PDP 11/24 - A Step Backwards

------ 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 installed. 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. C 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 termination/continuity. 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