Much swearing - was Re: Do-It-Yourself PDP-11 help!

From: Pete Turnbull <>
Date: Wed Feb 16 16:59:56 2005

On Feb 16 2005, 12:52, Vintage Computer Festival wrote:
> On Wed, 16 Feb 2005, Pete Turnbull wrote:
> > > How critical are the bus termination resistors?
> >
> > Depends. We discussed that just recently. In a lightly-loaded
> > usually you can manage with just the set on the CPU. Which might
be a
> > problem here!
> But is it a problem if some lines are terminated and some aren't?

Yes. All the bus signal lines need to have some termination, with the
possible exceptions of BEVENT, if it's tied low to disable it, and
perhaps the grant lines if nothing's using them. All the drivers are
open-collector, so effectively they're disconnected if not at logic 1.
 Unterminated lines will float to random values, carry lots of noise,
and even if they do average a high enough voltage to register as a
logic 0 (remember they're negative logic) they'll take so long to
return to a sensible level that the timing will be way way off.

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Wed Feb 16 2005 - 16:59:56 GMT

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