Diagnostics (Function Code 08). ProSoft Technology MVI69E MBS

MVI69E-MBS ♦ CompactLogix Platform

Modbus Serial Enhanced Communication Module

7.2.10 Diagnostics (Function Code 08)

Reference

User Manual

This function provides a series of tests for checking the communication system between a master device and a slave, or for checking various internal error conditions within a slave.

The function uses a two-byte sub-function code field in the query to define the type of test to be performed. The slave echoes both the function code and subfunction code in a normal response. Some of the diagnostics commands 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 monitors 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 MVI69E-MBS 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

Data Field (Request)

Any

Data Field (Response)

Echo Request Data

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 28, 2017

(Hex)

08

00

00

A5

37

Response

Field Name

Function

Sub-function Hi

Sub-function Lo

Data Hi

Data Lo

(Hex)

08

00

00

A5

27

Page 113 of 154

Reference

User Manual

MVI69E-MBS ♦ CompactLogix Platform

Modbus Serial Enhanced Communication Module

The data fields in responses to other kinds of queries could contain error counts or other data requested by the sub-function code.

Page 114 of 154 ProSoft Technology, Inc.

June 28, 2017