MVI69L-MBS ♦ Platform
Reference
Modbus Serial Lite Communication Module
User Manual
ProSoft Technology, Inc.
Page 97 of 130
7.2.10 Diagnostics (Function Code 08)
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 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
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 MVI69L-MBS.
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
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
Response
Field Name
(Hex)
Field Name
(Hex)
Function
08
Function
08
Sub-function Hi
00
Sub-function Hi
00
Sub-function Lo
00
Sub-function Lo
00
Data Hi
A5
Data Hi
A5
Data Lo
37
Data Lo
27
