Siemens S7-300 communication with Emerson Ovation DCS through Modbus TCP/IP


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I have a Siemens S7-300 PLC installed offsite. It communicates with Emerson Ovation DCS through Modbus TCP/IP. The problem is that PLC is not sending the data to DCS. Can anyone help me debug the program of PLC to make it communicate with the DCS, or any example of such situation or any manual regarding the diagnosing of such problem would be helpful. Any help would be very much appreciated.

You have to give a little more detail such as:

- has this communication link ever worked

- what is the Configuration at either end

- which is master and which is slave (Isn't the DCS sending data to the offsite PLC, not the other way round).
If I were to start troubleshooting the problem, I would probably start with a Modbus client and server to verify each end of the modbus communication link (Ovation, S7) I typically use modscan but there are a couple different options out there.

The modbus communication is discussed in the manuals for ovation.
Yeah the link worked for a very little time during commisioning and has been faulty since then. Secondly, DCS is the master while S7 is the slave. The offsite PLC is transmitting data to the DCS(installed in main complex). The communication means is wireless using Alvarion Breezenet. The TCP/IP is converted to RS-485 using a Remodaq-8578 module before connecting to the DCS.