RS232 signal splitting


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Issau Muchimwe

I have got a samsung plc with one rs232 port which i am using to communicate directly with my desktop with a mini scada system. Now i want to add another pc with similar scada system. how can i split the rs232 signals to feed the 2 pcs?
By assigning each PLC a different ID number or name. If the PLC does not support this, you can add another serial port to the PC.
You have 2 solutions to accomplish this: hardware or software. Software solution include the creation of virtual COM ports sharing only one hardware port. A quick look to google bring to me to

I have not used this tool but last year i were experimenting with COM-O-COM open source project ( with results similar to what your are looking for.

If your scada is OPC capable you can use OPC server instead to share data between your screens.

The last solution is hardware. I am sure that MOXA guys have a solution for your splitter.
I think you are saying you want two Modbus Hosts to communicate with one Modbus Slave. I think Calta used to offer a smart box that would do the job. Lorne (of Calta), have you got your ears on?

Jerry Miille

I do not completely understand your application so I am offering two possible solutions.

The first solution would be to add a "converter box" that is connected to the Samsung PLC that would "map" registers in the Samsung PLC to a "database" that then could be accessed from both your "mini SCADA systems". The protocols would not necessarily need to be the same. The Samsung PLC would connect using it's standard protocol and the "SCADA" systems could talk using their "standard" protocol. All three could be different.

The other solution would be a "Pass-Thru" type of product that would play "traffic cop" in allowing messages from SCADA system one to share time with messages from SCADA system two.

In either case, the "box" would need to be able to talk Samsung protocol, whatever that is. It would also need to be able to communicate with whatever protocols your SCADA systems use. There is the crunch!

We offer this type of module that supports both modes of operation but have only 60 or so protocols that we currently support. As far as I know, Samsung is not one of them but it could be added. What protocols do your "mini SCADA systems" support?

You can contact me off list at jmiille (at) miille (dot) com if you have more questions.


I believe B&B has an RS-232 sharing/splitting device. I would recommend echoing your data out through ethernet though, either repeating from PC A, or by using a protocol converter such as Modbus serial to Modbus TCP.

Yes we build a box which permits up to three Modbus hosts. Go to and look up 'PLC09 Arbitrator'