> On Jul 5, 2025, at 11:05 AM, Jon Elson via cctalk <cctalk@classiccmp.org>
> wrote:
>
> On 7/4/25 14:32, Mark Kahrs via cctalk wrote:
>> 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
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.
paul
