PLC Long distance comm. suggest


Thread Starter

Jody Gallant

Hi. Me again.

Another project we are working on with our local municipality requires long distance communications between a Master PLC (at a standpipe) to several slaves at the pumphouses.

In the past, the city has used 300 Baud telephone company circuits. Most PLC's don't even go this low anymore, so we are looking for suggestions on what would be the best method.

We have been looking at spread spectrum RF, but don't know about reliability, or vulnerability to vandilism. (big towers make great target practice, especially when some of the sites are in wooded areas.)

The only other affordable option I can think of would be leased lines. Problem is, the local phone company only knows how to provide you with the line. What type of hardware should we use? What type of topology (bus, star...?) and what are the pros/cons. Would we terminate everything at the master PLC and use a concentrator?

Just for more info, the PLC's will most likely be Modicon, using the modbus port (unless someone thinks the MB+ might be a better option), and the distances will vary from 1/4 mile to 4 miles or so.

Thanks in advance
Jody Gallant CET
Shadcomm Ltd.
A distance of 4 miles should be no problem for an RF link. Depending on terrain, you probably won't even need a tower at the pumping station. The absence of a tower should ease any vandalism concerns short of an all out pumping station assault.

If continuous monitoring is required, RF is probably the most economical but reliability falls a bit short of a leased line. One advantage is that you are in control of an RF link. If the unit fails you can drive to the station and slap in your spare (which I'm sure you will have on hand). With a leased line you're at the mercy of the phone company if things go bad.

Jerome W. Lavoie

If you can use RS-232C, then 910 MHZ radio modems might be the answer. You could look into such products as the Black Box corporation MDR100A-R4 with YAGI antenna MD3310-R2. These units are electronically coded for good security, and require no FCC license. They cover 20 mile distances in a line-of-sight application. Also, RS-484/422 to RS-232 interface module (optional accessory for the above modem) might be usable to connect your modbus to that radio-modem; that;s bb part number MD3310-R2. The YAGI antenna monts on a pipe, and would be a small surface area item, so not a likely target for pranksters, I would think. - Jerry Lavoie Nucor Steel Corporation