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3.3.2 Modbus response to a Modbus request
The Modus server generates a Modbus response after having pro-
cessed a Modbus request.
Depending on the type of processing, two types of Modbus responses
are possible:
•
Positive Modbus response
-
The "Function Code" in the Modbus response corresponds to
the "Function Code" in the Modbus request.
•
Negative Modbus response
-
The client receives pertinent information on error detection dur-
ing processing;
-
The "Function Code" in Modbus response corresponds to the
"Function Code" in the Modbus request + 80
h
.
-
The "Exception Code" indicates the cause of the error.
If a syntactically incorrect Modbus PDU (Protocol Data Unit) is trans-
mitted, the connection is terminated. In the case of other other error, a
negative Modbus response is sent.
Excep-
tion Code
Name Description
01 Illegal Function Code The "Function Code" is unknown to the
server.
02 Illegal Data Address Depends on the Modbus request
03 Illegal Data Value Depends on the Modbus request
04 Server Failure The server was unable to properly termi-
nate processing.
05 Acknowledge The server has accepted the Modbus
request. However, the execution takes a
relatively long time. The server therefore
only returns an acknowledgement con-
firming receipt of the service request.
06 Server Busy The server was unable to accept the
Modbus request. It is the responsibility of
the application on the client to determine
whether and when to re-send the request.
0A Gateway Problem The gateway path is unavailable.
0B Gateway Problem The targeted device does not respond.
The gateway generates this error.
3 Basics
LXM32M
20 Modbus-TCP module
0198441113843, V1.01, 01.2012