ProSoft Technology MVI69-MCM User Manual

Page 140

Reference

MVI69-MCM ♦ CompactLogix or MicroLogix Platform

User Manual

Modbus Communication Module

Page 140 of 167

ProSoft Technology, Inc.

March 22, 2011

Response
The response to a preset single register request is to re-transmit the query

message after the register has been altered.

Adr

Func

Data Reg Hi

Data Reg Lo

Data Input Reg Hi

Data Input Reg Lo

Error Check Field

CRC

5.6.8 Diagnostics (Function Code 08)

MODBUS function code 08 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 sub-

function 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

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 MVI69-MCM 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

Data Field (Request)

Data Field (Response)

00 00

Any

Echo Request Data