Diagnostics (Function Code 08) - ProSoft Technology ControlLogix inRAx MVI56E-MNETR User Manual

MVI56E-MNETR ♦ ControlLogix Platform
Modbus TCP/IP Interface Module with Reduced Data Block

5.5.8 Diagnostics (Function Code 08)

MODBUS function code 08 provides a series of tests for checking the
communication system between a Client device and a server, or for checking
various internal error conditions within a server.
The function uses a two-byte sub-function code field in the query to define the
type of test to be performed. The server echoes both the function code and sub-
function code in a normal response. Some of the diagnostics cause data to be
returned from the remote device in the data field of a normal response.
In general, issuing a diagnostic function to a remote device does not affect the
running of the user program in the remote device. Device memory bit and
register data addresses are not accessed by the diagnostics. However, certain
functions can optionally reset error counters in some remote devices.
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.
Sub-function codes supported
Only Sub-function 00 is supported by the MVI56E-MNETR module.
00 Return Query Data
The data passed in the request data field is to be returned (looped back) in the
response. The entire response message should be identical to the request.
Sub-function
00 00
Example and state diagram
Here is an example of a request to remote device to Return Query Data. This
uses a sub-function code of zero (00 00 hex in the two-byte field). The data to be
returned is sent in the two-byte data field (A5 37 hex).
Request
Field Name
Function
Sub-function Hi
Sub-function Lo
Data Hi
Data Lo
ProSoft Technology, Inc.
June 14, 2011
Data Field (Request)
Any
Response
(Hex) Field Name
08 Function
00 Sub-function Hi
00 Sub-function Lo
A5 Data Hi
37 Data Lo
Data Field (Response)
Echo Request Data
(Hex)
08
00
00
A5
27
Page 139 of 175
Reference
User Manual