Computer Interfacing
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Signal Ground / Signal Common necessary?


       Computer Interfacing Forum Index -> RS422 and RS485
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New User

Joined: 05 Apr 2011
Posts: 1

Apr 05, 2011 8:54 pm

Hi all, I'm a newbie to electronic engineering so forgive me if the answer to this question is obvious.

I'm experiencing a lot of noise in an RS485 network I've put together. The cable lengths are very short (<1m total cable length), I'm properly biasing the A and B signals and I've tried both terminated and unterminated lines, and I'm only running at 19200bp.

I suspect the problem is ground noise, as I'm using the RS485 input signals to set PWM levels for high-power LEDs (up to 1000mA momentarily at 3.8V). I'm using a four-wire connection between each node (+12V, GND, RS485-A, RS485-B, with the RS485-A & -B a self-twisted pair of hookup wire).

Would it be enough to add a signal ground wire between the RS485 device ground inputs, with 100Ω resistors in series between the RS485 device ground and the signal ground wire? Or should I think about adding full isolation to the RS485 device, including a transformer for floating power supply and opto-isolators on the A and B signal lines? Or is this excessive?

Any advice appreciated!

Thanks in advance,

       Computer Interfacing Forum Index -> RS422 and RS485
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