Hi all
For a point to point RS 485/half duplex point to point serial connection from my PLC Siemens S7 to a fiber optic line driver, what are the considerations for the end resistors ?
Shall be thankful for any informed opinions out there.

Rgds
B.Wills

 

Not a technical answer, just anecdotal:

We used all of three 1000' spools of twisted pair, shielded comm cable, plus a couple hundred feet from a 4th roll (1,000m total).

Had about 18% good comm at 38K baud. And only about 64% at 19.2K baud.

Then someone remembered the terminating resistors. We put a nominal 120 ohm resistor at each end (used a pick-up to drive the 1/2 mile plus distance) and the 19.2K rate went to 99.9% and 38.4K rate went to something over 90%.

I've run numerous Modbus 485 connections locally in the panel or between panels up to 50-100' with no terminating resistors without a hint of a problem, but on long runs, it does seem to make a difference.

Somewhere there's impedance matching calculations, but we used 120 ohm and it worked.

Bud

 

Generally speaking you don't need terminating resistors if the wiring is short. As the cables approach a significant fraction of the wavelength of the data transmission frequency; refelections will begin to pop up in the data window causing errors. At 115K the wavelength is about 6000 feet for typical cable with a propagation factor of 0.75. In this example I would expect to need terminators if the wire was much over 500 feet. The addition of termination resistors at each end will kill the reflections (use 120 ohms). However, some devices don't like the additional load. A pair of terminating resistors increases the load by a factor of 5 or more depending on the number of relays in the loop. Check out B&B Electronics website for more info.