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From: Paul Burke
Subject: Re: RS-485 termination
Date: Tue, 22 Oct 2002 16:45:29 +0000
Organization: Scazon Systems
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> Up to now I've been using a PIC'S software SEROUT RS-232 to drive an
> optocoupler at only 1200 baud to feed a group of receivers in sort of an
> optoisolated current loop situation and it's worked flawlessly.
> I need though to eliminate the RFI being generated by the unbalanced
> connection and want to change everything to
> RS-485, (something I've never "tutored myself" on).
I'd be very surprised if the RFI is coming from the current loop
switching edges. Are you sure the cable is screened properly, and that
you aren't letting out the processor clock or something else switching
fast? If it is the serial datastream, why not try slew- rate limiting
> One thing that puzzles me though, if I were to have these
> systems where the end-user can install anywhere from 1 receiver to a
> maximum of 10 receivers on the line, what kind of termination resistance
> could I put on each receiver so that there is a "window" that will be
> acceptable? ie..
> maximum resistance with one.....minimum with 10?
> I'm staying with a mere 1200 baud.
Use ONE 100 ohm resistor termination, somewhere at the far end of the
bus. Then just tag the receivers on with short stubs.
In most cases, you'll get away with no termination at all at 1200 baud.
If the ringing on the edges causes an RFI problem, put resistors in
series with the outputs.
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