UNIBUS/QBUS interface chips Was: Re: MEM11 update

Eric Smith spacewar at gmail.com
Tue Dec 6 00:34:09 CST 2016

On Mon, Dec 5, 2016 at 6:53 PM, allison <ajp166 at verizon.net> wrote:

> A bunch of us old digits (former dec engineers) got together and were
> talking
> about old systems and the thing that stood out is a general dislike for
> having
> to use the limited set of bus interface chips when there were newer
> parts.  It
> was a internal mandate not something that was better than could be had.
> The
> logic was the parts were known, the vendors vetted for quality and
> reliability
> and when you use hundreds of thousands to millions of a part like bus
> interface
> and ram quality is a critical thing.  Were they special, a flat no.

I don't fully agree. The receivers (and transceivers) had a threshold
voltage that is not available with modern parts, and that actually was
important for large systems with multiple bus segments.  That was
particularly important for large Unibus systems, but even Qbus with only
two bus segments can get finicky when heavily loaded.

DEC could easily have made custom interface ICs if they had needed them.

AFAIK, *no* current production interface ICs have the right threshold. It's
hard to meet the spec without using either NOS parts or comparators.

It would certainly be possible to build a functionally equivalent bus with
modern interface ICs, and it might have significantly better performance,
but it wouldn't be compatible with the legacy systems.

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