Spirax Sarco BC3250 - 11. Appendix: Summary of the Modbus Protocol

IM-P403-89 EMM Issue 8
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11. Appendix
Summary of the Modbus Protocol
Format: Byte
Start 1 bit
Data 8 bit
Parity 0 bit
Stop 1 bit
Format: Request frame
Address 1 byte
Function code 1 byte
Start address 2 bytes
Quantity of registers 2 bytes
Cyclic redundancy check (CRC) 2 bytes
Total 8 bytes
Format: Response frame
Address 1 byte
Function code 1 byte (or error code = function code plus 128)
Byte count 1 byte (or exception code, see below)
Register data 2 times the quantity of 16-bit register
Error check (CRC) 2 bytes
Total 9 bytes if correct (or 5 bytes if in error)
Only Function Code 03, 'Read holding registers' is allowed.