Technical Guide P92x/EN CT/E11
Modbus Protocol
MiCOM P921-P922-P923 Page 7/118
1.1.5 Address of the MiCOM relays
The address of the MiCOM relay on a same MODBUS network is situated between 1
and 255. The address 0 is reserved for the broadcast messages.
1.2 MODBUS functions of the MiCOM relays
The MODBUS functions implemented on the MiCOM relays are :
Function 1 or 2 : Reading of n bits
Function 3 or 4 : Reading of n words
Function 5 : Writing of 1 bit
Function 6 : Writing of 1 word
Function 7 : Fast reading of 8 bits
Function 8 : Reading of the diagnosis counters
Function 11 : Reading of the Event counter
Function 15: Writing of n bits
Function 16 : Writing of n words
1.3 Presentation of the MODBUS protocol
MODBUS is a master-slave protocol whereby every exchange involves a master query
and a slave response.
1.3.1 Frame size received by the MiCOM P92x relay
Frame transmitted by the master (query) :
Slave number Function code Information CRC16
1 byte 1 byte n bytes 2 bytes
Slave number :
The slave number is situated between 1 and 255.
Function code :
Requested MODBUS function (1 to 16).
Information :
Contains the parameters of the selected function.
CRC16 :
Value of the CRC16 calculated by the master.
NOTA : the MiCOM relay does not respond to globally broadcast frames
sent out by the master.
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