# FOCAS wire protocol — packed-buffer surface

Notes on the language-neutral packed-buffer encoding the FOCAS driver +
focas-mock simulator share. This format is **not** the FWLIB native struct
layout — Tier-C Fwlib32 backends marshal directly from the FANUC C struct.
The packed surface exists so the simulator (Python / FastAPI) and the .NET
wire client can speak a common format over IPC without piping a Win32 DLL
through both ends.

## Command id table

Each FOCAS-equivalent call gets a stable wire-protocol command id. Ids are
**append-only** — never renumber, never reuse.

| Id | FOCAS API | Surface |
| --- | --- | --- |
| `0x0001` | `cnc_rdcncstat` | ODBST 9-field status struct |
| `0x0002` | `cnc_rdparam` | parameter value (one number) |
| `0x0003` | `cnc_rdmacro` | macro variable value |
| `0x0004` | `cnc_rddiag` | diagnostic value |
| ... | ... | ... |
| **`0x0102`** | **`cnc_wrparam`** | **IODBPSD parameter-write packet (issue #269, plan PR F4-b)** |
| **`0x0103`** | **`cnc_wrmacro`** | **ODBM macro-write packet (issue #269, plan PR F4-b)** |
| `0x0F1A` | **`cnc_rdalmhistry`** | **ODBALMHIS alarm-history ring-buffer dump (issue #267, plan PR F3-a)** |

## ODBALMHIS — alarm history (`cnc_rdalmhistry`, command `0x0F1A`)

Issued by `FocasAlarmProjection` when
`FocasDriverOptions.AlarmProjection.Mode == ActivePlusHistory`. Returns up
to `depth` most-recent ring-buffer entries.

### Request

| Offset | Width | Field | Notes |
| --- | --- | --- | --- |
| 0 | `int16 LE` | `depth` | clamped client-side to `[1..250]` (`FocasAlarmProjectionOptions.MaxHistoryDepth`) |

### Response (packed buffer, little-endian)

| Offset | Width | Field |
| --- | --- | --- |
| 0 | `int16 LE` | `num_alm` — number of entries that follow. `< 0` indicates CNC error. |
| 2 | repeated | `ALMHIS_data alm[num_alm]` (see below) |

Each entry block:

| Offset (rel.) | Width | Field |
| --- | --- | --- |
| 0 | `int16 LE` | `year` |
| 2 | `int16 LE` | `month` |
| 4 | `int16 LE` | `day` |
| 6 | `int16 LE` | `hour` |
| 8 | `int16 LE` | `minute` |
| 10 | `int16 LE` | `second` |
| 12 | `int16 LE` | `axis_no` (1-based; 0 = whole-CNC) |
| 14 | `int16 LE` | `alm_type` (P/S/OT/SV/SR/MC/SP/PW/IO encoded numerically) |
| 16 | `int16 LE` | `alm_no` |
| 18 | `int16 LE` | `msg_len` (0..32 typical) |
| 20 | `msg_len` | ASCII message (no null terminator) |
| `20 + msg_len` | 0..3 | pad to 4-byte boundary so per-entry blocks stay self-delimiting |

The CNC stamps `year..second` in **its own local time**. The deployment
guide instructs operators to keep CNC clocks on UTC so the projection's
dedup key `(OccurrenceTime, AlarmNumber, AlarmType)` stays stable across
DST transitions. The .NET decoder
(`Wire/FocasAlarmHistoryDecoder.Decode`) constructs each
`DateTimeOffset` with `TimeSpan.Zero` (UTC) on that assumption.

### Error handling

- A negative `num_alm` short-circuits decode to an empty list — the
  projection treats it as "no history this tick" and the next poll
  retries.
- Malformed timestamps (e.g. month=0) are skipped per-entry instead of
  faulting the whole decode; the dedup key for malformed entries would be
  unstable anyway.
- `msg_len` overrunning the payload truncates the entry list at the
  malformed entry rather than throwing.

## IODBPSD — parameter write (`cnc_wrparam`, command `0x0102`)

Issue #269, plan PR F4-b. The write-side payload is the **byte-symmetric
inverse of the `cnc_rdparam` read** — the same `IODBPSD` struct shape, and
the .NET wire client uses the read-side decoder reversed (`EncodeParamValue`
in `FwlibFocasClient.cs`) so the encoder/decoder are guaranteed to stay in
lock-step.

### Request

| Offset | Width | Field |
| --- | --- | --- |
| 0 | `int16 LE` | `datano` — parameter number (e.g. `1815`) |
| 2 | `int16 LE` | `type` — axis index (1-based; `0` = whole-CNC parameter) |
| 4 | `length` | `data` payload — width depends on parameter type |

`length` (request frame trailer, drives `data` width):

| FocasDataType | `length` | Payload encoding |
| --- | --- | --- |
| `Byte` | `4 + 1` | one signed byte at offset 4 |
| `Int16` | `4 + 2` | int16 LE at offset 4 |
| `Int32` | `4 + 4` | int32 LE at offset 4 |

Bit-addressed parameters (`PARAM:1815/0` form) are not supported by F4-b
and surface as `BadNotSupported`; F4-c will land the read-modify-write
helper alongside the PMC bit RMW path.

### Response

Single `int16 LE` return code per the standard FWLIB convention:

- `0` → `Good`
- `11` (`EW_PASSWD`) → **`BadUserAccessDenied`** (was `BadNotWritable`
  pre-F4-b — see `FocasStatusMapper`). Means the parameter-write switch is
  off or the CNC isn't in MDI mode; the F4-d unlock workflow will close
  the loop on this from the OPC UA side.
- Other `EW_*` codes map per
  [`FocasStatusMapper.MapFocasReturn`](../../src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasStatusMapper.cs).

## ODBM — macro write (`cnc_wrmacro`, command `0x0103`)

Issue #269, plan PR F4-b. The write-side payload mirrors the
`cnc_rdmacro` read shape: the same `(mcr_val, dec_val)` (integer +
decimal-point count) split, but emitted from the .NET side rather than
decoded.

### Request

| Offset | Width | Field |
| --- | --- | --- |
| 0 | `int16 LE` | `number` — macro variable number (e.g. `500`) |
| 2 | `int16 LE` | `length` — fixed at `8` for ODBM |
| 4 | `int32 LE` | `mcr_val` — scaled integer value |
| 8 | `int16 LE` | `dec_val` — decimal-point count |

F4-b ships **integer-only writes** (`dec_val = 0`) to match the most
common HMI pattern; a future `WriteMacroScaled` overload will land if the
field calls for fractional macro setpoints. Read-side decoders apply
`mcr_val / 10^dec_val`, so a `dec_val = 0` write surfaces back as the
integer it was emitted as.

### Response

Same single-int16 envelope as `cnc_wrparam`. `EW_PASSWD` is rare on macro
writes (the gate-switch protection is parameter-specific) but the mapper
treats both kinds identically.

### Symmetry note

The plan carries a "byte layout symmetry" requirement — the encoder for
each kind is the read-side decoder reversed. Adding a new parameter type
(e.g. `Int64` parameters, when they ship) means extending both sides in
the same PR; the unit test
`FocasWriteParameterTests.ParameterWrite_round_trip_stores_value_visible_to_subsequent_read`
exercises encode → store → decode with the fake wire client and is the
canary for symmetry regressions.