Connectivity RS-422 to RS-232


Thread Starter

Rajesh Sharma

I have serious problem in networking. My Existing system (DATA-Concentrator- Microsol Make) works on RS-422 (4 Wire)Modbus RTU. I want to connect one MODICON Quantum PLC on same cable from DCS. I change RS-422 to RS-232 & RS-232 to RS-485 (2 wire) so I can now multidrop. Again I had convert RS-485 to RS-232 for PLC & RS-485 to RS-232 & RS-232 (Cross cable)to RS-422(4 wire) to Data Concentrator. Am using ADAM-4520 Convertor for conversion.

Now Problem starts when I give query to Data Concentrator (Existing system). It is responding in proper way as desired, but when I give query to PLC I am not getting response. Address of both system are diffrent. I check that when I remove RS-422 Link PLC starts responding & on connecting RS-422 link it does not work.

One more observation that on issuing query to PLC my Data concentrator TXDA & RXDA LED blinks & also responds. I had changed address of both systems but same problem persists.
Is it the problem of Half Duplex Full Duplex or 4 wire & 2 wire, Am in saturation stage, waiting for solution from expert what needs to be done.

Please help.

Dale Whitchurch

That is a ton of translation. It sounds to me like both of your converters are driving the other line. In 485, usually a converter is always driving one side.

Is there a way for you to simplify this to three converters? It sounds to me like you have two devices at 422 (One master, and one slave) and then another device at 232. So, three converters are necessary. Two converters will be 422->485. The other will be 485->232. Then, have a multidrop from the master, with termination set appropritely.

Let me know how it turns out.

Dale Whitchurch
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You can do other alternative is bypass all the converter & connect RS422 & RS 485 on the same network.RS422 wiring can be carried out as under.

TX + }
RX + }--------> TX +

TX -}
RX- }---------> TX -

grn ----------> grn

This is stupid suggestion, but it should not be much problem since both units are non interacting, if Voltage levels are same for both RS485/RS422.

we have never tried such combinations, but for one of our project on Modbus RTU WORKING on RS 422 [4 wire] & USED RS 485 [2 wire] to RS 232 converter. Converter was from Acromag. The above configuration is still working.