PDP 11/24 - A Step Backwards

Joshua Rice 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, 
then stuff.

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!

Josh Rice

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