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

Dennis Tomlinson (det@tellabs.com)
Wed, 01 Jul 1998 09:57:03 -0500

Hi List, yesterday I posted a reply on the Bi-directional bus
termination thread. Unfortunately, I sent it in HTML format.
Some of you got gobbledygook, and for that I apologize. Others
of you will get this posting twice - again, sorry for that.

I believe I have the HTML option turned off, so let me try
that again:

Dennis Tomlinson wrote:
>
> Hi All,
>
> George Harris wrote:
>
> > Hi Susan,
> >
> > Your note has a lot of information so I'll intersperse my answers
> > with your text.
> >
> > ----------
> > 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.
>
> I'll second that. There's a discontinuity at the junction between daughter
> card and backplane (for mid span cards) - with the backplane looking
> at best, like 30 Ohms. The round trip time from driver to this discontinuity
> and back to driver is almost one rise/fall time. This distorts transitions
> even before launching onto the backplane and having to contend with all the
> other ghoulies.
>
> Again, if it's not too late, could you consider using one of the low voltage
> bus interface logic families. In particular, GTL swings approximately
> 1.2V peak-peak, and slews slower than AC, ACT, or ACTQ by
> a factor of 3 or so.
> GTL would allow shunt termination on the backplane at both
> extreme ends of your bus. This would do wonders for signal
> beautification, and cut EMI emissions from your bus by a factor
> of about 17.4 (-12.4 dB) due to the lowered voltage swing, and
> decreased spectral bandwidth would decrease emissions further still.
>
> Another caveat of using AC, ACT, or ACTQ as a bus interface
> are those clamp diodes you'll find on the inputs. If your
> system has a requirement for "hot card insertion", or individually
> fused power on each daughter card, you are susceptible to a single
> point failure causing an entire system to go down.
> (Note, you may have a redundancy scheme to cover such things,
> but it's a concern none-the-less).
>
> Cheers, and good luck,
>
> Dennis
>
>
> >
> >
> > 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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> >
> > ------------------------------------------------------------------------
> >
> > Part 1.2 Type: application/ms-tnef
> > Encoding: base64
>
>
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