292 • Common MODBUS functions
Function code FC11 (Get comm event counter)
Modular I/O System
ETHERNET TCP/IP
6.2.10 Function code FC11 (Get comm event counter)
This function returns a status word and an event counter from the slave’s
communication event counter. By reading the current count before and after a
series of messages, a master can determine whether the messages were handled
normally by the slave.
Following each successful new processing, the counter counts up. This count-
ing process is not performed in the case of exception replies, poll commands
or counter inquiries.
Request
Byte Field name Example
Byte 0, 1 Transaction identifier 0x0000
Byte 2, 3 protocol identifier 0x0000
Byte 4, 5 length field 0x0002
Byte 6 Unit identifier 0x01 not used
Byte 7 MODBUS function code 0x0B
Response
The reply contains a 2-byte status word and a 2-byte event counter. The status
word only contains zeroes.
Byte Field name Example
.....
Byte 7 MODBUS function code 0x10
Byte 8, 9 Status 0x0000
Byte 10, 11 Event Count 0x0003
The event counter shows that 3 (0x0003) events were counted.
Exception
Byte Field name Example
.....
Byte 7 MODBUS function code 0x85
Byte 8 Exception code 0x01 or 0x02
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