PLX30 Series
MBS Protocol
Ethernet and Serial Gateways
User Manual
ProSoft Technology, Inc.
Page 147 of 200
September 19, 2012
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
11
06
00
01
00
03
CRC
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 gateway.
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
