MVI56-MCM ♦ ControlLogix Platform Reference
Modbus Communication Module
Page 132 of 159 ProSoft Technology, Inc.
July 24, 2008
A server device can, however, be forced into 'Listen Only Mode' in which it will
monitor the messages on the communications system but not respond to them.
This can affect the outcome of your application program if it depends upon any
further exchange of data with the remote device. Generally, the mode is forced to
remove a malfunctioning remote device from the communications system.
The following diagnostic functions are dedicated to serial line devices.
The normal response to the Return Query Data request is to loopback the same
data. The function code and sub-function codes are also echoed.
Request
Function code 1 Byte 0x08
Sub-function 2 Bytes
Data N x 2 Bytes
Response
Function code 1 Byte 0x08
Sub-function 2 Bytes
Data N x 2 Bytes
Error
Error code 1 Byte 0x88
Exception code 1 Byte 01 or 03 or 04
Sub-function codes supported by the serial line devices
Here the list of sub-function codes supported by the serial line devices. Each
sub-function code is then listed with an example of the data field contents that
would apply for that diagnostic.
Sub-functions supported by the MVI56-MCM module are highlighted in BOLD.
Sub-function code Name
Hex Dec
00 00 Return Query Data
01 01 Restart Communications Option
02 02 Return Diagnostic Register
03 03 Change ASCII Input Delimiter
04 04 Force Listen Only Mode
05..09 05..09 RESERVED
0A 10 Clear Counters and Diagnostic Register
0B 11 Return Bus Message Count
0C 12 Return Bus Communication Error Count
0D 13 Return Bus Exception Error Count
0E 14 Return Slave Message Count
0F 15 Return Slave No Response Count
10 16 Return Slave NAK Count
11 17 Return Slave Busy Count
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