UNIBUS/QBUS interface chips Was: Re: MEM11 update

Toby Thain toby at telegraphics.com.au
Tue Dec 6 09:05:44 CST 2016

On 2016-12-06 1:34 AM, Eric Smith wrote:
> 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

I'm an electronics noob, but do you mean a threshold of 1.5V, as with 

I'm referring to this part of October's thread:


> 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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