282 • Common MODBUS functions
Description of the MODBUS functions
Modular I/O System
ETHERNET TCP/IP
6.2 Description of the MODBUS functions
All MODBUS functions in the WAGO ETHERNET fieldbus coupler and
controller are executed as follows:
When a function code is entered, the MODBUS master (i.e. PC) makes a re-
quest to the coupler/controller of the fieldbus node.
Subsequently, the coupler/controller sends a datagram to the master as a re-
sponse.
If the coupler receives an incorrect request, it sends an error datagram (Excep-
tion) to the master.
The exception code contained in the exception has the following meaning:
Exception Code
Meaning
0x01
Illegal Function
0x02
Illegal Data Address
0x03
Illegal Data Value
0x04
Slave Device Failure
The following chapters describe the datagram architecture of request, response
and exception with examples for each function code.
Note
In the case of the read functions (FC1 – FC 4) the outputs can be additionally
written and read back by adding an offset of 200
hex
(0x0200) to the MODBUS
address.