lmxopcua/docs/v2/implementation/focas-wire-protocol.md

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.

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.