Modems for Communication Between PLCs

Seems like I am putting up a new post just about every day here. A wealth of valuable expertise here and I really appreciate everyone's comments and advice.

I need to connect 4 PLCs together and have them communicate with Modbus. The distance is about 4000 feet. I have done this many times in the past (several years ago) using data modems (FSK Bell). I can't find anyone who manufactures these anymore. The whole world has gone to wireless spread spectrum, wireless Ethernet, etc. Ten years ago there were several companies who made such a thing.

These are used for a golf course irrigation system. We are using RS485 with poor results. The distance is a bit too great and also the existing wiring is in a 'star' arrangement which is not good for RS485.

Does anyone know where to get dedicated wireline modems? I don't mean the dial-up kind. The ones I need are full-time dedicated FSK tone modems.

Here are just a few. But why not wireless?

http://www.blackbox.com/Store/Resul...SUs-Muxes/Leased-Line-Analog/n-4294966688/p-0

http://www.blackbox.com/Store/Results.aspx/search-short+haul+modems%5E%5E%5E/Industrial/Modems-CSU-DSUs-Muxes/Leased-Line-Analog

http://search.bb-elec.com/?s=&q=lease+line+modems

http://www.data-linc.com/privdedlines.htm



RS485 stuff should work with proper wiring ( not star) and maybe repeaters.

http://www.bb-elec.com/Subcategory.asp?SubCategoryId=19&Trail=2&TrailType=Top

Thanks for that information.

Cost is the main issue. Last time I used them, modems were about $400, while wireless runs $1000 or more. Also, there is an issue of getting good line-of-sight. Two of the PLCs are down in a river valley.

Advantech make RS485 repeaters and isolators. Check at www.advantech.net.

rwy34ils said:

Seems like I am putting up a new post just about every day here. A wealth of valuable expertise here and I really appreciate everyone's comments and advice.

I need to connect 4 PLCs together and have them communicate with Modbus. The distance is about 4000 feet. I have done this many times in the past (several years ago) using data modems (FSK Bell). I can't find anyone who manufactures these anymore. The whole world has gone to wireless spread spectrum, wireless Ethernet, etc. Ten years ago there were several companies who made such a thing.

These are used for a golf course irrigation system. We are using RS485 with poor results. The distance is a bit too great and also the existing wiring is in a 'star' arrangement which is not good for RS485.

Does anyone know where to get dedicated wireline modems? I don't mean the dial-up kind. The ones I need are full-time dedicated FSK tone modems.

Click to expand...

Have you looked at the Westermo products ?

http://www.westermo.com/Resource.phx/content/products/productGroups.htx