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

Roy McCammon ([email protected])
Tue, 30 Jun 1998 17:23:56 -0500

I had a similar problem. Nineteen inches of bus. Sixteen
modules that could be placed in any position on the bus
or be missing. 74ACxx and 74HCxx logic. In my case, the
following worked for me:
1. Minimized the trace on the module between the connector
and the transceiver to minimize extra capacitance.
2. Series terminate of one half the expected loaded
bus impedence.
3. Parallel terminate the busses for twice the expected
unloaded impedence.

I'm making up numbers now, but suppose the bus with no
modules plugged in is 60 ohms and with all modules plugged
in (considering the added capacitance to be distributed) is
30 ohms, then I series terminated the drivers with 15 ohms
and parallel terminated with 120 ohms (two 240 ohm
resisters). In my application, that was adequate to handles
the reflections.

M. Susan Tweeton wrote:
> 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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