Series resistors can be placed within 1/5 of the fastest edge
rate of the drivers and still be effective. Further than that
far away limits the effectiveness due to the transmission line
effect of the stub from the driver to the resistor.
Having said that, however, resistors can be placed within a
bus, acting as mismatch and energy dissipating elements.
The general idea is to divide the bus into one or two sections
and join the sections with a resistor. By "tuning" the resistor
value, you can achieve a compromise between overshoot/undershoot,
ringback and delay. Generally, you push out the delay on the
first edge of the waveform, yet gain overall margin by
Try it ... you'll like the results.
M. Susan Tweeton wrote:
> 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
> 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.
> 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.
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