ProSoft Technology 5201-MNET-ASCII - Manual (Page 62)

Modbus Protocol Specification MNET ♦ ProLinx Gateway
Driver Manual Modbus TCP/IP Interface Module
Page 62 of 78 ProSoft Technology, Inc.
September 1, 2010
4.7 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.