120 • Fieldbus CommunicationMODBUS FunctionsWAGO-I/O-SYSTEM 750ETHERNET TCP/IP4.2.3 Description of the MODBUS FunctionsAll MODBUS functions are executed as follows:A MODBUS TCP master (e.g., a PC) makes a request to the WAGO fieldbusnode using a specific function code based on the desired operation. TheWAGO fieldbus node receives the datagram and then responds to the masterwith the proper data, which is based on the master’s request.If the WAGO fieldbus node receives an incorrect request, it sends an errordatagram (Exception) to the master.The exception code contained in the exception has the following meaning:Exception Code Meaning0x01 Illegal Function0x02 Illegal Data Address0x03 Illegal Data Value0x04 Slave Device Failure0x05 Acknowledge0x06 Server Busy0x08 Memory Parity Error0x0A Gateway Path Unavailable0x0B Gateway Target Device Failed To RespondThe following chapters describe the datagram architecture of request, responseand exception with examples for each function code.NoteIn the case of the read functions (FC1 – FC4) the outputs can be additionallywritten and read back by adding an offset of 200 hex (0x0200) to the MODBUSaddresses in the range of [0 hex - FF hex ] and an offset of 1000 hex (0x01000) to theMODBUS addresses in the range of [6000 hex - 62FC hex ].
