Thread Starter

Jay Kirsch

I have an application that uses 2-wire multi-drop comm circuits. The circuits extend up 10 miles. The PLC's on these circuits use a timing dependent protocol ( i.e., no devise that use buffering and/or error correction will work ). Currently, Black Box line drivers are being used, but this product has been discontinued, and never
worked very well to begin with. I have tried to find a product that will work reliably in this application, but have found nothing.

Any ideas ?


Jay Kirsch
Macro Automatics
2985 E. Hillcrest Drive, Ste 101
Thousand Oaks, CA 91362
[email protected]
What is the distance between the nodes on the system? Also, when you say that it is timing dependent, what exactly do you mean (the time difference between one end of the wire to the other is going to be pretty large).

Al Pawlowski, PE

If you don't want to change the physical links to something else, your best bet will probably be "dumb", 300 to 1200 baud FSK modems. They have
about as rugid, and reliable, as you can get for copper circuits.

The last ones I used were private labelled units from Harris radio. I forget who actually manufactured them.
First the bad news:

Long 2-wire omnibus (multi-drop) circuits are very difficult. I have been involved in railway telecoms were such circuits abound. Traditionaly they are the lineside phone cisrcuits, but nowdays everbody wants to stick signals down the esisting copper circuits as well. As I was working in the telecomms sector at the time I read a lot of trade press and visited trade shows and always had an eye out for equipment capable of this sort of thing. Alas nothing. Equipment claiming to work 'a few kilometres' never work reliably at such distances.

The not so bad news:

I have heard of (but never held in my hand) dedicated line modems with high impedance 2-4 wire circuits, if you can find them they would work if you could manage a reasonable compromise with line balance.

At very low speeds you can get away with almost anything (just look at what people do with carrier suppression), but I have the impression your application requires a certain speed. Are you able to give more details? In particular the baud rate and the nature of the interface between modem and PLC.