QBus termination?

From: Paul Koning <pkoning_at_equallogic.com>
Date: Tue Feb 8 12:27:17 2005

>>>>> "joe" == joe heck <joeheck_at_splab.cas.neu.edu> writes:

 joe> The "definitive requirement" I suppose is that a bus is a
 joe> transmission line and is modelled like one. That means that it
 joe> should (in this instance) be terminated on each end. ...

 joe> In reality, I rarely paid attention to termination when using a
 joe> single backplane. ... Yes, I have regularly broken the rules.
 joe> Things still worked.

Sure.

The transmission line nature of things matters if the total wire
length is more than a small fraction of the wavelength of the highest
frequency signal you care about. Qbus rise time is specified as 10
ns, which translates to about 6 feet. So for a single backplane the
reflections aren't going to mess up the pulse shapes much. You'd
still get ringing and overshoot, but no excess jitter. And on a small
config there would be lots of margin. So yes, you can get away with
various forms of abuse...

Re too many terminators -- I still have a copy of an article (in "RSTS
Professional") that showed an Ethernet with a terminator in the
middle. They were showing how to convert thick to thin, and did that
with a T connector complete with terminator...

     paul
Received on Tue Feb 08 2005 - 12:27:17 GMT