Function
code
Name Description
05 Force single coil Sets the status of a discrete output.
06 Preset single register Sets the value of a holding register.
08 Diagnostics Checks the communication system
between the master and the slave.
15 Force multiple coils Sets the status of multiple discrete outputs.
16 Preset multiple registers Sets the value of multiple holding registers.
23 Read/write holding registers Exchanges holding registers in one query.
3.3.2 Diagnostic functions
The diagnostic functions are only intended for serial communication. However, the
serial diagnostic counters can be read, but not reset, via the Modbus TCP/IP interface.
The serial line cannot be forced to the listen mode via the Modbus TCP/IP interface.
Table 8: Supported diagnostic subfunctions
Function
code
Name Description
00 Return query data The data in the query data field is returned
(looped back) in the response. The entire
response is identical to the query.
01
Restart communication option The slaves peripheral port is initialized and
restarted and the communication event
counters are cleared. Before this, a normal
response will be sent provided that the port
is not in the listen only mode. If the port is in
the listen only mode, no response will be
sent.
04
Force listen only mode The slave is forced to enter the listen only
mode for Modbus communication.
10 Clear counters and diagnostic register All counters and the diagnostic register are
cleared.
11 Return bus message count The response returns the number of
messages in the communication system
detected by the slave since its last restart,
clear counters operation or power up.
12
Return bus communication error count The response returns the number of CRC
errors encountered by the slave since its
last restart, clear counters operation or
power up.
13
Return bus exception error count The response returns the number of
Modbus exception responses sent by the
slave since its last restart, clear counters
operation or power up.
14
Return slave message count The response returns the number of
messages addressed to the slave or
broadcast which the slave has processed
since its last restart, clear counters
operation or power up.
Table continues on next page
Section 3 1MRS757645 B
Vendor-specific implementation
20 620 series
Communication Protocol Manual
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