5 Dec
2016
5 Dec
'16
10:34 p.m.
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.