lmxopcua/docs/v2/modbus-addressing.md

Modbus tag-addressing reference

Foundational doc for the Modbus addressing grammar shipped across #136–#144. Covers the address-string parser (ModbusAddressParser) that the wire driver and the Admin UI both consume, the per-tag suffix modifiers, and the family- native branch.

Grammar

<region><offset>[.<bit>][:<type>[<len>]][:<order>][:<count>]

Each field is optional from left to right; the parser fills defaults.

Region + offset

Three accepted forms — pick whichever matches your tag spreadsheet's convention. All three resolve to the same (Region, ushort PduOffset) on the wire.

Form	Example	Means
Modicon 5-digit	40001	Holding register 1 (PDU 0)
Modicon 6-digit	400001	Holding register 1 (PDU 0); supports up to 465536 (PDU 65535)
Mnemonic	HR1, IR1, C100, DI1	Same regions; 1-based register number

Modicon leading-digit → region:

Digit	Region	OPC UA wire FC
0	Coils	FC01 / FC05 / FC15
1	DiscreteInputs	FC02 (read-only)
3	InputRegisters	FC04 (read-only)
4	HoldingRegisters	FC03 / FC06 / FC16

Bit suffix .N

40001.5 = bit 5 (LSB-first) of HR[0]. Implies DataType=BitInRegister; mixing with an explicit type or array-count is rejected.

Type code :T

Code	Type	Registers
BOOL	Boolean	1 (region must be Coils / DiscreteInputs)
I	Int16	1
UI	UInt16	1
DI, L	Int32	2
UDI, UL	UInt32	2
LI	Int64	4
ULI	UInt64	4
F	Float32	2
D	Float64	4
BCD	16-bit BCD	1
LBCD	32-bit BCD	2
STR<len>	ASCII string, len chars (2 chars / register)	ceil(len/2)

Default when omitted:

• Coils / DiscreteInputs → BOOL
• HoldingRegisters / InputRegisters → I (Int16)

Byte order :O

Mnemonic	Meaning	Wire
ABCD	Big-endian (Modbus spec default)	[A,B,C,D]
CDAB	Word swap (Siemens, several AB)	[C,D,A,B]
BADC	Byte swap (legacy little-endian-internal devices)	[B,A,D,C]
DCBA	Full reverse (some EtherNet/IP gateways)	[D,C,B,A]

For 8-byte values (Int64 / Float64) the same labels apply pairwise.

Array count :N

40001:F:5 = Float32[5] (consumes HR[0..9]). Array + bit suffix is rejected. Strings are not arrays.

Composition

The 3-field shorthand 40001:F:5 is parsed as (type=F, count=5) because 5 isn't a valid byte-order mnemonic. Use the explicit 4-field form 40001:F:CDAB:5 when you need a non-default order.

Family-native syntax (#144)

When the driver instance has Family != Generic, the parser tries the family's native syntax FIRST, then falls back to Modicon / mnemonic.

DL205 (AutomationDirect DirectLOGIC)

Form	Example	Mapping
Vnnnn (octal)	V2000	HoldingRegisters[1024] (octal 2000 = decimal 1024)
Ynn (octal)	Y17	Coils[2048 + 15] (Y-output base + offset)
Cnn (octal)	C100	Coils[3072 + 64] (C-relay base + offset)
Xnn (octal)	X17	DiscreteInputs[15]
SPnn (octal)	SP10	DiscreteInputs[1024 + 8]

Cross-family ambiguity: C100 means Coils[99] under Generic (mnemonic) but Coils[3136] under DL205. Per-driver Family choice disambiguates.

MELSEC (Mitsubishi)

Form	Example	Mapping (sub-family Q_L_iQR / F_iQF)
Dnnn (decimal)	D100	HoldingRegisters[100]
Mnnn (decimal)	M50	Coils[50]
Xnn	X20	DiscreteInputs[32 hex / 16 octal]
Ynn	Y20	Coils[32 hex / 16 octal]

X / Y digit interpretation depends on MelsecSubFamily:

• Q_L_iQR → hex (default)
• F_iQF → octal

Bank-base offsets default to 0 in the grammar string. Sites with non-zero "Modbus Device Assignment" bases use the structured tag form.

Driver-instance options

Beyond per-tag addressing, ModbusDriverOptions exposes (#139–#143):

Connection (#139)

• KeepAlive { Enabled, Time, Interval, RetryCount } — TCP-level probes. Defaults match the historical PR 53 wire output (Enabled=true, Time=30s, Interval=10s, RetryCount=3).
• IdleDisconnectTimeout — proactively close + reconnect after this much socket idle time. Default null = disabled.
• Reconnect { InitialDelay, MaxDelay, BackoffMultiplier } — geometric backoff for the post-drop reconnect loop. Default (0, 30s, 2.0) = immediate first retry, geometric thereafter.

Protocol (#140)

• MaxCoilsPerRead (default 2000) — separate cap for FC01/FC02 coil reads.
• UseFC15ForSingleCoilWrites — force FC15 (write multiple coils qty=1) for single-coil writes. Safety/audit PLCs may require this.
• UseFC16ForSingleRegisterWrites — same for FC16 vs FC06.
• DisableFC23 — kill switch for FC23 (currently unused; reserved).

Subscribe (#141)

• Per-tag Deadband — suppress sub-threshold publishes on numeric tags.
• WriteOnChangeOnly (driver-level) — short-circuit identical-value writes. Cache invalidates on read-divergence.

Multi-unit (#142)

• Per-tag UnitId — overrides the driver-level UnitId in the MBAP header. Required for one-Ethernet-gateway / N-RTU-slave deployments.
• IPerCallHostResolver.ResolveHost returns host:port/unitN per tag so per-PLC circuit breakers fire per slave.
• Per-tag CoalesceProhibited — escape hatch for #143's planner (read this tag in isolation regardless of MaxReadGap).

Block-read coalescing (#143)

• MaxReadGap (default 0 = off) — gap budget the planner is willing to bridge between adjacent register tags. With MaxReadGap=10, three tags at HR 100/102/110 collapse into one FC03 of quantity 11.

JSON DTO shape

The factory accepts both the structured form (legacy) and the new AddressString form per-tag. Mix freely — newer pasted rows use the grammar string; legacy rows keep the structured fields.

{
  "host": "10.1.2.3",
  "port": 502,
  "unitId": 1,
  "family": "DL205",
  "keepAlive": { "enabled": true, "timeMs": 30000, "intervalMs": 10000, "retryCount": 3 },
  "idleDisconnectMs": 120000,
  "reconnect": { "initialDelayMs": 0, "maxDelayMs": 30000, "backoffMultiplier": 2.0 },
  "maxCoilsPerRead": 2000,
  "writeOnChangeOnly": false,
  "maxReadGap": 8,
  "tags": [
    { "name": "Temp",        "addressString": "V2000:F:CDAB" },
    { "name": "Setpoint",    "addressString": "40001:I" },
    { "name": "Outputs",     "addressString": "Y0:5" },
    { "name": "AlarmCount",  "region": "HoldingRegisters", "address": 200, "dataType": "Int16", "deadband": 5.0 }
  ]
}

Vendor compatibility caveat

The exact spelling of type codes (e.g. I vs INT, BCD vs L_BCD) and the byte-order mnemonics were synthesised from training-era vendor docs (Wonderware DASMBTCP, Kepware KEPServerEX, Ignition, Matrikon, OAS). Before locking the grammar for a production deployment, verify against the current Kepware "Modbus Ethernet Driver Help" PDF and Ignition's "Modbus Addressing" user-manual page — if a critical tool's mnemonics have shifted, add aliases in ModbusAddressParser.TryParseType rather than asking users to rewrite spreadsheets.