[SI-LIST] Re: rs-485 reflection problem

All -

I noticed that you didn't get the screen shot attachment....probably too big
- try this one.

Jim

-----Original Message-----
From: Peterson, James F (FL51) [mailto:james.f.peterson@xxxxxxxxxxxxx]
Sent: Wednesday, June 12, 2002 8:14 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] rs-485 reflection problem


Hello all - 


I have an RS-485 multipoint system with the fail-safe biasing at one end and
a 120 ohm resistor at the other end of a 15 foot twisted pair interconnect.
(the fail-safe circuit is : Vcc->536ohm<->130ohm<->536ohm->gnd).

Since I have a multipoint system, the RS485's must tri-state (duh).

When the rs485 driver at the failsafe end goes from a "1" to tristate, the
high end trickles down to the failsafe bias point, but the low end 'slams'
up close to the high end and then trickles down with the high end to the
bias point. I've attached a screen shot of this (note: you will not see the
below problem in the screen shot, but you do see the slam).

Now to my problem : when this low end slams up, it is sending a
non-differential wave down the t-line all the way to the end, where it
reflects ABOVE the high point and causes a 60ns glitch at the single-ended
output of the 485 receiver - and that is a bummer.

Any suggestions for a fix?

regards,
jim peterson







-- Binary/unsupported file stripped by Ecartis --
-- Type: image/bmp
-- File: 485_hiz.bmp


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-- Binary/unsupported file stripped by Ecartis --
-- Type: image/bmp
-- File: 485_hiz.bmp


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