Connecting RS485 and RS422


Thread Starter

John Elliott

I have looked and looked all over, and I can find people saying that it CAN be done, just no how-tos or lists of pitfalls to avoid, etc. I have a Twido Compact 24 PLC that I am trying to figure out how to connect to a ProFace ST400 HMI. The Twido uses RS485 on it's com port, and the Proface uses RS-422. I can make the cable, I just need to know what the pinout should be, do I need any extra components, etc? If someone is willing to help me, and needs them to figure it out, I can send PDFs of the manuals for both devices.

These devices were already chosen for me by my predecessor, who left the company before completing this machine (a medical gel dispensing system). I don't know if he realized that the two were different interfaces, or what. The built-in serial port will have the Proface hooked to it, as the only device. We have an add-on port that is connected to four motor drives.

I have been having to learn about all of this rather quickly. My main background is in computers, with a fair bit of electronics (TV/VCR/DVD/whatever) repair knowledge thrown in the mix. I can learn quickly, and I have been learning a lot. Prior to 5 weeks ago, I had no idea what Modbus was. Now I understand quite a bit, I just can't get my head wrapped around this, as I have been able to find no definitive answers.
Yes it can be done, here is a link to an informative paper:

The trick I use to interface between 2 unknown devices is to connect an old laptop's RS-232 RX Data pin to the negative (B I think) and Grnd to Grnd with the laptop running a terminal emulation program. This allows me to monitor the communication flow.

Set the Master device to send a simple command that requires a response from the slave. Start at a low baud rate, e.g. 1200 as it's more forgiving, and play with the parity and stop bits until you see the comms working without any garbage characters. In theory you should be able to look up both manuals and set the parameters, but I have never had that work first time. Once you get the two devices talking to each other you can switch to a higher baud rate.

There is not much point in sending the manual, by the time I get back to the office you will have it working.

Good Luck,

Ken, I would measure the voltage from each device first. You often find the actual voltage is less than the maximum allowed by spec. I think the termination resistors are different also but I doubt that matters for a short range.

I have also used RS-232 wired directly to 422 without additional hardware. The laptop method I use for example.

Dear John,
I suggest you to go for a solution that can save your time. But if your project is not so critical that introduction of components jeopardize the reliability of the system, it is worth trying. Just get a RS-485 to RS-232 converter and interface it with a RS-232 to RS-422 converter. Just ensure to use cross cabling and follow the same serial comm settings everywhere.

C Potdar