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Transmission Line Issue (422 over CAT5)

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cwrasmussen

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I am communicating using RS422 over CAT5 at a pretty low baud rate (4800bps), but I get horrible noise if I use a cable much longer than 3 or 4 feet. I think the reason for this is that the differential pairs are not pinned to twisted pairs in the cable causing capacitance issues.

This is for a peripheral I am designing for a system that is already in place with these incorrect pinouts, so I cannot change those. Also, I am making so many of these, that custom cables are out of the question.

I have already tried several termination methods which help somewhat, but not much. Any ideas on how to approach this problem on the peripheral's side aside from using termination resistors?

If you guys need any more info let me know. I appreciate the help!! thanks
 

kmr

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Have you looked at the signals levels on your peripheral (ideally with an oscilloscope)? Perhaps you need termination plus an amplifier if the receiver is not driving the cable with adequate voltage to overcome cable losses.
 

IanP

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CAT-5 cable should be good enough for the RS-422 for several hundreds of meters ..
Try to connect bias and termination resistors at both receivers (see picture below) ..

Regards,
IanP
 

cwrasmussen

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IanP, Would biasing resistors help on the transmitter rather than the receiver? I've seen examples of biasing those lines on both the transmitter and the receiver. The peripheral receives just fine, but the receiver on the other board can't be changed at this point.

kmr, I will take a look at those signal levels. I have looked at them briefly, but not since I got it working on the shorter cable (before I tried longer lengths). I'm using the MC3487 chip as my transmitter and MC3486 as my receiver with optional 100 ohm parallel termination through a jumper. Switching the termination on and off doesn't make much difference. I'll look into an amplifier. I was under the impression that the 3487 would be sufficient, but good point, maybe that's where the issue is.

Added after 4 hours 19 minutes:

Well, thanks for everyone's help, but actually I figured out the problem. Just had a break in the power line to the 3487. Weird thing is that it still worked without power on the 3 foot cable. Oh well, figured it out.
 

kmr

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Glad to hear that you solved the problem.
 

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