PDP 11/24 - A Step Backwards
rice43 at btinternet.com
Tue Mar 29 03:28:56 CDT 2022
------ Original Message ------
From: "Chris Zach via cctalk" <cctalk at classiccmp.org>
To: rob at jarratt.me.uk; "'General Discussion: On-Topic and Off-Topic
Posts'" <cctalk at classiccmp.org>
Sent: Tuesday, 29 Mar, 2022 At 00:11
Subject: Re: PDP 11/24 - A Step Backwards
I have been reluctant to put everything back in, in case the PSU fries
something. And the ripple I noticed is...
For the record, right now I have only the M7133, M7134 and G7273
Ok, I do recall that my 11/24 wasn't doing any ODT without some form of
memory. When I configured a (broken-ish) MS11-PL in it did at least come
up and allow me to load memory addresses and the like.
But no memory, no deal. I'd say figure out your PSU first, then memory,
Just to chip in my 5 pennies worth. At least in the QBUS world, the only
chipset that wouldn't ODT without memory is the original LSI-11 (and the
T11 i believe, but that's moot because they came with RAM onboard). You
should still get ODT on F11 and J11 chipsets even without RAM.
Since the 11/24 uses the same chipset as the 11/23, i assume that this
would still apply. Not sure of the intricacies of UNIBUS, having had no
experience, but i assume that if the bus was terminated or continuity
cards inserted correctly, the CPU board should ODT without memory. It's
likely that the lack of ODT is due to issues with other boards, the
backplane (an often missed source of issues), or the bus
I'd start with the basic stuff first. Whilst you're waiting on your PSU
to be serviced in warranty, check the backplane and cabling for foreign
objects, damage, and corrosion. Shorts and intermittent connections are
inherently difficult to troubleshoot assembled, but can be trivial to
identify when the machine is torn down. Then i'd reinstall the PSU and
fans etc, then work on bringing up the machine, starting with the CPU
and memory, then other devices.
One notable thing, is i believe most PDP-11 PSUs, regardless of bus
type, won't function correctly without a load on them. It might be hard
to troubleshoot without a load on it.
If you're short of grant continuity cards, they're extremely simple, and
i'm sure with some googling, gerber files can be found for them. It
should be trivial to make some from copper plated PCB stock and some
patience if time is pressing.
Good luck in your endeavors!
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