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

M. Susan Tweeton (mstweeto@collins.rockwell.com)
Tue, 30 Jun 1998 11:43:48 -0500

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. 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.

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?

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. How about ferrites?
Any ferrite experts out there?

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.

Thanks for all the responses so far.

Susan

Rockwell-Collins
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