Re: [SI-LIST] : daisy chain

Dennis Tomlinson (det@tellabs.com)
Tue, 11 Aug 1998 09:58:01 -0500

Hi Shimon,

Shimon_Turgeman@amat.com wrote:
>
> waht you think about daisy chain configuration with parallel termination in
> the far end ?

It can work very well. Your driver will need the capability to drive logic
levels into the load at DC, and you should keep your stub lengths short. The
round trip delay for each stub (include loading effects) should be no more than
a small fraction of the driver's transition times.

> is that prefered way to terminate control signal that connect to few
> consumer ?

It's one way. Thevenin termination is a special case of parallel termination
which can be used to balance the high/low DC current requirements at the driver.
AC termination can be used in some situations - saving the DC current drain
when the control signal is static.

> is that prncple to connect the consumer with equal length ?

I don't know of a good "general case" answer for this question. Equal load spacing
would make the transmission medium more uniform, but all it achieves is equal time
intervals between reflection components. A large number of loads placed electrically
close to one another can cause a "single" reflection whose amplitude is larger than if
the loads had been more diversely placed. (Randon thoughts. Not answers)
I concern myself more with making each load cause as little discontinuity as possible.
But then, I work on product whose load spacings are generally fixed by system
mechanical requirements, not electrical concerns.

Opinions from others?

> have you any better way to terminate control signal ?

Which termination technique to use depends strongly on signal function and, in
some cases, delay requirements. Also, some logic types require pull-up or pull-down
resistors for proper operation (ECL, GTL). In that event, it is customary to use this
resistor as a parallel termination of the net as well.
If you have no edge sensitive loads, no requirement for a DC path to a power rail,
and lots of slack in the timing budget, series termination at the driver would be
my first choice. Waveforms at intermediate loads can get quite ugly, but in some
cases ugly is good enough.

> what the penaly of this termination ?

Power dissipation, and driver strength requirements.

> have you experiance with termination to control signals ?
>
> Best regards
> Shimon Turgeman
> ISRAEL

Cheers,

Dennis