HMI/PLC Comm over leased line


Thread Starter

Don Zunti

We are upgrading a control system for a client whose original system was a dial-up connection between two PLC's. If you can believe it, the only way to reconnect a dropped phone connection was to re-start the man PLC - which controlled the city's water treatment plant!

We are changing the system to use GE-Fanuc's Cimplicity Machine Edition (View HMI component) to connect to the remote PLC. However, we cannot get the dial-up phone line to stay connected for more than 6-8 hours. Something always causes the phone line to hang up.

The client is considering using a leased-line from the local phone company. I'd like to hear from others who have used leased lines in such applications, with ideas on setting up the leased-line modems and other factors to be aware of.

Don Zunti. P. Eng.
Delco Automation Canada
The PLC and SCADA PC are located across the city from each other, and there is no LAN connection to the building housing the PLC, only a phone line. An ethernet line would require installation of a couple miles of fiber, media converters, etc. There is no PC at the remote location, can't use PCAnywhere. Thanks anyway.



Dialup connections always fail over time. I did a similar application for a remote water pumping station; you have to test the connection periodically, and include logic in the PLC to hang up and redial when a failure is detected. Do you have PLCs at both ends of the line?
I used a leased line to provide communication between two PLCs about 5 miles apart. It was very reliable. The PLCs had RS-232 cards, and the modem was a long-haul async RS-232 modem from Black Box.

You need to specify the grade of leased line; a voice grade will work but you have to keep the baud rate down around 2400. You can get a T1 data line but it costs more.

The phone company didn't do anything to maintain the leased line unless we had a problem. It was easiest to let our communications department deal with the phone company, they knew how to and specify test it.

Pierre R. Hinse CET

In Canada dial up telephone lines cannot stay connected permanently, if you inquire with Bell you will find that they even discourage permanent dial up connections. Some reasons are technical, others are financial. You have several options accessible to you:
* If you can use Ethernet, then maybe DSL or Cable Internet with routers configured to be IP pipes, are inexpensive compared to throughput speed.
* With leased lines, the modems take care of the connection, but don't expect 100% availability. Even leased line drop out sometimes.
* If the locations are close to line of sight, radio modems may be a cheaper solution, considering the recurring cost of telephone and cable connections.
* If you want to stick with dial up, look into more sophisticated modems, some of the top of the line can be configured with permanent telephone
numbers internally, and to automatically re-connect on drop out.

Pierre R. Hinse CET
Industrial Automation Specialist
We use leased lines for Shell to Shell to/from Petro-Canada to/from TMPL etc. here in Vancouver. Telus provided the modems and like any serial connect you have to have the settings right (baud etc.) when you set them up. Dedicated lines are essentially locked in and not switched, or multiplexed. There have been one maybe two occasions over the years where Telus did switch them by mistake, but they run without problem 99.9%.

Paul Jager, CEO

From time to time I give presentations for the Water Environment Association of Ontario about instrumentation. The participants include water and wastewater operators, electricians, consulting engineers and suppliers to the industry. Whenever the topic of leased-line communications is presented, the feedback is pretty consistent:

1. The phone company cut our line again and we lost communications for another (4 hours, 4 days, 1 week, etc.).
2. We pay for a dedicated link even though we only need communications for alarm conditions and daily summaries (alarm conditions may never happen, or may only happen monthly).

It might be worthwhile to investigate why the dial-up communications isn't working and fix that. It usually isn't that hard to initiate a dial-up when the line is dropped. Some other alternatives to leased lines include ISDN if available, radio, satellite, or Bell's "hyperstream" for higher bandwidths. If you stick with a leased line then you should have a contingency plan for the leased line failure - even as simple as having somebody drive to the remote site from time to time.


Mark Wells
Runfactory Systems Inc.
http://www.runfactory.com1235 Bay Street, Suite 400
Toronto, Ontario, Canada M5R 3K4
Ph. 416-934-5038
Fax 416-352-5206