Modbus RTU CRC & frame helper

Every Modbus RTU frame ends with a 16-bit CRC (polynomial 0xA001, init 0xFFFF), transmitted low byte first. If the CRC is wrong, the slave stays silent — there is no error reply, only a timeout. This tool computes the CRC for any frame you paste, so you can verify what your master sent or build test frames by hand.

FIG. 02From machine to data

The two classic mistakes

Byte order. The CRC goes on the wire low byte first. A frame ending … C5 CD carries CRC 0xCDC5. Swapping the bytes is the most common reason a hand-built frame gets no answer. Register data follows different rules: each register is sent high byte first, but a 32-bit float or counter spread across two registers has no standard word order — check it with the Modbus register decoder.

Off-by-one addressing. Documentation says "holding register 40001"; the protocol address in the frame is usually 0 (some vendors mean 1 — check the manual). The address converter below translates between documentation numbering (40001-style, 30001-style, 00001, 10001) and the protocol address with the matching function code.

Function code quick reference

• 01 read coils · 02 read discrete inputs · 03 read holding registers · 04 read input registers
• 05 write single coil · 06 write single register · 15 write multiple coils · 16 write multiple registers

See also

• Modbus RTU vs Modbus TCP: a practical guide
• Modbus register decoder & byte-order check — two registers to float32/int32/uint32 in ABCD, CDAB, BADC and DCBA, tested against known readings
• Tag-based addressing — Shopfloor glossary