Technical data

MODBUS Functions • 147
Description of the MODBUS Functions
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
6.3 Description of the MODBUS Functions
All MODBUS functions in the WAGO ETHERNET fieldbus coupler and
controller are executed as follows:
A MODBUS TCP master (e.g., a PC) makes a request to the fieldbus
controller using a specific function code based on the desired operation. The
fieldbus controller receives the datagram and then responds to the master with
the proper data, which is based on the master’s request.
If the coupler receives an incorrect request, it sends an error datagram
(Exception) 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
0x05 Acknowledge
0x06 Server Busy
0x08 Memory Parity Error
0x0A Gateway Path Unavailable
0x0B Gateway Target Device Failed To Respond
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 – FC4) the outputs can be additionally
written and read back by adding an offset of 200
hex
(0x0200) to the MODBUS
addresses in the range of [0
hex
- FF
hex
] and an offset of 1000
hex
(0x01000) to the
MODBUS addresses in the range of [6000
hex
- 62FC
hex
].