connecting two PLCs via RS422, >4000 ft


Thread Starter

Sasha Benim

I have a problem connecting two PLCs via RS422 interface because of the 4000 ft distance limitation. I need to connect at about 2.5 km. Can
anyone advise on real solutions (repeaters, converters, etc.) to transmit the RS422 data to longer distances in a reasonably noisy

Thanks in advance.

Tom Fenimore

We have used fiber optic converters to convert the RS422 signals to fiber and then back at the other end. We have this working at sever al sites due to noise and distance. It works well and is simple to implement. B&B electronics makes a nice DIN rail mount product for $229.00 that will send the data 2.5 miles. Check them out at 815-433-5100

Tom Fenimore
You could put a repeater in the middle, but that is a bad idea in the long run - you will need power and an enclosure etc. Lots of opportunity for regrets here!

The best solution is to convert the RS-422 to RS-232 & use some form of short-haul modem with a range of 10Km or more. They are cheap & easy to find (or use a spread-spectrum radio pair) Do the same at the other end. This solution should be regret free.

Best Regards

Lynn August Linse, Senior Product Application Engineer
15353 Barranca Parkway, Lantronix Inc, Irvine CA 92618
[email protected]
Tel: (949)450-7272 Fax: (949)453-7132

Ian B. Milne

You have not indicated the baud rate you are using (there is an inverse relationship between speed and distance) however you should look into
the Black Box Async Short-Haul Modem-B Plus. These units effectively give you an RS-232 to RS-232 connection between two widely separated
devices and are good for 9600 baud at up to 3.2km. The modems operate using two shielded pairs (Tx & Rx). I have used these units in various industrial applications and to date have had good results.

The Black Box web site is Follow the Product
Listings and search for "short haul"

Ian B. Milne, P.Eng.
Upside Engineering Ltd.
Calgary, Canada