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Reference MVI56-MCM
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ControlLogix Platform
Modbus Communication Module
ProSoft Technology, Inc.
Page 133 of 159
July 24, 2008
Sub-function code
Name
Hex Dec
12
18
Return Bus Character Overrun Count
13 19
RESERVED
14
20
Clear Overrun Counter and Flag
N.A. 21..65535
Reserved
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
The data fields in responses to other kinds of queries could contain error counts
or other data requested by the sub-function code.