ABB AC 31 Technical Manual page 175

Page 7-4
Communication
The MODBUS frames transmitted by the master contain the following information:
- The MODBUS address of the interrogated slave (1 byte)
- The function code defining the master request (1 byte)
- The data to exchange (N bytes)
- The CRC16 control code (2 bytes)
The frame has a maximum total length of 240 bytes thereby enabling the exchange of a
maximum of 100 data words or 255 binary data.
The slave's reply contains the request confirmation, the data to be returned and also a frame
control code. The slave returns an error code in the case of an error.
Other protocol information is supplied in the annex.
Only the following MODBUS operation codes may be processed by the 40 and 50 series
central units:
Function codes
In hexadecimal In decimal
01 or 02 01 or 02
03 or 04 03 or 04
05 05
06 06
07 07
08 08
0F 15
10 16
The codes generated on error are:
Error codes
01
02
03
Description
Read n bits
Read n words
Write a bit
Write a word
Fast reading of 8 bits
Diagnosis / initialization
Write n bits
Write n words
Description
Unknown function code
Address error
Data error
1SBC 260400 R1001-b
ABB Control - AC 31
06.99