Areva MiCOM P921, Technical Manual

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P92x/EN CT/E11 Technical Guide
Modbus Protocol
Page 8/118 MiCOM P921-P922-P923
1.3.2 Format of frames sent by the MiCOM P92x relay
Frame sent by the MiCOM relay (response) :
Slave number Function code Data CRC16
1 byte 1 byte n bytes 2 bytes
Slave number :
The slave number is situated between 1 and 255.
Function code :
Processed MODBUS function (1 to 16).
Data :
Contains the response data to master query.
CRC16 :
Value of the CRC16 calculated by the MiCOM relay.
1.3.3 Messages validity check
When the MiCOM relay receives a master query, it validates the frame :
· If the CRC is false, the frame is invalid. The MiCOM relay does not reply to the
query. The master must retransmit its query. Excepting a broadcast message, this
is the only case of non-reply by the MiCOM relay to a master query.
· If the CRC is correct but the MiCOM relay can not process the query, it sents to
the master a exception response.
Exception frame sent by the MiCOM relay (response) :
Slave number Function code Error code CRC16
1 byte 1 byte 1 byte 2 bytes
Slave number :
The slave number is situated between 1 and 255.
Function code :
The function code returned by the MiCOM relay in the exception frame is the code in
which the most significant bit (bit7) is forced to 1.