# TwinCAT test fixture

Coverage map + gap inventory for the Beckhoff TwinCAT ADS driver.

**TL;DR:** Integration-test scaffolding lives at
`tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/` (task #221).
`TwinCATXarFixture` probes TCP 48898 on an operator-supplied VM; three
smoke tests (read / write / native notification) run end-to-end through
the real ADS stack when the VM is reachable, skip cleanly otherwise.
**Remaining operational work**: stand up a TwinCAT 3 XAR runtime in a
Hyper-V VM, author the `.tsproj` project documented at
`TwinCatProject/README.md`, rotate the 7-day trial license (or buy a
paid runtime). Unit tests via `FakeTwinCATClient` still carry the
exhaustive contract coverage.

TwinCAT is the only driver outside Galaxy that uses **native
notifications** (no polling) for `ISubscribable`, and the fake exposes a
fire-event harness so notification routing is contract-tested rigorously
at the unit layer.

## What the fixture is

**Integration layer** (task #221, scaffolded):
`tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/` —
`TwinCATXarFixture` TCP-probes ADS port 48898 on the host specified by
`TWINCAT_TARGET_HOST` + requires `TWINCAT_TARGET_NETID` (AmsNetId of the
VM). No fixture-owned lifecycle — XAR can't run in Docker because it
bypasses the Windows kernel scheduler, so the VM stays
operator-managed. `TwinCatProject/README.md` documents the required
`.tsproj` project state; the file itself lands once the XAR VM is up +
the project is authored. Three smoke tests:
`Driver_reads_seeded_DINT_through_real_ADS`,
`Driver_write_then_read_round_trip_on_scratch_REAL`, and
`Driver_subscribe_receives_native_ADS_notifications_on_counter_changes`
— all skip cleanly via `[TwinCATFact]` when the runtime isn't
reachable.

**Unit layer**: `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/` is
still the primary coverage. `FakeTwinCATClient` also fakes the
`AddDeviceNotification` flow so tests can trigger callbacks without a
running runtime.

## What it actually covers

### Integration (XAR VM, task #221 — code scaffolded, needs VM + project)

- `TwinCAT3SmokeTests.Driver_reads_seeded_DINT_through_real_ADS` — real AMS
  handshake + ADS read of `GVL_Fixture.nCounter` (seeded at 1234, MAIN
  increments each cycle)
- `TwinCAT3SmokeTests.Driver_write_then_read_round_trip_on_scratch_REAL` —
  real ADS write + read on `GVL_Fixture.rSetpoint`
- `TwinCAT3SmokeTests.Driver_subscribe_receives_native_ADS_notifications_on_counter_changes`
  — real `AddDeviceNotification` against the cycle-incrementing counter;
  observes `OnDataChange` firing within 3 s of subscribe

All three gated on `TWINCAT_TARGET_HOST` + `TWINCAT_TARGET_NETID` env
vars; skip cleanly via `[TwinCATFact]` when the VM isn't reachable or
vars are unset.

### Unit

- `TwinCATAmsAddressTests` — `ads://<netId>:<port>` parsing + routing
- `TwinCATCapabilityTests` — data-type mapping (primitives + declared UDTs),
  read-only classification
- `TwinCATReadWriteTests` — read + write through the fake, status mapping
- `TwinCATSymbolPathTests` — symbol-path routing for nested struct members
- `TwinCATSymbolBrowserTests` — `ITagDiscovery.DiscoverAsync` via
  `ReadSymbolsAsync` (#188) + system-symbol filtering
- `TwinCATNativeNotificationTests` — `AddDeviceNotification` (#189)
  registration, callback-delivery-to-`OnDataChange` wiring, unregister on
  unsubscribe
- `TwinCATDriverTests` — `IDriver` lifecycle

Capability surfaces whose contract is verified: `IDriver`, `IReadable`,
`IWritable`, `ITagDiscovery`, `ISubscribable`, `IHostConnectivityProbe`,
`IPerCallHostResolver`.

## What it does NOT cover

### 1. AMS / ADS wire traffic

No real AMS router frame is exchanged. Beckhoff's `TwinCAT.Ads` NuGet (their
own .NET SDK, not libplctag-style OSS) has no in-process fake; tests stub
the `ITwinCATClient` abstraction above it.

### 2. Multi-route AMS

ADS supports chained routes (`<localNetId> → <routerNetId> → <targetNetId>`)
for PLCs behind an EC master / IPC gateway. Parse coverage exists; wire-path
coverage doesn't.

### 3. Notification reliability under jitter

`AddDeviceNotification` delivers at the runtime's cycle boundary; under high
CPU load or network jitter real notifications can coalesce. The fake fires
one callback per test invocation — real callback-coalescing behavior is
untested.

### 4. TC2 vs TC3 variant handling

TwinCAT 2 (ADS v1) and TwinCAT 3 (ADS v2) have subtly different
`GetSymbolInfoByName` semantics + symbol-table layouts. Driver targets TC3;
TC2 compatibility is not exercised.

### 5. Cycle-time alignment for `ISubscribable`

Native ADS notifications fire on the PLC cycle boundary. The fake test
harness assumes notifications fire on a timer the test controls;
cycle-aligned firing under real PLC control is not verified.

### 6. Alarms / history

Driver doesn't implement `IAlarmSource` or `IHistoryProvider` — not in
scope for this driver family. TwinCAT 3's TcEventLogger could theoretically
back an `IAlarmSource`, but shipping that is a separate feature.

## When to trust TwinCAT tests, when to reach for a rig

| Question | Unit tests | Real TwinCAT runtime |
| --- | --- | --- |
| "Does the AMS address parser accept X?" | yes | - |
| "Does notification → `OnDataChange` wire correctly?" | yes (contract) | yes |
| "Does symbol browsing filter TwinCAT internals?" | yes | yes |
| "Does a real ADS read return correct bytes?" | no | yes (required) |
| "Do notifications coalesce under load?" | no | yes (required) |
| "Does a TC2 PLC work the same as TC3?" | no | yes (required) |

## Follow-up candidates

1. **XAR VM live-population** — scaffolding is in place (this PR); the
   remaining work is operational: stand up the Hyper-V VM, install XAR,
   author the `.tsproj` per `TwinCatProject/README.md`, configure the
   bilateral ADS route, set `TWINCAT_TARGET_HOST` + `TWINCAT_TARGET_NETID`
   on the dev box. Then the three smoke tests transition skip → pass.
   Tracked as #221.
2. **License-rotation automation** — XAR's 7-day trial expires on
   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. The doc at
   `TwinCatProject/README.md` §License rotation walks through both.
3. **Lab rig** — cheapest IPC (CX7000 / CX9020) on a dedicated network;
   the only route that covers TC2 + real EtherCAT I/O timing + cycle
   jitter under CPU load.

## Key fixture / config files

- `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/TwinCATXarFixture.cs`
  — TCP probe + skip-attributes + env-var parsing
- `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/TwinCAT3SmokeTests.cs`
  — three wire-level smoke tests
- `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/TwinCatProject/README.md`
  — project spec + VM setup + license-rotation notes
- `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/FakeTwinCATClient.cs` —
  in-process fake with the notification-fire harness used by
  `TwinCATNativeNotificationTests`
- `src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs` — ctor takes
  `ITwinCATClientFactory`
