# Beckhoff TwinCAT (ADS) Driver

Getting-started guide for the Beckhoff TwinCAT driver. This is the short path —
for the full per-field spec read [`docs/v2/driver-specs.md §6`](../v2/driver-specs.md),
for hands-on CLI testing read [Driver.TwinCAT.Cli.md](../Driver.TwinCAT.Cli.md),
and for the test-harness map read [TwinCAT-Test-Fixture.md](TwinCAT-Test-Fixture.md).

## What it talks to

Beckhoff PLC runtimes — **TwinCAT 2 and TwinCAT 3** — over the Beckhoff **ADS**
protocol carried by **AMS** routing. The driver runs in-process in the OtOpcUa
server's .NET 10 AnyCPU host. It compiles and runs without a local AMS router,
but every wire call returns `BadCommunicationError` until a router is reachable
(the router translates an AMS Net ID to an IP route).

Addressing is **symbol-based**: tags are referenced by their TwinCAT symbolic
name (e.g. `MAIN.bStart`, `GVL.Counter`, `Motor1.Status.Running`) rather than by
raw memory offset. One driver instance fans out to N targets, each identified by
an AMS Net ID + port.

## Project split

| Project | Target | Role |
|---------|--------|------|
| `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/` | net10.0 | In-process driver — hosts the ADS client, symbol-path parser, and per-device probe loops |
| `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Contracts/` | net10.0 | Config records + the `TwinCATDataType` enum bound from `DriverConfig` JSON |

## Minimum deployment

```jsonc
"Drivers": {
  "twincat-cell-1": {
    "Type": "TwinCAT",
    "Config": {
      "Devices": [ { "HostAddress": "ads://5.23.91.23.1.1:851", "DeviceName": "Cell1" } ],
      "Tags": [
        { "Name": "Start",  "DeviceHostAddress": "ads://5.23.91.23.1.1:851",
          "SymbolPath": "MAIN.bStart", "DataType": "Bool", "Writable": true },
        { "Name": "Count",  "DeviceHostAddress": "ads://5.23.91.23.1.1:851",
          "SymbolPath": "GVL.Counter", "DataType": "Int32", "Writable": false }
      ]
    }
  }
}
```

### AMS address form

`HostAddress` is an `ads://{netId}:{port}` URI parsed by
`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATAmsAddress.cs`. The Net ID
is six dot-separated octets (NOT an IP — a Beckhoff-specific identifier the
router maps to a route); the port is the AMS service port (851 = TC3 PLC runtime
1, 852 = runtime 2, 801 / 811 / 821 = TC2 PLC runtimes). Port defaults to 851
when omitted (`ads://5.23.91.23.1.1`).

### Symbol path form

Symbol paths are parsed by
`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATSymbolPath.cs`, which
mirrors IEC 61131-3 structured-text identifiers: global-variable-list
(`GVL.Counter`), program variable (`MAIN.bStart`), struct member access
(`Motor1.Status.Running`), array subscripts (`Data[5]`, `Matrix[1,2]`), and
bit-access (`Flags.0`).

## Tag discovery

`DiscoverAsync` always emits the pre-declared `Tags` as the authoritative config
path, under `TwinCAT/{device}/`. When `EnableControllerBrowse` is set, the
driver also walks each device's symbol table and surfaces controller-resident
globals / program locals under a `Discovered/` sub-folder; any symbol-loader
error falls back to pre-declared-only so a flaky symbol download never blocks
discovery.

## Capability surface

`TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver, IRediscoverable`
(`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs`).

| Capability | Path | Notes |
|------------|------|-------|
| `IReadable` | `ReadAsync` → ADS `ReadValueAsync` | Per-device client, lazily connected and serialized per device |
| `IWritable` | `WriteAsync` → ADS `WriteValueAsync` | Read-only tags return `BadNotWritable` |
| `ITagDiscovery` | `DiscoverAsync` | Pre-declared tags + opt-in controller symbol browse |
| `ISubscribable` | native ADS notifications (default), poll fallback | `UseNativeNotifications=true` registers device notifications so the PLC pushes changes; `false` uses the shared `PollGroupEngine` |
| `IHostConnectivityProbe` | per-device probe loop | One `HostConnectivityStatus` per configured device; `Running`/`Stopped` transitions raise `OnHostStatusChanged` |
| `IPerCallHostResolver` | `ResolveHost` lookup in the tag map | Routes each call to the device of the referenced tag; returns an empty-string sentinel when unresolved |
| `IRediscoverable` | symbol-version-changed callback | A PLC re-download fires `OnRediscoveryNeeded` so the address space is rebuilt |

### Rediscovery on PLC re-download

`IRediscoverable` is the distinguishing capability. When the ADS client detects
`DeviceSymbolVersionInvalid` (1809 / 0x0711) — the documented TwinCAT
symbol-version-changed signal, raised when a PLC program is re-downloaded —
every symbol and notification handle is invalidated. The driver raises
`OnRediscoveryNeeded` with a `TwinCAT` scope hint so Core rebuilds the address
space rather than treating it as a transient connection error.

### Native notifications

By default the driver registers native ADS device notifications: the PLC pushes
value changes on its own cycle, which is strictly better for latency and CPU
than polling. `NotificationMaxDelayMs` lets TwinCAT coalesce notifications up to
a batching delay for high-churn signals. Set `UseNativeNotifications=false` for
deployments where the AMS router has notification limits you can't raise — then
the driver falls through to the shared poll engine.

## Single-connection-per-device

Each device's ADS client is lazily connected and serialized by a per-device
connect gate, so a concurrent read / write / probe can't race a client
create-or-dispose. Probe-initiated connects use the probe timeout; reads and
writes use the driver-wide `Timeout`.

## Testing

- **Unit tests** — `tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/` cover
  the AMS / symbol-path parsers, the status mapper, and the driver lifecycle via
  a fake ADS client factory.
- **Integration fixture** — see
  [TwinCAT-Test-Fixture.md](TwinCAT-Test-Fixture.md) for the harness map.
- **CLI** — [Driver.TwinCAT.Cli.md](../Driver.TwinCAT.Cli.md) documents the
  standalone read/write/browse/probe CLI for manual checks.

## Further reading

- [`docs/v2/driver-specs.md §6`](../v2/driver-specs.md) — full per-field spec and
  DriverConfig JSON shape
- [Driver.TwinCAT.Cli.md](../Driver.TwinCAT.Cli.md) — standalone TwinCAT driver CLI
- [TwinCAT-Test-Fixture.md](TwinCAT-Test-Fixture.md) — test-harness map
