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01Modbus CRC-16 calculator

Paste the frame without CRC. Polynomial 0xA001 (reflected), init 0xFFFF, appended low byte first — the Modbus RTU standard.

Frame bytes (hex, spaces optional)

Paste a full ADU. MBAP header and PDU are split into named fields — exceptions are decoded automatically.

Frame bytes (hex)

The codes you'll actually meet in the field, with the request → response shape.

FC	Name	Request → response
01	Read Coils 0x area	start + quantity → packed bits
02	Read Discrete Inputs 1x area	start + quantity → packed bits
03	Read Holding Registers 4x area	start + quantity → byte count + register values
04	Read Input Registers 3x area	start + quantity → byte count + register values
05	Write Single Coil	address + FF00/0000 → echo
06	Write Single Register	address + value → echo
0F	Write Multiple Coils	start + quantity + packed bits → start + quantity
10	Write Multiple Registers	start + quantity + values → start + quantity
16	Mask Write Register	address + AND + OR masks → echo
17	Read/Write Multiple	write block + read block in one transaction
2B	Encapsulated Interface	MEI 0x0E: read device identification

Response FC = request FC + 0x80, one data byte carries the code. Definitions are everywhere; the third column is what matters.

Code	Name	Most likely cause · check first
01	Illegal Function	Device doesn't implement that FC. Check the manual's supported-FC list; try 03 instead of 04 (or vice versa).
02	Illegal Data Address	Start address or quantity runs past the map. Nine times out of ten it's the 40001-vs-0-based offset — subtract 1 and retry.
03	Illegal Data Value	Quantity of 0, >125 registers, or a write value outside limits. Check the count field before the payload.
04	Slave Device Failure	Device-internal fault while answering. Power-cycle the slave; check its own error indicators.
05	Acknowledge	Long-running command accepted, not finished. Poll again later — not an error.
06	Slave Device Busy	Retry with backoff. If constant, another master or a firmware task is hogging it.
08	Memory Parity Error	Device memory fault on file-record access. Usually a dying device.
0A	Gateway Path Unavailable	The TCP→RTU gateway can't route. Check gateway config and the serial side's wiring.
0B	Gateway Target Failed	Gateway is fine, the serial slave behind it didn't answer. Debug the RTU segment: address, baud, A/B lines.