Areva MiCOM P921 - Page 195

Technical Guide P92x/EN CT/E11
Modbus Protocol
MiCOM P921-P922-P923 Page 5/118
1. MODBUS PROTOCOL
The MiCOM P921-P922-P923 relay offer MODBUS TM RTU mode communication via
a rear RS485 port.
1.1 MODBUS communication characteristics
1.1.1 MODBUS network characteristics
The MODBUS protocol is based on the master-slave principle with the MiCOM P921
or P922-P923 relay as slave.
The MODBUS protocol allows the master to read and to write one or several bits, one
or several words and to remote the event logging data.
The access to the network can be :
- either according to a query/response principle
Slave
MiCOM P921 n˚1
Slave
MiCOM P923 n˚2
Slave
MiCOM P122 n˚3
Response
Query
Master
P0410 ENa
- or according to a broadcast message sent from the master to all the slaves.
Slave
MiCOM P921 n˚1
Slave
MiCOM P923 n˚2
Slave
MiCOM P123 n˚3
Broadcast
Master
P0411ENa
in that case :
- compulsory, the broadcast message is a writing order,
- the slaves return no response,
- the protocol is RTU mode. Each byte of the data frame is coded according to a
hexadecimal base.
- At the end of each frame, two bytes of CRC16 validity checksum are applied on
the whole of the frame content.