QBus termination?

From: Paul Koning <pkoning_at_equallogic.com>
Date: Tue Feb 8 08:46:16 2005

> Patrick Finnegan wrote:
> > On Monday 07 February 2005 18:18, Vintage Computer Festival wrote:
> >>So what's the definitive requirement for terminating QBus? I get
> >>various non-answers from various people and it's quite annoying.
> >>
> >>How does one properly terminate Qbus?
> >
> > The CPU terminates one end, and no terminator is required at the far
> > end, unless you connect chassis together. No machine I've ever seen
> > has had a QBUS terminator in it (other than the termination the CPU
> > provides)

That's incorrect.

The PDP11 Architecture Handbook (page E-31) says:

  The 120 ohm LSI-11 Bus must be terminated at each end by an
  appropriate terminator. ... This is to be done as a voltage
  divider with its Thevenin equivalent equal to 120 ohms and 3.4
  nominal. This type of termination is provided by an REV11-A
  refresh/boot/terminator, BDV11-AA, KPV11-B, TEV11, or by certain
  backplanes and expansion cards.

That all makes sense. DEC was very much aware of the transmission
line nature of buses, and carefully specified all the implications.
It's more visible in the Unibus world, because those tend to be larger
systems with longer wires, but it applies in both.

        paul
Received on Tue Feb 08 2005 - 08:46:16 GMT