5 Jul
2025
5 Jul
'25
8:55 p.m.
On 2025-07-05 1:34 p.m., Paul Koning via cctalk wrote:
And all the wires tuned to have the same delay I bet.
On Jul 5, 2025, at 11:05 AM, Jon Elson via cctalk
<cctalk(a)classiccmp.org> wrote:
On 7/4/25 14:32, Mark Kahrs via cctalk wrote:
If you understand the machine well enough and trace the broken data path around the
machine, you can get there without too much trouble. That assumes the issue is reasonably
repeatable. If it only messes up once a day or so, it's harder, or at least more time
consuming.
Compared to CDC 6000 mainframes, the 780 (and other DEC computers) are marvels of
simplicity. Consider the 6600: 15 chassis each with 750-ish module slots, each with 28
signal pins. A bunch of slots were filled instead by memory modules (5 slots wide), but
still you're looking at maybe 5000 wires (or rather, twisted pairs) per chassis, plus
30 or so cable assemblies each with 19 coax inside, to run signals from one chassis to
another (or to I/O devices). I assume those were all done by hand; it's not obvious
how a robot could do that in the early 1960s, unlike wire wrap backplanes.
Jon Elson's take hits home. A 780 was
delivered and VMS was running. We
installed 4.1BSD and it ran fine until it crashed. Field service insisted
we needed a full set of RS-232 wires in our cable. Still crashed
(surprise!). Switched to VMS, still crashed after a while. Local field
service couldn't find it. The big guns flew in from Maynard. First day:
Couldn't find it. Second day: "What, what's that wire doing there? Have a
wire-wrap tool?". Removed wire from backplane. Boots, runs. Engineer
flies home.
Holy cow, HOW did he find it!!?? The KA780 backplane was a HUGE mass of wires!
Jon paul
Ben.