Communication
Page 7-4 ABB Control - AC 31
1SBC 260400 R1001-b 06.99
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 Description
In hexadecimal In decimal
01 or 02 01 or 02 Read n bits
03 or 04 03 or 04 Read n words
05 05 Write a bit
06 06 Write a word
07 07 Fast reading of 8 bits
08 08 Diagnosis / initialization
0F 15 Write n bits
10 16 Write n words
The codes generated on error are:
Error codes Description
01 Unknown function code
02 Address error
03 Data error
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