Linking Controllogix units over modem


Thread Starter

Carl Burgess

I've got two Controllogix racks that need to exchange data. Racks are several miles apart, and we are looking at getting them to talk over leased line / radio modem.

What is the best protocol / hardware to use ?
- My guess so far is use a couple of Prosoft modbus cards, or use native AB protocol like DF1 ?
Data rate will not be too high.


Jason Valenzuela

If you get a ISDN, or some other form of private point to point WAN link, you can use Ethernet/IP and a pair of routers.

Jerry Miille

If you only have two racks then a 4-wire leased line would be a simple, low cost solution. You can use standard MSG instructions as long as you have a full duplex connection. A dial-up connection could also be used if you have phone circuits to both locations. You could set up one modem to call the
other and the second modem as auto answer only. If the modems can dial based on a previously stored number and can "auto connect", then you don't need any ladder logic to do the dialing.

If you only have two wires and a DF-1 Half Duplex connection, then one of the PLCs would have to be set up as a Polling Master and the other a Slave.

Going to Modbus with only a point to point connection would not provide a significant advantage over DF-1 Full Duplex. If you are forced to go to Half Duplex, then there are some improvements using Modbus.

If you are going to use the Dial Up modems be careful to select modems that are compatible with the DF-1 protocol.

Please see our DIN Rail mounted Modems and Protocol converters at for some equipment that is compatible with Allen-Bradley as well as Modbus protocols.

You are welcome to contact me directly if you have further questions.

Jerry Miille
[email protected]
If you have line of site then, I would use radios and the DF1 protocol. You wont have any monthly or annual costs with the radios.

Kevin Zamzow

If line-of-site connection is possible (either directly, with increased antenna height or with a repeater), an industrial wireless solution is the best choice. No monthly fee to pay year after year, a faster connection, and it's your network! You can also be up and running within a days of placing an order vs. a couple of months or more waiting for the local phone company.

If you wish to program one of the ControlLogix PLCs over the link, suggest the full-duplex DF1 connection. Both full and half duplex DF1 are supported with our RL-2400S industrial grade wireless modems. We can also assist you in determining line-of-site.

Visit or contact me for more assistance.

Kevin Zamzow
[email protected]