IM-P403-89
EMM Issue 8
73
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.
Summary of Contents for BC3250
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