Micom P721, Technical Manual

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P72x/EN CT/A11 Communication database
Page 8/78 MiCOM P721/P723
2.6.2 Format of frames sent by the MiCOM relays
Frame sent by the MiCOM relay (response)
Slave number Function code Data CRC16
1 byte 1 byte n bytes 2 bytes
1 à FFh 1 à 10h
Slave number:
The slave number is situated between 1 and 255.
Function code:
Processed MODBUS function (1 to 16).
Data:
Contains reply data to master query .
CRC 16:
Value of the CRC 16 calculated by the slave.
2.6.3 Messages validity check
When MiCOM P721 and P723 relays receive a master query, it validates the frame:
−
If the CRC is false, the frame is invalid. MiCOM 721 and P723 relays do not reply to the
query. The master must retransmit its query. Excepting a broadcast message, this is the
only case of non-reply by
MiCOM P721 and P723 relays to a master query.
−
If the CRC is good but the MiCOM relay can not process the query, it sends an
exception response.
Warning frame sent by the MiCOM relay (response)
Slave number Function code Warning code CRC16
1 byte 1 byte 1 byte 2 bytes
1 to FFh 81h or 83h or 8Ah or 8Bh pf ... PF
Slave number:
The slave number is situated between 1 and 255.
Function code:
The function code returned by the MiCOM relay in the warning frame is the code in which the
most significant bit (b7) is forced to 1.
Warning code:
On the 8 warning codes of the MODBUS protocol, the MiCOM relay manages two of them:
•
code 01: function code unauthorised or unknown.
•
code 03: a value in the data field is unauthorised ( incorrect data ).
−
Control of pages being read
−
Control of pages being written
−
Control of addresses in pages
−
Length of request messages
CRC16:
Value of the CRC16 calculated by the slave.