Modbus TCP through a fibre/serial media converter

  • Thread starter Jonathan Whelan
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Jonathan Whelan


I'm working on a project where I'm connecting a Momentum PLC to a SCADA system. The PLC was installed by an outside contractor and comes to an HMI via ethernet->fibre->ethernet. The original discussion with my site contact led me to believe that I'd be getting an ethernet connection from the PLC (so Modbus TCP) and use a small Prolinx protocol converter to dump DNP 3.0 over serial to the SCADA.

Now they want to re-do the fibre connection to make it the same as everything else in the plant. The fibre will come into a patch panel converting to serial, then they want to install another media converter to give me back the ethernet. So from the PLC to the protocol converter it will be:


I guess what my question is, what happens to the Modbus TCP when it's put through the serial to ethernet media converter (not a protocol converter? What comes out the other side? Will it still work with the protocol converter?
If it's just a media converter, then the same thing that went in one end should could come out the other end. The Modbus/TCP messages themselves don't care about the media, they're just data. Modbus/TCP is an application protocol. It doesn't control routing or timing. *However*, the TCP/IP stack drivers might not be happy about the extra signal delays. Some embedded Ethernet systems have some odd implementations that don't matter under normal circumstances, but which turn up in unusual applications. It's not something you can really tell for sure until you try it. This whole idea sounds like a problem waiting to happen.

Also, you said you would "use a small Prolinx protocol converter to dump DNP 3.0 over serial to the SCADA". So that's:

1) Modbus/TCP on Ethernet
2) to fiber
3) to serial
4) to Ethernet
5) to DNP 3.0 on serial

I would really hate to be the one left holding the bag on this one. Is there no way to just go:

1) Modbus/TCP on Ethernet
2) to fiber
3) to Ethernet and straight into the SCADA computer?

That's one protocol and the media transitions are minimized and transparent.
Thanks for the insight. I clarifies some of the issues I was looking at.

This is the way the site owner wants to do it now:

>1) Modbus/TCP on Ethernet
>2) to fiber
>3) to serial
>4) to Ethernet
>5) to DNP 3.0 on serial

and I agree its a mess. The reason this is even an option is because fibre to serial through a patch panel is their 'standard' way of doing things. This is the way I want to do it and the way it was originally tables as being done:

1) Modbus/TCP on Ethernet
2) to fiber
3) to Ethernet
4) to DNP 3.0 over serial

Step four is an unfortunate necessity because the SCADA doesn't handled Modbus and I can't find a protocol converter that goes Modbus TCP over ethernet to DNP 3.0 over ethernet without requiring an additional switch.