21 Mar
2014
21 Mar
'14
5:35 a.m.
On 03/20/2014 02:16 PM, Vincent Slyngstad wrote:
From: Philipp Hachtmann: Thursday, March 20, 2014 4:55
AM
I think your perspective may be Omnibus-centric. I basically agree
with you for the Omnibus, Posibus, and Posibus Data Break busses.
Those busses are relatively poorly specified, and generally fairly
short. Indeed, the Omnibus configuration guide only discusses systems
with 1 or 2 boxes.
However, discussions of bus drivers for Unibus and Qbus still seem
relevant.
Vince
What is being forgotten here is that even DEC had problems around this.
Using the
DEC parts did NOT assure nominal operation. It leads to both QA and a
whole process
creation called FA/T (factory assemble and test) because machines were
being configured
sent to the field and while it all plugged in it far to often didn't
work. Sometimes it was bad
boards and cables but more often it was what DEC would call in later years
"unsupported configurations". People that build and use multi box
systems understand
this.
Omnibus wasn't well specified and back then things like slew rates were
just becoming
a question and far from controlled. Same for the Utililogic and TTL
parts. Multi cabinet
systems were because boards were big and technology hadn't made them
smaller but
things like propagation delay, crosstalk, and cable lengths were
issues. Its up side
Omnibuss made it easy for users to make their own boards and it was
often done. DEC
made kits to aid that process but people did often go off the campus and
do it their way
sometimes good and occasionally not.
The fact that we have seen a few useful boards and an attempt to put in
parts that should
work fine is critical. A simple thing like memory has stopped many
pdp-8s from seeing
use or even bother to power them up.
Allison/An old mill rat
but I
think it is entirely appropriate
to point out that that they almost certainly don't meet the bus
specs, even
though that probably will affect only a small fraction of users.
Who is that fraction? For me it currently looks like the empty set.
So for me the bus driver discussion is as much of secondary interest
as it could be.
And I insist: Throwing up this bus driver thing again and again
discourages people.