P92x/EN CT/E11 Technical Guide
Modbus Protocol
Page 6/118 MiCOM P921-P922-P923
1.1.2 Parameters of the MODBUS connection
The different parameters of the MODBUS connection are as follows :
- Isolated two-point RS485 connection (2kV 50Hz).
- MODBUS line protocol in RTU mode.
- The baud rate can be configurated by operator dialogue in the front panel of
the relay :
Baud rate
300
600
1200
2400
4800
9600
19200
38400
- Mode de transmission des characters paramétrable par dialogue opérateur
Transmission mode
1 start / 8 bits / 1 stop : total 10 bits
1 start / 8 bits / even parity / 1 stop : total 11 bits
1 start / 8 bits / odd parity / 1 stop : total 11 bits
1 start / 8 bits / 2 stop : total 11 bits
1.1.3 Synchronisation of exchanged messages
Any character received after a silence on the line with more or equal to a
transmission time of 3 bytes is considered as a frame start.
1.1.4 Message validity check
The validation of a trame is performed with a 16-bit cyclical redundancy check (CRC).
The generator polynomial is :
1 + x² + x
15
+ x
16
= 1010 0000 0000 0001 binary = A001h
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