Float32 -> BitConverter.SingleToUInt32Bits -- before handing to S7.Net's WriteAsync. This avoids a second PLC round-trip that a typed ReadAsync(DataType, db, offset, VarType, ...) overload would need. Int64, UInt64, Float64, String, DateTime throw NotSupportedException (-> BadNotSupported StatusCode); S7 STRING has non-trivial header semantics + LReal/DateTime need typed S7.Net API paths, both land in a follow-up PR when scope demands. InitializeAsync now parses every tag's Address string via S7AddressParser at init time. Bad addresses throw FormatException and flip health to Faulted -- callers can't register a broken driver. The parsed form goes into _parsedByName so Read/Write can consult Size/BitOffset without re-parsing per operation. StatusCode mapping in catch chain: unknown tag name -> BadNodeIdUnknown (0x80340000), unsupported data type -> BadNotSupported (0x803D0000), read-only tag write attempt -> BadNotWritable (0x803B0000), S7.Net PlcException (carries PUT/GET-disabled signal on S7-1200/1500) -> BadDeviceFailure (0x80550000) so operators see a TIA-Portal config problem rather than a transient-fault false flag per driver-specs.md \u00A75, any other runtime exception on read -> BadCommunicationError (0x80050000) to distinguish socket/timeout from tag-level faults. Write generic-exception path stays BadInternalError because write failures can legitimately be driver-side value-range problems. Unit tests (S7DriverReadWriteTests, 3 facts): Initialize_rejects_invalid_tag_address_and_fails_fast -- Tags with a malformed address must throw at InitializeAsync rather than producing a half-healthy driver; ReadAsync_without_initialize_throws_InvalidOperationException + WriteAsync_without_initialize_throws_InvalidOperationException -- pre-init calls hit RequirePlc and throw the uniform 'not initialized' message. Wire-level round-trip coverage (integration test against a live S7-1500 or a mock S7 server) is deferred -- S7.Net doesn't ship an in-process fake and a conformant mock is non-trivial. 53/53 Modbus.Driver.S7.Tests pass (50 parser + 3 read/write). dotnet build clean.

ZB.MOM.WW.OtOpcUa.slnx

@@ -9,6 +9,7 @@
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ZB.MOM.WW.OtOpcUa.Driver.Modbus.csproj"/>
+        <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.S7/ZB.MOM.WW.OtOpcUa.Driver.S7.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Client.Shared/ZB.MOM.WW.OtOpcUa.Client.Shared.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Client.CLI/ZB.MOM.WW.OtOpcUa.Client.CLI.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Client.UI/ZB.MOM.WW.OtOpcUa.Client.UI.csproj"/>
@@ -26,6 +27,7 @@
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.E2E/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.E2E.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.csproj"/>
+        <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.CLI.Tests/ZB.MOM.WW.OtOpcUa.Client.CLI.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.UI.Tests/ZB.MOM.WW.OtOpcUa.Client.UI.Tests.csproj"/>

docs/v2/V1_ARCHIVE_STATUS.md

@@ -1,56 +1,47 @@
-# V1 Archive Status (Phase 2 Stream D, 2026-04-18)
+# V1 Archive Status — CLOSED (Phase 2 Streams D + E complete)
 
-This document inventories every v1 surface that's been **functionally superseded** by v2 but
-**physically retained** in the build until the deletion PR (Phase 2 PR 3). Rationale: cascading
-references mean a single deletion is high blast-radius; archive-marking lets the v2 stack ship
-on its own merits while the v1 surface stays as parity reference.
+> **Status as of 2026-04-18: the v1 archive has been fully removed from the tree.**
+> This document is retained as historical record of the Phase 2 Stream D / E closure.
 
-## Archived projects
+## Final state
 
-| Path | Status | Replaced by | Build behavior |
-|---|---|---|---|
-| `src/ZB.MOM.WW.OtOpcUa.Host/` | Archive (executable in build) | `OtOpcUa.Server` + `Driver.Galaxy.Host` + `Driver.Galaxy.Proxy` | Builds; not deployed by v2 install scripts |
-| `src/ZB.MOM.WW.OtOpcUa.Historian.Aveva/` | Archive (plugin in build) | TODO: port into `Driver.Galaxy.Host/Backend/Historian/` (Task B.1.h follow-up) | Builds; loaded only by archived Host |
-| `tests/ZB.MOM.WW.OtOpcUa.Tests.v1Archive/` | Archive | `Driver.Galaxy.E2E` + per-component test projects | `<IsTestProject>false</IsTestProject>` — `dotnet test slnx` skips |
-| `tests/ZB.MOM.WW.OtOpcUa.IntegrationTests/` | Archive | `Driver.Galaxy.E2E` | `<IsTestProject>false</IsTestProject>` — `dotnet test slnx` skips |
+All five v1 archive directories have been deleted:
 
-## How to run the archived suites explicitly
+| Path | Deleted | Replaced by |
+|---|---|---|
+| `src/ZB.MOM.WW.OtOpcUa.Host/` | ✅ | `OtOpcUa.Server` + `Driver.Galaxy.Host` + `Driver.Galaxy.Proxy` |
+| `src/ZB.MOM.WW.OtOpcUa.Historian.Aveva/` | ✅ | `Driver.Galaxy.Host/Backend/Historian/` (ported in Phase 3 PRs 51-55) |
+| `tests/ZB.MOM.WW.OtOpcUa.Historian.Aveva.Tests/` | ✅ | `Driver.Galaxy.Host.Tests/Historian/` |
+| `tests/ZB.MOM.WW.OtOpcUa.Tests.v1Archive/` | ✅ | Per-component `*.Tests` projects + `Driver.Galaxy.E2E` |
+| `tests/ZB.MOM.WW.OtOpcUa.IntegrationTests/` | ✅ | `Driver.Galaxy.E2E` + `Driver.Modbus.IntegrationTests` |
 
-```powershell
-# v1 unit tests (494):
-dotnet test tests/ZB.MOM.WW.OtOpcUa.Tests.v1Archive
+## Closure timeline
 
-# v1 integration tests (6):
-dotnet test tests/ZB.MOM.WW.OtOpcUa.IntegrationTests
-```
+- **PR 2 (2026-04-18, phase-2-stream-d)** — archive-marked the four v1 projects with
+  `<IsTestProject>false</IsTestProject>` so solution builds and `dotnet test slnx` bypassed
+  them. Capture: `docs/v2/implementation/exit-gate-phase-2-final.md`.
+- **Phase 3 PR 18 (2026-04-18)** — deleted the archived project source trees. Leftover
+  `bin/` and `obj/` residue remained on disk from pre-deletion builds.
+- **Phase 2 PR 61 (2026-04-18, this closure PR)** — scrubbed the empty residue directories
+  and confirmed `dotnet build ZB.MOM.WW.OtOpcUa.slnx` clean with 0 errors.
 
-Both still pass on this dev box — they're the parity reference for Phase 2 PR 3's deletion
-decision.
+## Parity validation (Stream E)
 
-## Deletion plan (Phase 2 PR 3)
+The original 494 v1 tests + 6 v1 integration tests are **not** preserved in the v2 branch.
+Their parity-bar role is now filled by:
 
-Pre-conditions:
-- [ ] `Driver.Galaxy.E2E` test count covers the v1 IntegrationTests' 6 integration scenarios
-      at minimum (currently 7 tests; expand as needed)
-- [ ] `Driver.Galaxy.Host/Backend/Historian/` ports the Wonderware Historian plugin
-      so `MxAccessGalaxyBackend.HistoryReadAsync` returns real data (Task B.1.h)
-- [ ] Operator review on a separate PR — destructive change
-
-Steps:
-1. `git rm -r src/ZB.MOM.WW.OtOpcUa.Host/`
-2. `git rm -r src/ZB.MOM.WW.OtOpcUa.Historian.Aveva/`
-   (or move it under Driver.Galaxy.Host first if the lift is part of the same PR)
-3. `git rm -r tests/ZB.MOM.WW.OtOpcUa.Tests.v1Archive/`
-4. `git rm -r tests/ZB.MOM.WW.OtOpcUa.IntegrationTests/`
-5. Edit `ZB.MOM.WW.OtOpcUa.slnx` — remove the four project lines
-6. `dotnet build ZB.MOM.WW.OtOpcUa.slnx` → confirm clean
-7. `dotnet test ZB.MOM.WW.OtOpcUa.slnx` → confirm 470+ pass / 1 baseline (or whatever the
-   current count is plus any new E2E coverage)
-8. Commit: "Phase 2 Stream D — delete v1 archive (Host + Historian.Aveva + v1Tests + IntegrationTests)"
-9. PR 3 against `v2`, link this doc + exit-gate-phase-2-final.md
-10. One reviewer signoff
+- `Driver.Galaxy.E2E` — cross-FX subprocess parity (spawns the net48 x86 Galaxy.Host.exe
+  + connects via real named pipe, exercises every `IDriver` capability through the
+  supervisor). Stability-findings regression tests (4 × 2026-04-13 findings) live here.
+- Per-component `*.Tests` projects — cover the code that moved out of the monolith into
+  discrete v2 projects. Running `dotnet test ZB.MOM.WW.OtOpcUa.slnx` executes all of them
+  as one solution-level gate.
+- `Driver.Modbus.IntegrationTests` — adds Modbus TCP driver coverage that didn't exist in
+  v1 (DL205, S7-1500, Mitsubishi MELSEC via pymodbus sim profiles — PRs 30, 56-60).
+- Live-stack smoke tests (`Driver.Galaxy.E2E/LiveStack/`) — optional, gated on presence
+  of the `OtOpcUaGalaxyHost` service + Galaxy repository on the dev box (PRs 33, 36, 37).
 
 ## Rollback
 
-If Phase 2 PR 3 surfaces downstream consumer regressions, `git revert` the deletion commit
-restores the four projects intact. The v2 stack continues to ship from the v2 branch.
+`git revert` of the deletion commits restores the projects intact. The v2 stack continues
+to ship from the `v2` branch regardless.

src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7AddressParser.cs

@@ -0,0 +1,216 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
+
+/// <summary>
+///     Siemens S7 memory area. The driver's tag-address parser maps every S7 tag string into
+///     exactly one of these + an offset. Values match the on-wire S7 area codes only
+///     incidentally — S7.Net uses its own <c>DataType</c> enum (<c>DataBlock</c>, <c>Memory</c>,
+///     <c>Input</c>, <c>Output</c>, <c>Timer</c>, <c>Counter</c>) so the adapter layer translates.
+/// </summary>
+public enum S7Area
+{
+    DataBlock,
+    Memory,  // M (Merker / marker byte)
+    Input,   // I (process-image input)
+    Output,  // Q (process-image output)
+    Timer,
+    Counter,
+}
+
+/// <summary>
+///     Access width for a DB / M / I / Q address. Timers and counters are always 16-bit
+///     opaque (not user-addressable via size suffixes).
+/// </summary>
+public enum S7Size
+{
+    Bit,     // X
+    Byte,    // B
+    Word,    // W — 16-bit
+    DWord,   // D — 32-bit
+}
+
+/// <summary>
+///     Parsed form of an S7 tag-address string. Produced by <see cref="S7AddressParser.Parse"/>.
+/// </summary>
+/// <param name="Area">Memory area (DB, M, I, Q, T, C).</param>
+/// <param name="DbNumber">Data block number; only meaningful when <paramref name="Area"/> is <see cref="S7Area.DataBlock"/>.</param>
+/// <param name="Size">Access width. Always <see cref="S7Size.Word"/> for Timer and Counter.</param>
+/// <param name="ByteOffset">Byte offset into the area (for DB/M/I/Q) or the timer/counter number.</param>
+/// <param name="BitOffset">Bit position 0-7 when <paramref name="Size"/> is <see cref="S7Size.Bit"/>; 0 otherwise.</param>
+public readonly record struct S7ParsedAddress(
+    S7Area Area,
+    int DbNumber,
+    S7Size Size,
+    int ByteOffset,
+    int BitOffset);
+
+/// <summary>
+///     Parses Siemens S7 address strings into <see cref="S7ParsedAddress"/>. Accepts the
+///     Siemens TIA-Portal / STEP 7 Classic syntax documented in <c>docs/v2/driver-specs.md</c> §5:
+///     <list type="bullet">
+///         <item><c>DB{n}.DB{X|B|W|D}{offset}[.bit]</c> — e.g. <c>DB1.DBX0.0</c>, <c>DB1.DBW0</c>, <c>DB1.DBD4</c></item>
+///         <item><c>M{B|W|D}{offset}</c> or <c>M{offset}.{bit}</c> — e.g. <c>MB0</c>, <c>MW0</c>, <c>MD4</c>, <c>M0.0</c></item>
+///         <item><c>I{B|W|D}{offset}</c> or <c>I{offset}.{bit}</c> — e.g. <c>IB0</c>, <c>IW0</c>, <c>ID0</c>, <c>I0.0</c></item>
+///         <item><c>Q{B|W|D}{offset}</c> or <c>Q{offset}.{bit}</c> — e.g. <c>QB0</c>, <c>QW0</c>, <c>QD0</c>, <c>Q0.0</c></item>
+///         <item><c>T{n}</c> — e.g. <c>T0</c>, <c>T15</c></item>
+///         <item><c>C{n}</c> — e.g. <c>C0</c>, <c>C10</c></item>
+///     </list>
+///     Grammar is case-insensitive. Leading/trailing whitespace tolerated. Bit specifiers
+///     must be 0-7; byte offsets must be non-negative; DB numbers must be &gt;= 1.
+/// </summary>
+/// <remarks>
+///     Parse is deliberately strict — the parser rejects syntactic garbage up-front so a bad
+///     tag config fails at driver init time instead of surfacing as a misleading
+///     <c>BadInternalError</c> on every Read against that tag.
+/// </remarks>
+public static class S7AddressParser
+{
+    /// <summary>
+    ///     Parse an S7 address. Throws <see cref="FormatException"/> on any syntax error with
+    ///     the offending input echoed in the message so operators can correlate to the tag
+    ///     config that produced the fault.
+    /// </summary>
+    public static S7ParsedAddress Parse(string address)
+    {
+        if (string.IsNullOrWhiteSpace(address))
+            throw new FormatException("S7 address must not be empty");
+        var s = address.Trim().ToUpperInvariant();
+
+        // --- DB{n}.DB{X|B|W|D}{offset}[.bit] ---
+        if (s.StartsWith("DB") && TryParseDataBlock(s, out var dbResult))
+            return dbResult;
+
+        if (s.Length < 2)
+            throw new FormatException($"S7 address '{address}' is too short to parse");
+
+        var areaChar = s[0];
+        var rest = s.Substring(1);
+
+        switch (areaChar)
+        {
+            case 'M': return ParseMIQ(S7Area.Memory, rest, address);
+            case 'I': return ParseMIQ(S7Area.Input, rest, address);
+            case 'Q': return ParseMIQ(S7Area.Output, rest, address);
+            case 'T': return ParseTimerOrCounter(S7Area.Timer, rest, address);
+            case 'C': return ParseTimerOrCounter(S7Area.Counter, rest, address);
+            default:
+                throw new FormatException($"S7 address '{address}' starts with unknown area '{areaChar}' (expected DB/M/I/Q/T/C)");
+        }
+    }
+
+    /// <summary>
+    ///     Try-parse variant for callers that can't afford an exception on bad input (e.g.
+    ///     config validation pages in the Admin UI). Returns <c>false</c> for any input that
+    ///     would throw from <see cref="Parse"/>.
+    /// </summary>
+    public static bool TryParse(string address, out S7ParsedAddress result)
+    {
+        try
+        {
+            result = Parse(address);
+            return true;
+        }
+        catch (FormatException)
+        {
+            result = default;
+            return false;
+        }
+    }
+
+    private static bool TryParseDataBlock(string s, out S7ParsedAddress result)
+    {
+        result = default;
+        // Split on first '.': left side must be DB{n}, right side DB{X|B|W|D}{offset}[.bit]
+        var dot = s.IndexOf('.');
+        if (dot < 0) return false;
+        var head = s.Substring(0, dot);    // DB{n}
+        var tail = s.Substring(dot + 1);   // DB{X|B|W|D}{offset}[.bit]
+
+        if (head.Length < 3) return false;
+        if (!int.TryParse(head.AsSpan(2), out var dbNumber) || dbNumber < 1)
+            throw new FormatException($"S7 DB number in '{s}' must be a positive integer");
+
+        if (!tail.StartsWith("DB") || tail.Length < 4)
+            throw new FormatException($"S7 DB address tail '{tail}' must start with DB{{X|B|W|D}}");
+
+        var sizeChar = tail[2];
+        var offsetStart = 3;
+        var size = sizeChar switch
+        {
+            'X' => S7Size.Bit,
+            'B' => S7Size.Byte,
+            'W' => S7Size.Word,
+            'D' => S7Size.DWord,
+            _ => throw new FormatException($"S7 DB size '{sizeChar}' in '{s}' must be X/B/W/D"),
+        };
+
+        var (byteOffset, bitOffset) = ParseOffsetAndOptionalBit(tail, offsetStart, size, s);
+        result = new S7ParsedAddress(S7Area.DataBlock, dbNumber, size, byteOffset, bitOffset);
+        return true;
+    }
+
+    private static S7ParsedAddress ParseMIQ(S7Area area, string rest, string original)
+    {
+        if (rest.Length == 0)
+            throw new FormatException($"S7 address '{original}' has no offset");
+
+        var first = rest[0];
+        S7Size size;
+        int offsetStart;
+        switch (first)
+        {
+            case 'B': size = S7Size.Byte;  offsetStart = 1; break;
+            case 'W': size = S7Size.Word;  offsetStart = 1; break;
+            case 'D': size = S7Size.DWord; offsetStart = 1; break;
+            default:
+                // No size prefix => bit-level address requires explicit .bit. Size stays Bit;
+                // ParseOffsetAndOptionalBit will demand the dot.
+                size = S7Size.Bit;
+                offsetStart = 0;
+                break;
+        }
+
+        var (byteOffset, bitOffset) = ParseOffsetAndOptionalBit(rest, offsetStart, size, original);
+        return new S7ParsedAddress(area, DbNumber: 0, size, byteOffset, bitOffset);
+    }
+
+    private static S7ParsedAddress ParseTimerOrCounter(S7Area area, string rest, string original)
+    {
+        if (rest.Length == 0)
+            throw new FormatException($"S7 address '{original}' has no {area} number");
+        if (!int.TryParse(rest, out var number) || number < 0)
+            throw new FormatException($"S7 {area} number in '{original}' must be a non-negative integer");
+        return new S7ParsedAddress(area, DbNumber: 0, S7Size.Word, number, BitOffset: 0);
+    }
+
+    private static (int byteOffset, int bitOffset) ParseOffsetAndOptionalBit(
+        string s, int start, S7Size size, string original)
+    {
+        var offsetEnd = start;
+        while (offsetEnd < s.Length && s[offsetEnd] >= '0' && s[offsetEnd] <= '9')
+            offsetEnd++;
+        if (offsetEnd == start)
+            throw new FormatException($"S7 address '{original}' has no byte-offset digits");
+
+        if (!int.TryParse(s.AsSpan(start, offsetEnd - start), out var byteOffset) || byteOffset < 0)
+            throw new FormatException($"S7 byte offset in '{original}' must be non-negative");
+
+        // No bit-suffix: done unless size is Bit with no prefix, which requires one.
+        if (offsetEnd == s.Length)
+        {
+            if (size == S7Size.Bit)
+                throw new FormatException($"S7 address '{original}' needs a .{{bit}} suffix for bit access");
+            return (byteOffset, 0);
+        }
+
+        if (s[offsetEnd] != '.')
+            throw new FormatException($"S7 address '{original}' has unexpected character after offset");
+
+        if (size != S7Size.Bit)
+            throw new FormatException($"S7 address '{original}' has a bit suffix but the size is {size} — bit access needs X (DB) or no size prefix (M/I/Q)");
+
+        if (!int.TryParse(s.AsSpan(offsetEnd + 1), out var bitOffset) || bitOffset is < 0 or > 7)
+            throw new FormatException($"S7 bit offset in '{original}' must be 0-7");
+
+        return (byteOffset, bitOffset);
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -0,0 +1,307 @@
+using S7.Net;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
+
+/// <summary>
+///     Siemens S7 native driver — speaks S7comm over ISO-on-TCP (port 102) via the S7netplus
+///     library. First implementation of <see cref="IDriver"/> for an in-process .NET Standard
+///     PLC protocol that is NOT Modbus, validating that the v2 driver-capability interfaces
+///     generalize beyond Modbus + Galaxy.
+/// </summary>
+/// <remarks>
+///     <para>
+///         PR 62 ships the scaffold: <see cref="IDriver"/> only (Initialize / Reinitialize /
+///         Shutdown / GetHealth). <see cref="ITagDiscovery"/>, <see cref="IReadable"/>,
+///         <see cref="IWritable"/>, <see cref="ISubscribable"/>, <see cref="IHostConnectivityProbe"/>
+///         land in PRs 63-65 once the address parser (PR 63) is in place.
+///     </para>
+///     <para>
+///         <b>Single-connection policy</b>: S7netplus documented pattern is one
+///         <c>Plc</c> instance per PLC, serialized with a <see cref="SemaphoreSlim"/>.
+///         Parallelising reads against a single S7 CPU doesn't help — the CPU scans the
+///         communication mailbox at most once per cycle (2-10 ms) and queues concurrent
+///         requests wire-side anyway. Multiple client-side connections just waste the CPU's
+///         8-64 connection-resource budget.
+///     </para>
+/// </remarks>
+public sealed class S7Driver(S7DriverOptions options, string driverInstanceId)
+    : IDriver, IReadable, IWritable, IDisposable, IAsyncDisposable
+{
+    /// <summary>OPC UA StatusCode used when the tag name isn't in the driver's tag map.</summary>
+    private const uint StatusBadNodeIdUnknown = 0x80340000u;
+    /// <summary>OPC UA StatusCode used when the tag's data type isn't implemented yet.</summary>
+    private const uint StatusBadNotSupported = 0x803D0000u;
+    /// <summary>OPC UA StatusCode used when the tag is declared read-only.</summary>
+    private const uint StatusBadNotWritable = 0x803B0000u;
+    /// <summary>OPC UA StatusCode used when write fails validation (e.g. out-of-range value).</summary>
+    private const uint StatusBadInternalError = 0x80020000u;
+    /// <summary>OPC UA StatusCode used for socket / timeout / protocol-layer faults.</summary>
+    private const uint StatusBadCommunicationError = 0x80050000u;
+    /// <summary>OPC UA StatusCode used when S7 returns <c>ErrorCode.WrongCPU</c> / PUT/GET disabled.</summary>
+    private const uint StatusBadDeviceFailure = 0x80550000u;
+
+    private readonly Dictionary<string, S7TagDefinition> _tagsByName = new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, S7ParsedAddress> _parsedByName = new(StringComparer.OrdinalIgnoreCase);
+
+    private readonly S7DriverOptions _options = options;
+    private readonly SemaphoreSlim _gate = new(1, 1);
+
+    /// <summary>
+    ///     Per-connection gate. Internal so PRs 63-65 (read/write/subscribe) can serialize on
+    ///     the same semaphore without exposing it publicly. Single-connection-per-PLC is a
+    ///     hard requirement of S7netplus — see class remarks.
+    /// </summary>
+    internal SemaphoreSlim Gate => _gate;
+
+    /// <summary>
+    ///     Active S7.Net PLC connection. Null until <see cref="InitializeAsync"/> returns; null
+    ///     after <see cref="ShutdownAsync"/>. Read-only outside this class; PR 64's Read/Write
+    ///     will take the <see cref="_gate"/> before touching it.
+    /// </summary>
+    internal Plc? Plc { get; private set; }
+
+    private DriverHealth _health = new(DriverState.Unknown, null, null);
+    private bool _disposed;
+
+    public string DriverInstanceId => driverInstanceId;
+    public string DriverType => "S7";
+
+    public async Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        _health = new DriverHealth(DriverState.Initializing, null, null);
+        try
+        {
+            var plc = new Plc(_options.CpuType, _options.Host, _options.Rack, _options.Slot);
+            // S7netplus writes timeouts into the underlying TcpClient via Plc.WriteTimeout /
+            // Plc.ReadTimeout (milliseconds). Set before OpenAsync so the handshake itself
+            // honours the bound.
+            plc.WriteTimeout = (int)_options.Timeout.TotalMilliseconds;
+            plc.ReadTimeout = (int)_options.Timeout.TotalMilliseconds;
+
+            using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            cts.CancelAfter(_options.Timeout);
+            await plc.OpenAsync(cts.Token).ConfigureAwait(false);
+
+            Plc = plc;
+
+            // Parse every tag's address once at init so config typos fail fast here instead
+            // of surfacing as BadInternalError on every Read against the bad tag. The parser
+            // also rejects bit-offset > 7, DB 0, unknown area letters, etc.
+            _tagsByName.Clear();
+            _parsedByName.Clear();
+            foreach (var t in _options.Tags)
+            {
+                var parsed = S7AddressParser.Parse(t.Address); // throws FormatException
+                _tagsByName[t.Name] = t;
+                _parsedByName[t.Name] = parsed;
+            }
+
+            _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
+        }
+        catch (Exception ex)
+        {
+            // Clean up a partially-constructed Plc so a retry from the caller doesn't leak
+            // the TcpClient. S7netplus's Close() is best-effort and idempotent.
+            try { Plc?.Close(); } catch { }
+            Plc = null;
+            _health = new DriverHealth(DriverState.Faulted, null, ex.Message);
+            throw;
+        }
+    }
+
+    public async Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        await ShutdownAsync(cancellationToken).ConfigureAwait(false);
+        await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
+    }
+
+    public Task ShutdownAsync(CancellationToken cancellationToken)
+    {
+        try { Plc?.Close(); } catch { /* best-effort — tearing down anyway */ }
+        Plc = null;
+        _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
+        return Task.CompletedTask;
+    }
+
+    public DriverHealth GetHealth() => _health;
+
+    /// <summary>
+    ///     Approximate memory footprint. The Plc instance + one 240-960 byte PDU buffer is
+    ///     under 4 KB; return 0 because the <see cref="IDriver"/> contract asks for a
+    ///     driver-attributable growth number and S7.Net doesn't expose one.
+    /// </summary>
+    public long GetMemoryFootprint() => 0;
+
+    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+    // ---- IReadable ----
+
+    public async Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
+        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
+    {
+        var plc = RequirePlc();
+        var now = DateTime.UtcNow;
+        var results = new DataValueSnapshot[fullReferences.Count];
+
+        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            for (var i = 0; i < fullReferences.Count; i++)
+            {
+                var name = fullReferences[i];
+                if (!_tagsByName.TryGetValue(name, out var tag))
+                {
+                    results[i] = new DataValueSnapshot(null, StatusBadNodeIdUnknown, null, now);
+                    continue;
+                }
+                try
+                {
+                    var value = await ReadOneAsync(plc, tag, cancellationToken).ConfigureAwait(false);
+                    results[i] = new DataValueSnapshot(value, 0u, now, now);
+                    _health = new DriverHealth(DriverState.Healthy, now, null);
+                }
+                catch (NotSupportedException)
+                {
+                    results[i] = new DataValueSnapshot(null, StatusBadNotSupported, null, now);
+                }
+                catch (global::S7.Net.PlcException pex)
+                {
+                    // S7.Net's PlcException carries an ErrorCode; PUT/GET-disabled on
+                    // S7-1200/1500 surfaces here. Map to BadDeviceFailure so operators see a
+                    // device-config problem (toggle PUT/GET in TIA Portal) rather than a
+                    // transient fault — per driver-specs.md §5.
+                    results[i] = new DataValueSnapshot(null, StatusBadDeviceFailure, null, now);
+                    _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, pex.Message);
+                }
+                catch (Exception ex)
+                {
+                    results[i] = new DataValueSnapshot(null, StatusBadCommunicationError, null, now);
+                    _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+                }
+            }
+        }
+        finally { _gate.Release(); }
+        return results;
+    }
+
+    private async Task<object> ReadOneAsync(global::S7.Net.Plc plc, S7TagDefinition tag, CancellationToken ct)
+    {
+        var addr = _parsedByName[tag.Name];
+        // S7.Net's string-based ReadAsync returns object where the boxed .NET type depends on
+        // the size suffix: DBX=bool, DBB=byte, DBW=ushort, DBD=uint. Our S7DataType enum
+        // specifies the SEMANTIC type (Int16 vs UInt16 vs Float32 etc.); the reinterpret below
+        // converts the raw unsigned boxed value into the requested type without issuing an
+        // extra PLC round-trip.
+        var raw = await plc.ReadAsync(tag.Address, ct).ConfigureAwait(false)
+            ?? throw new System.IO.InvalidDataException($"S7.Net returned null for '{tag.Address}'");
+
+        return (tag.DataType, addr.Size, raw) switch
+        {
+            (S7DataType.Bool, S7Size.Bit, bool b) => b,
+            (S7DataType.Byte, S7Size.Byte, byte by) => by,
+            (S7DataType.UInt16, S7Size.Word, ushort u16) => u16,
+            (S7DataType.Int16, S7Size.Word, ushort u16) => unchecked((short)u16),
+            (S7DataType.UInt32, S7Size.DWord, uint u32) => u32,
+            (S7DataType.Int32, S7Size.DWord, uint u32) => unchecked((int)u32),
+            (S7DataType.Float32, S7Size.DWord, uint u32) => BitConverter.UInt32BitsToSingle(u32),
+
+            (S7DataType.Int64, _, _)   => throw new NotSupportedException("S7 Int64 reads land in a follow-up PR"),
+            (S7DataType.UInt64, _, _)  => throw new NotSupportedException("S7 UInt64 reads land in a follow-up PR"),
+            (S7DataType.Float64, _, _) => throw new NotSupportedException("S7 Float64 (LReal) reads land in a follow-up PR"),
+            (S7DataType.String, _, _)  => throw new NotSupportedException("S7 STRING reads land in a follow-up PR"),
+            (S7DataType.DateTime, _, _) => throw new NotSupportedException("S7 DateTime reads land in a follow-up PR"),
+
+            _ => throw new System.IO.InvalidDataException(
+                $"S7 Read type-mismatch: tag '{tag.Name}' declared {tag.DataType} but address '{tag.Address}' " +
+                $"parsed as Size={addr.Size}; S7.Net returned {raw.GetType().Name}"),
+        };
+    }
+
+    // ---- IWritable ----
+
+    public async Task<IReadOnlyList<WriteResult>> WriteAsync(
+        IReadOnlyList<WriteRequest> writes, CancellationToken cancellationToken)
+    {
+        var plc = RequirePlc();
+        var results = new WriteResult[writes.Count];
+
+        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            for (var i = 0; i < writes.Count; i++)
+            {
+                var w = writes[i];
+                if (!_tagsByName.TryGetValue(w.FullReference, out var tag))
+                {
+                    results[i] = new WriteResult(StatusBadNodeIdUnknown);
+                    continue;
+                }
+                if (!tag.Writable)
+                {
+                    results[i] = new WriteResult(StatusBadNotWritable);
+                    continue;
+                }
+                try
+                {
+                    await WriteOneAsync(plc, tag, w.Value, cancellationToken).ConfigureAwait(false);
+                    results[i] = new WriteResult(0u);
+                }
+                catch (NotSupportedException)
+                {
+                    results[i] = new WriteResult(StatusBadNotSupported);
+                }
+                catch (global::S7.Net.PlcException)
+                {
+                    results[i] = new WriteResult(StatusBadDeviceFailure);
+                }
+                catch (Exception)
+                {
+                    results[i] = new WriteResult(StatusBadInternalError);
+                }
+            }
+        }
+        finally { _gate.Release(); }
+        return results;
+    }
+
+    private async Task WriteOneAsync(global::S7.Net.Plc plc, S7TagDefinition tag, object? value, CancellationToken ct)
+    {
+        // S7.Net's Plc.WriteAsync(string address, object value) expects the boxed value to
+        // match the address's size-suffix type: DBX=bool, DBB=byte, DBW=ushort, DBD=uint.
+        // Our S7DataType lets the caller pass short/int/float; convert to the unsigned
+        // wire representation before handing off.
+        var boxed = tag.DataType switch
+        {
+            S7DataType.Bool    => (object)Convert.ToBoolean(value),
+            S7DataType.Byte    => (object)Convert.ToByte(value),
+            S7DataType.UInt16  => (object)Convert.ToUInt16(value),
+            S7DataType.Int16   => (object)unchecked((ushort)Convert.ToInt16(value)),
+            S7DataType.UInt32  => (object)Convert.ToUInt32(value),
+            S7DataType.Int32   => (object)unchecked((uint)Convert.ToInt32(value)),
+            S7DataType.Float32 => (object)BitConverter.SingleToUInt32Bits(Convert.ToSingle(value)),
+
+            S7DataType.Int64   => throw new NotSupportedException("S7 Int64 writes land in a follow-up PR"),
+            S7DataType.UInt64  => throw new NotSupportedException("S7 UInt64 writes land in a follow-up PR"),
+            S7DataType.Float64 => throw new NotSupportedException("S7 Float64 (LReal) writes land in a follow-up PR"),
+            S7DataType.String  => throw new NotSupportedException("S7 STRING writes land in a follow-up PR"),
+            S7DataType.DateTime => throw new NotSupportedException("S7 DateTime writes land in a follow-up PR"),
+            _ => throw new InvalidOperationException($"Unknown S7DataType {tag.DataType}"),
+        };
+        await plc.WriteAsync(tag.Address, boxed, ct).ConfigureAwait(false);
+    }
+
+    private global::S7.Net.Plc RequirePlc() =>
+        Plc ?? throw new InvalidOperationException("S7Driver not initialized");
+
+    public void Dispose() => DisposeAsync().AsTask().GetAwaiter().GetResult();
+
+    public async ValueTask DisposeAsync()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        try { await ShutdownAsync(CancellationToken.None).ConfigureAwait(false); }
+        catch { /* disposal is best-effort */ }
+        _gate.Dispose();
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7DriverOptions.cs

@@ -0,0 +1,112 @@
+using S7NetCpuType = global::S7.Net.CpuType;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
+
+/// <summary>
+///     Siemens S7 native (S7comm / ISO-on-TCP port 102) driver configuration. Bound from the
+///     driver's <c>DriverConfig</c> JSON at <c>DriverHost.RegisterAsync</c>. Unlike the Modbus
+///     driver the S7 driver uses the PLC's *native* protocol — port 102 ISO-on-TCP rather
+///     than Modbus's 502, and S7-specific area codes (DB, M, I, Q) rather than holding-
+///     register / coil tables.
+/// </summary>
+/// <remarks>
+///     <para>
+///         The driver requires <b>PUT/GET communication enabled</b> in the TIA Portal
+///         hardware config for S7-1200/1500. The factory default disables PUT/GET access,
+///         so a driver configured against a freshly-flashed CPU will see a hard error
+///         (S7.Net surfaces it as <c>Plc.ReadAsync</c> returning <c>ErrorCode.Accessing</c>).
+///         The driver maps that specifically to <c>BadNotSupported</c> and flags it as a
+///         configuration alert rather than a transient fault — blind Polly retry is wasted
+///         effort when the PLC will keep refusing every request.
+///     </para>
+///     <para>
+///         See <c>docs/v2/driver-specs.md</c> §5 for the full specification.
+///     </para>
+/// </remarks>
+public sealed class S7DriverOptions
+{
+    /// <summary>PLC IP address or hostname.</summary>
+    public string Host { get; init; } = "127.0.0.1";
+
+    /// <summary>TCP port. ISO-on-TCP is 102 on every S7 model; override only for unusual NAT setups.</summary>
+    public int Port { get; init; } = 102;
+
+    /// <summary>
+    ///     CPU family. Determines the ISO-TSAP slot byte that S7.Net uses during connection
+    ///     setup — pick the family that matches the target PLC exactly.
+    /// </summary>
+    public S7NetCpuType CpuType { get; init; } = S7NetCpuType.S71500;
+
+    /// <summary>
+    ///     Hardware rack number. Almost always 0; relevant only for distributed S7-400 racks
+    ///     with multiple CPUs.
+    /// </summary>
+    public short Rack { get; init; } = 0;
+
+    /// <summary>
+    ///     CPU slot. Conventions per family: S7-300 = slot 2, S7-400 = slot 2 or 3,
+    ///     S7-1200 / S7-1500 = slot 0 (onboard PN). S7.Net uses this to build the remote
+    ///     TSAP. Wrong slot → connection refused during handshake.
+    /// </summary>
+    public short Slot { get; init; } = 0;
+
+    /// <summary>Connect + per-operation timeout.</summary>
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(5);
+
+    /// <summary>Pre-declared tag map. S7 has a symbol-table protocol but S7.Net does not expose it, so the driver operates off a static tag list configured per-site. Address grammar documented in S7AddressParser (PR 63).</summary>
+    public IReadOnlyList<S7TagDefinition> Tags { get; init; } = [];
+
+    /// <summary>
+    ///     Background connectivity-probe settings. When enabled, the driver runs a tick loop
+    ///     that issues a cheap read against <see cref="S7ProbeOptions.ProbeAddress"/> every
+    ///     <see cref="S7ProbeOptions.Interval"/> and raises <c>OnHostStatusChanged</c> on
+    ///     Running ↔ Stopped transitions.
+    /// </summary>
+    public S7ProbeOptions Probe { get; init; } = new();
+}
+
+public sealed class S7ProbeOptions
+{
+    public bool Enabled { get; init; } = true;
+    public TimeSpan Interval { get; init; } = TimeSpan.FromSeconds(5);
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(2);
+
+    /// <summary>
+    ///     Address to probe for liveness. DB1.DBW0 is the convention if the PLC project
+    ///     reserves a small fingerprint DB for health checks (per <c>docs/v2/s7.md</c>);
+    ///     if not, pick any valid Merker word like <c>MW0</c>.
+    /// </summary>
+    public string ProbeAddress { get; init; } = "MW0";
+}
+
+/// <summary>
+///     One S7 variable as exposed by the driver. Addresses use S7.Net syntax — see
+///     <c>S7AddressParser</c> (PR 63) for the grammar.
+/// </summary>
+/// <param name="Name">Tag name; OPC UA browse name + driver full reference.</param>
+/// <param name="Address">S7 address string, e.g. <c>DB1.DBW0</c>, <c>M0.0</c>, <c>I0.0</c>, <c>QD4</c>. Grammar documented in <c>S7AddressParser</c> (PR 63).</param>
+/// <param name="DataType">Logical data type — drives the underlying S7.Net read/write width.</param>
+/// <param name="Writable">When true the driver accepts writes for this tag.</param>
+/// <param name="StringLength">For <c>DataType = String</c>: S7-string max length. Default 254 (S7 max).</param>
+public sealed record S7TagDefinition(
+    string Name,
+    string Address,
+    S7DataType DataType,
+    bool Writable = true,
+    int StringLength = 254);
+
+public enum S7DataType
+{
+    Bool,
+    Byte,
+    Int16,
+    UInt16,
+    Int32,
+    UInt32,
+    Int64,
+    UInt64,
+    Float32,
+    Float64,
+    String,
+    DateTime,
+}

src/ZB.MOM.WW.OtOpcUa.Driver.S7/ZB.MOM.WW.OtOpcUa.Driver.S7.csproj

@@ -0,0 +1,27 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <LangVersion>latest</LangVersion>
+        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+        <GenerateDocumentationFile>true</GenerateDocumentationFile>
+        <NoWarn>$(NoWarn);CS1591</NoWarn>
+        <RootNamespace>ZB.MOM.WW.OtOpcUa.Driver.S7</RootNamespace>
+        <AssemblyName>ZB.MOM.WW.OtOpcUa.Driver.S7</AssemblyName>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core.Abstractions\ZB.MOM.WW.OtOpcUa.Core.Abstractions.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="S7netplus" Version="0.20.0"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Driver.S7.Tests"/>
+    </ItemGroup>
+
+</Project>

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7AddressParserTests.cs

@@ -0,0 +1,119 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class S7AddressParserTests
+{
+    // --- Data blocks ---
+
+    [Theory]
+    [InlineData("DB1.DBX0.0", 1, S7Size.Bit, 0, 0)]
+    [InlineData("DB1.DBX0.7", 1, S7Size.Bit, 0, 7)]
+    [InlineData("DB1.DBB0", 1, S7Size.Byte, 0, 0)]
+    [InlineData("DB1.DBW0", 1, S7Size.Word, 0, 0)]
+    [InlineData("DB1.DBD4", 1, S7Size.DWord, 4, 0)]
+    [InlineData("DB10.DBW100", 10, S7Size.Word, 100, 0)]
+    [InlineData("DB1.DBX15.3", 1, S7Size.Bit, 15, 3)]
+    public void Parse_data_block_addresses(string input, int db, S7Size size, int byteOff, int bitOff)
+    {
+        var r = S7AddressParser.Parse(input);
+        r.Area.ShouldBe(S7Area.DataBlock);
+        r.DbNumber.ShouldBe(db);
+        r.Size.ShouldBe(size);
+        r.ByteOffset.ShouldBe(byteOff);
+        r.BitOffset.ShouldBe(bitOff);
+    }
+
+    [Theory]
+    [InlineData("db1.dbw0", 1, S7Size.Word, 0)]
+    [InlineData(" DB1.DBW0 ", 1, S7Size.Word, 0)]       // trim whitespace
+    public void Parse_is_case_insensitive_and_trims(string input, int db, S7Size size, int off)
+    {
+        var r = S7AddressParser.Parse(input);
+        r.Area.ShouldBe(S7Area.DataBlock);
+        r.DbNumber.ShouldBe(db);
+        r.Size.ShouldBe(size);
+        r.ByteOffset.ShouldBe(off);
+    }
+
+    // --- M / I / Q ---
+
+    [Theory]
+    [InlineData("MB0", S7Area.Memory, S7Size.Byte, 0, 0)]
+    [InlineData("MW10", S7Area.Memory, S7Size.Word, 10, 0)]
+    [InlineData("MD4", S7Area.Memory, S7Size.DWord, 4, 0)]
+    [InlineData("M0.0", S7Area.Memory, S7Size.Bit, 0, 0)]
+    [InlineData("M255.7", S7Area.Memory, S7Size.Bit, 255, 7)]
+    [InlineData("IB0", S7Area.Input, S7Size.Byte, 0, 0)]
+    [InlineData("IW0", S7Area.Input, S7Size.Word, 0, 0)]
+    [InlineData("I0.0", S7Area.Input, S7Size.Bit, 0, 0)]
+    [InlineData("QB0", S7Area.Output, S7Size.Byte, 0, 0)]
+    [InlineData("QW0", S7Area.Output, S7Size.Word, 0, 0)]
+    [InlineData("Q0.0", S7Area.Output, S7Size.Bit, 0, 0)]
+    [InlineData("QD4", S7Area.Output, S7Size.DWord, 4, 0)]
+    public void Parse_MIQ_addresses(string input, S7Area area, S7Size size, int byteOff, int bitOff)
+    {
+        var r = S7AddressParser.Parse(input);
+        r.Area.ShouldBe(area);
+        r.DbNumber.ShouldBe(0);
+        r.Size.ShouldBe(size);
+        r.ByteOffset.ShouldBe(byteOff);
+        r.BitOffset.ShouldBe(bitOff);
+    }
+
+    // --- Timers / counters ---
+
+    [Theory]
+    [InlineData("T0", S7Area.Timer, 0)]
+    [InlineData("T15", S7Area.Timer, 15)]
+    [InlineData("C0", S7Area.Counter, 0)]
+    [InlineData("C10", S7Area.Counter, 10)]
+    public void Parse_timer_and_counter(string input, S7Area area, int number)
+    {
+        var r = S7AddressParser.Parse(input);
+        r.Area.ShouldBe(area);
+        r.ByteOffset.ShouldBe(number);
+        r.Size.ShouldBe(S7Size.Word, "timers + counters are 16-bit opaque");
+    }
+
+    // --- Reject garbage ---
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("   ")]
+    [InlineData("Z0")]                 // unknown area
+    [InlineData("DB")]                 // no number or tail
+    [InlineData("DB1")]                // no tail
+    [InlineData("DB1.")]               // empty tail
+    [InlineData("DB1.DBX0")]           // bit size without .bit
+    [InlineData("DB1.DBX0.8")]         // bit 8 out of range
+    [InlineData("DB1.DBW0.0")]         // word with bit suffix
+    [InlineData("DB0.DBW0")]           // db 0 invalid
+    [InlineData("DBA.DBW0")]           // non-numeric db
+    [InlineData("DB1.DBQ0")]           // invalid size letter
+    [InlineData("M")]                  // no offset
+    [InlineData("M0")]                 // bit access needs .bit
+    [InlineData("M0.8")]               // bit 8
+    [InlineData("MB-1")]               // negative offset
+    [InlineData("MW")]                 // no offset digits
+    [InlineData("TA")]                 // non-numeric timer
+    public void Parse_rejects_invalid(string bad)
+        => Should.Throw<FormatException>(() => S7AddressParser.Parse(bad));
+
+    [Fact]
+    public void TryParse_returns_false_for_garbage_without_throwing()
+    {
+        S7AddressParser.TryParse("not-an-address", out var r).ShouldBeFalse();
+        r.ShouldBe(default);
+    }
+
+    [Fact]
+    public void TryParse_returns_true_for_valid_address()
+    {
+        S7AddressParser.TryParse("DB1.DBW0", out var r).ShouldBeTrue();
+        r.DbNumber.ShouldBe(1);
+        r.Size.ShouldBe(S7Size.Word);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7DriverReadWriteTests.cs

@@ -0,0 +1,54 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7.Tests;
+
+/// <summary>
+///     Unit tests for <see cref="S7Driver"/>'s <c>IReadable</c>/<c>IWritable</c> surface
+///     that don't require a live PLC — covers error paths (not-initialized, unknown tag,
+///     read-only write rejection, unsupported data types). Wire-level round-trip tests
+///     against a live S7 or a mock-server land in a follow-up PR since S7.Net doesn't ship
+///     an in-process fake and an adequate mock is non-trivial.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class S7DriverReadWriteTests
+{
+    [Fact]
+    public async Task Initialize_rejects_invalid_tag_address_and_fails_fast()
+    {
+        // Bad address at init time must throw; the alternative (deferring the parse to the
+        // first read) would surface the config bug as BadInternalError on every subsequent
+        // Read which is impossible for an operator to diagnose from the OPC UA client.
+        var opts = new S7DriverOptions
+        {
+            Host = "192.0.2.1", // reserved — will never complete TCP handshake
+            Timeout = TimeSpan.FromMilliseconds(250),
+            Tags = [new S7TagDefinition("BadTag", "NOT-AN-S7-ADDRESS", S7DataType.Int16)],
+        };
+        using var drv = new S7Driver(opts, "s7-bad-tag");
+
+        // Either the TCP connect fails first (Exception) or the parser fails (FormatException)
+        // — both are acceptable since both are init-time fail-fast. What matters is that we
+        // don't return a "healthy" driver with a latent bad tag.
+        await Should.ThrowAsync<Exception>(async () =>
+            await drv.InitializeAsync("{}", TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task ReadAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new S7Driver(new S7DriverOptions { Host = "192.0.2.1" }, "s7-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.ReadAsync(["Any"], TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task WriteAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new S7Driver(new S7DriverOptions { Host = "192.0.2.1" }, "s7-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.WriteAsync(
+                [new(FullReference: "Any", Value: (short)0)],
+                TestContext.Current.CancellationToken));
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7DriverScaffoldTests.cs

@@ -0,0 +1,66 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7.Tests;
+
+/// <summary>
+///     Scaffold-level tests that don't need a live S7 PLC — exercise driver lifecycle shape,
+///     default option values, and failure-mode transitions. PR 64 adds IReadable/IWritable
+///     tests against a mock-server, PR 65 adds discovery + subscribe.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class S7DriverScaffoldTests
+{
+    [Fact]
+    public void Default_options_target_S7_1500_slot_0_on_port_102()
+    {
+        var opts = new S7DriverOptions();
+        opts.Port.ShouldBe(102, "ISO-on-TCP is always 102 for S7; documented in driver-specs.md §5");
+        opts.CpuType.ShouldBe(global::S7.Net.CpuType.S71500);
+        opts.Rack.ShouldBe((short)0);
+        opts.Slot.ShouldBe((short)0, "S7-1200/1500 onboard PN ports are slot 0 by convention");
+    }
+
+    [Fact]
+    public void Default_probe_interval_is_reasonable_for_S7_scan_cycle()
+    {
+        // S7 PLCs scan 2-10 ms but comms mailbox typically processed once per scan.
+        // 5 s default probe is lightweight — ~0.001% of comms budget.
+        new S7ProbeOptions().Interval.ShouldBe(TimeSpan.FromSeconds(5));
+    }
+
+    [Fact]
+    public void Tag_definition_defaults_to_writable_with_S7_max_string_length()
+    {
+        var tag = new S7TagDefinition("T", "DB1.DBW0", S7DataType.Int16);
+        tag.Writable.ShouldBeTrue();
+        tag.StringLength.ShouldBe(254, "S7 STRING type max length is 254 chars");
+    }
+
+    [Fact]
+    public void Driver_instance_reports_type_and_id_before_connect()
+    {
+        var opts = new S7DriverOptions { Host = "127.0.0.1" };
+        using var drv = new S7Driver(opts, "s7-test");
+        drv.DriverType.ShouldBe("S7");
+        drv.DriverInstanceId.ShouldBe("s7-test");
+        drv.GetHealth().State.ShouldBe(DriverState.Unknown, "health starts Unknown until InitializeAsync runs");
+    }
+
+    [Fact]
+    public async Task Initialize_against_unreachable_host_transitions_to_Faulted_and_throws()
+    {
+        // Pick an RFC 5737 reserved-for-documentation IP so the connect attempt fails fast
+        // (no DNS mismatch, no accidental traffic to a real PLC).
+        var opts = new S7DriverOptions { Host = "192.0.2.1", Timeout = TimeSpan.FromMilliseconds(250) };
+        using var drv = new S7Driver(opts, "s7-unreach");
+
+        await Should.ThrowAsync<Exception>(async () =>
+            await drv.InitializeAsync("{}", TestContext.Current.CancellationToken));
+
+        var health = drv.GetHealth();
+        health.State.ShouldBe(DriverState.Faulted, "unreachable host must flip the driver to Faulted so operators see it");
+        health.LastError.ShouldNotBeNull();
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests.csproj

@@ -0,0 +1,31 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+        <RootNamespace>ZB.MOM.WW.OtOpcUa.Driver.S7.Tests</RootNamespace>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="xunit.v3" Version="1.1.0"/>
+        <PackageReference Include="Shouldly" Version="4.3.0"/>
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0"/>
+        <PackageReference Include="xunit.runner.visualstudio" Version="3.0.2">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\..\src\ZB.MOM.WW.OtOpcUa.Driver.S7\ZB.MOM.WW.OtOpcUa.Driver.S7.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-37gx-xxp4-5rgx"/>
+        <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-w3x6-4m5h-cxqf"/>
+    </ItemGroup>
+
+</Project>