PLCs & Distances


Thread Starter

Ion Voinea

Please tell me if it is possible to have a Master PLC controlling 3-4 I/O modules located on about 4 km(1km between each). Can I do it with Profibus modules? Is it possible on 2 wires(RS485) because Optical fiber is expansive. Please help !

Gilles Allard

According to my Profibus documentation, there is no problem with such a distance. You can have up to 187Kbps data rate for a segment of 1km. Using repeaters, you can cover up to 12km. Profibus is not designed for very small stations so the cost per I/O will not be negligeable in your case. The least expensive solutions may be the ET200S product family from Siemens. I suppose your network will be installed outdoor. Be sure your cable is adequately protected (surge protectors). Also, because of Earth ground voltage differences, you will probably need repeaters with isolation feature (the Siemens model has such a feature). I hope this help Gilles

Ralphsnyder, Grayg

Actually, you can have relays/repeaters to do comms any distance that you want. The only big problem is when you have to be concerned about signal propagation delays. Grayg Ralphsnyder

D. C. Pittendrigh`

Hi All Sorry to have missed the original posting, the distances are not a problem only be sure to select the correct baud rate. I recommend you consider the Bechoff and Wago product ranges which are very cost effective for small digital stations, far more so than ET200S. I have not as yet seen prices from my suppliers, but I have also seen some very impressive small scale I/O station gear from Moeller called the XI/ON concept, and also seen some very impressive outdoor profibus stations from Hirschmann that you may want to consider as well. Optical fibre should not be more expensive than copper, I can get glass FO installed for R9.00 per meter, and if I am not mistaken, at last call Siemens charges around R13.00 per meter for Profibus cable (R8.00 = US$1.00). There is unfortunately one cost associated with FO that you don't have with CU cable and that is the CU/FO converters, and if someone has a cheap solution to that problem i would like to hear about it. Regards Donald Pittendrigh
Have you considered Spread Spectrum Radio? The radios are not teribly expensive. Around $1,400 apiece plus antena and such. I don't think you can get cable (fiber or copper) installed for anything near that. The modems would usually use the RS-232 port on the PLC for communicatons. Several manufacturers make "Plug and Play" models made for the type of PLC you are using. They come already setup for Full Duplex communications. Baud rates are usually at least 9600 or above. I know you can go to 19200 for Modicon which is the fasted their modbus port can go. If you are interested try Data Link. There are others out there but I've never used them.