RE: [SI-LIST] : Terminations scheme for bi-directional bus

George Harris (George.Harris@digital.com)
Tue, 30 Jun 1998 14:21:26 -0400

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Hi Susan,

Your note has a lot of information so I'll intersperse my answers
with your text.

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From: M. Susan Tweeton[SMTP:mstweeto@collins.rockwell.com]
Sent: Tuesday, June 30, 1998 12:43 PM
To: si-list@silab.Eng.Sun.COM
Subject: Re: [SI-LIST] : Terminations scheme for
bi-directional bus

All, sorry I haven't responded sooner, I've been out of
town.
In response to some of the questions and suggestions.
First of all of these measurements are in SIMULATION.

Have these boards been sent out for etch, or built, or otherwise
committed? The best solution would be re-layout to place the
transceivers next to the backplane connectors.

My system isn't
in operation, and even if it were operating properly, I'd still
have the
same concerns. The biggest obstacle is the varied loading.
There can
be as few as 4 transceivers and as many as 8 transceivers on the
bus at
once.

First incidence switching isn't necessary. The overshoot
shouldn't be
much of a problem either. It's about 5.93V and can always be
clamped.
I'm investigating the timing budget, but the signals are ugly,
and I'd
like to have them as robust as possible. Timing on this
particular
build may operate with these signals, but the first time we get
an IC
die shrink, we're in trouble. The thing I'm most concerned with
is the
ringback and EMI. The ringback crosses or comes near to crossing
the
threshold. Even if the timing is robust enough to accommodate
the
ringback, what's the EMI going to look like? Keep in mind this
is an
avionics system which has an extremely rigid EMI requirement.
^
Now I am worried...

When you refer to "terminating the backplane bus at each end of
the bus"
(ref Giovanni DiBenedetto & George Harris) are you referring to
placing
terminators on the backplane itself, physically at each end of
the
board? Or on the transceiver boards?

In view of the 3 inch stubs, terminations would be more effective
on the boards next to transceivers, but what if the end boards are not
used? If configuration rules are acceptable, then end boards should have
the terminations, but that would make them different from the middle
boards, which is probably unacceptable.

For now I have a 10 Ohm series resistors on each transceiver
which looks
the best so far. I'm looking into using schottky diode
terminations
like Roland Portman and Paul Thompson suggested.

I don't think Schottky terminations will work here because of the
distributed loads -- they work best for point-to-point signals, placed
near the receiver.

How about ferrites?
Any ferrite experts out there?

I never used ferrites in applications of this kind.

How about some more discussion on placing the series terminators
close
to the transceiver vs placing them next to the connector to the
backplane. As Larry Smith said, placing them next to the
connectors
will reduce the loading effects on the driver.
Yet, others are adamant
that they should be close to the driver. In simulation, there
isn't a
whole lot of difference. It is slightly (but not significantly)
better
with the resistor near the driver.
For series terminations to be effective they have to be close to
the driver. You didn't see a lot of difference because 10 ohms is not
enough to make much difference.

Parting thought:

Simulation should be done with the driver in all possible positions with
all possible load combinations. My guess is that the worst case will be
full load with the driver in next to the last slot.

Slow and fast silicon should also be simulated, not only typical.

Thanks for all the responses so far.

Susan

Rockwell-Collins
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