Services Supported by a Modbus Link Slave Processor
(Original Document)
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At a Glance
When used as a slave processor in a Modbus link, the following processors support several services:
Data Exchanges
A slave processor manages the following requests:
Modbus request
Function code
PLC object
Read n output bits
16#01
%M
Read n input bits
16#02
%M
Read n output words
16#03
%MW
Read n input words
16#04
%MW
Write an output bit
16#05
%M
Write an output word
16#06
%MW
Write n output bits
16#0F
%M
Write n output words
16#10
%MW
Diagnostics and Maintenance
The diagnostics and maintenance information accessible from a Modbus link is listed below:
Designation
Function code/sub-function code
Echo
16#08 / 16#00
Read the PLC diagnostic registers
16#08 / 16#02
Reset PLC diagnostic registers and counters to 0
16#08 / 16#0A
Read number of messages on the bus
16#08 / 16#0B
Read number of detected communication errors on the bus
16#08 / 16#0C
Read number of detected exception errors on the bus
16#08 / 16#0D
Read number of messages received from the slave
16#08 / 16#0E
Read number of "no responses" from the slave
16#08 / 16#0F
Read number of negative acknowledgements from the slave
16#08 / 16#10
Read number of exception responses from the slave
16#08 / 16#11
Read number of overflowing characters on the bus
16#08 / 16#12
Read event counter
16#0B
Read connection event
16#0C
Read identification
16#11
Read device identification
16#2B / 16#0E