RE: [SI-LIST] : RE: RC terminations on multidrop busses

Bob Davis (bob@scsi.com)
Fri, 13 Nov 1998 20:05:03 -0800

Magnus,

The RC termination is usually the same as the AC termination that you think
about. It does not matter which is first in the R - C coupling to
ground(signal return). When this is designed, the Resistor is the energy
absorption terminating element and the Capacitor is for the DC isolation.
The Resistor is chosen to match the line impedance at the point of
termination and the capacitor is chosen to couple the resistor for the high
frequencies of the edge and discharge prior to the next edge.

I like the resistor at the ground(signal return) end to make the observation
of the termination voltage more easily seen. The circuit still works the
same with either order of R and C.

Using this termination on stubs has worked for me to reduce multiple
reflections back to the normal line provided the stub is long enough to
adopt a transmission line propagation mode at the frequencies.

Good luck.

Bob Davis, Consulting Engineer.
Summit Computer Systems, Inc
Signal Integrity Specialists - High Speed, Critical PCB Design
408-353-2706
bob@scsi.com , www.scsi.com

-----Original Message-----
From: owner-si-list@silab.Eng.Sun.COM
[mailto:owner-si-list@silab.Eng.Sun.COM] On Behalf Of Magnus Homann
Sent: Friday, November 13, 1998 8:37 AM
To: Roy_Leventhal@mw.3com.com
Cc: Si-list@silab.Eng.Sun.COM
Subject: Re: [SI-LIST] : RE: RC terminations on multidrop busses

> Abe,
>
> Thanks for your reply.
>
> If I read it correctly, you are talking about RC terminations at each end
> of the backplane line. Have you ever looked into RC terminations at the
> receivers / drivers on the daughter board stubs?

I have to ask,

Is the RC termination you mentioned the same as an AC-termination (cap+res
to ground from signal) or is it a low-pass filter (res in series with
signal,
cap to ground). I've seen both used, but have never found a reason to use
either in my designs. Yet.

Magnus Homann
Ericsson Mobile Data Design AB

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