Looking for Unibus core

[mfeberhard]

I'm looking for core memory for my PDP11/20, including especially the required backplane. (This machine delivers power to the backplane sections with paddle-boards, not with the Molex connectors on later Unibus machines.) If you have any parts for Unibus core memory that's compatible with an old 11/20, please let me know - I am happy to pay for it.

Thanks!
Martin

 

[pbirkel@gmail.com]

Is your objective specifically a paddleboard-era backplane with a core memory that mounts in it? Period-appropriate would be a MM11-E (4Kw) that I believe shipped with the 11/20 (or perhaps a MM11-F). That's a 4-slot backplane.

Easier would be something a bit more modern like a MM11-L (16Kw) with a rewired power-interface. Also a 4-slot backplane.

After that I believe that you're into a 9-slot backplane (various possibilities) which may not fit your available space, and core memory (MF11 variants) that require voltages that the 11/20 doesn't supply. I believe that you're limited to one of the MM11 variants. Suggest familiarizing yourself with those (see Bitsavers, etc.) for their peculiarities and backplane ("system unit" = SU) requirements.

 

[mfeberhard]

Thanks for the feedback!

Yes, I would prefer to keep it period appropriate, though I might settle for something else in the short term so that I can begin debugging the CPU. (I've already repaired the power supplies in the CPU chassis and the expansion chassis.) So ideally yes - an MM11-E or -F would be perfect, but I would make an MM11-L work with a modified wiring harness.

Any chance you have a core memory set that you'd consider selling?

Of course I've been poring over the available documentation at Bitsavers and elsewhere...

I have some experience restoring core memory as well as the DEC way of doing things, having fully restored a PDP8e with 32Kx12 core. (Also with high-speed reader/punch, DECtape and RX01.)

I was also once upon a time pretty familiar with the PDP-11 instruction set, as this was the CPU for the assembly language course that I took in college, back in the bronze age. I remember it being an incredibly elegant language, compared to the 8080 and Z80, with which I was already familiar

 

[Roland Huisman]

So you are looking for these parts. I've been searching for it as well and got it complete. But there are some differences between memory modules for the pdp11/20. So make a part list and try to find the boards. The hardest part is the backplane... But with patience you could wrap your own from a standard panel. I still have to test and repair my memory module, it is not in one of my machines at the moment.

Regards, Roland

 

[pbirkel@gmail.com]

I can't recall ever seeing the H207 stack used in a MM11-E show up on eBay. Maybe even less likely than finding a complete MM11-E!

Plessey made memory units for various PDP-11 models. I suspect that you'll have better luck looking for a DEC MM11-L or Plessey (or perhaps another third-party like DATARAM) equivalent. DEC memory might be listed as a ME11-L, which was a separate 3U (9-slot) subsystem holding up to three MM11-L. I have seen MM11-L show up on eBay, possibly listed as some combination of G110/G231.

There are two PM-1116B (16Kw) listed on eBay at the moment (https://www.ebay.com/itm/315063367621 and https://www.ebay.com/itm/226001813404) that should get you much closer to your goal. The complete manual, with schematics, was recently recovered (by Roland?). It states "The PM-1116B may be operated with, or in place of, the DEC model MM11-L in applications where -15V at 6 amps is provided." and "The PM-1116B memory module requires the same DC voltages as the DEC MM11-L memory." The H720 is spec'd for 10A on the -15Vdc rail so I expect that you'd be good for one of these. Hosting two would be ideal, and that might actually work with the H720 as-is.

The PDP-11/20 was capable of supporting a _pair_ of MM11-E. While an individual MM11-E stack was 8A worst-case active at -15Vdc (2.5A at +5Vdc) only one stack would be active at a time. The PM-1116B was only 5.3A worst-case active at -15Vdc, so a pair of stacks should work fine on the -15Vdc rail. Note however that the worst-case requirement on the PM-1116B at +5Vdc is 3.3A (0.8A higher) so that rail might become problematic, even with a single stack, depending on what other modules you have installed. You'll want to check your power budget ...

There are options for solving the backplane/SU part of the puzzle if you decide to go down the Plessey path. I may be able to help in that case.

 

[pbirkel@gmail.com]

So you are looking for these parts. I've been searching for it as well and got it complete. But there are some differences between memory modules for the pdp11/20. So make a part list and try to find the boards. The hardest part is the backplane... But with patience you could wrap your own from a standard panel. I still have to test and repair my memory module, it is not in one of my machines at the moment.

Judging by the position of what I presume to be a M225, either that's a MM11-F that you're assembling or you've put it in the wrong position for a MM11-E.

I'm looking at the module layouts specified in: https://gunkies.org/wiki/MM11-E_and_MM1-F_core_memories Which is it?

 

[Roland Huisman]

Boards are in the correct positions, it is the MM11F. Here are some pictures in a better resolution. (one board is missing a transistor) But now you know what the boards look like... There is a seller in the UK who had all the boards a while ago. I already had a broken H207 which is now on display on school for our students to show core memory. So I bought another two of these H207 boards. Still untested and I've been thinking to wire wrap a second back plane if both modules are working. In that way I can have a MM11F in both of my PDP11/20 machines

The Unibone is also an ideal solution for the 4 slot memory in the PDP11/20. But you have to do some wire wrapping as well on the DD11A.

Regards, Roland

 

[pbirkel@gmail.com]

Boards are in the correct positions, it is the MM11F. Here are some pictures in a better resolution.

Aha, thank you :->.

The Unibone is also an ideal solution for the 4 slot memory in the PDP11/20. But you have to do some wire wrapping as well on the DD11A.

Not core, but still a useful "contrivance"! What's the shortfall on the DD11A?

 

[Roland Huisman]

Not core, but still a useful "contrivance"! What's the shortfall on the DD11A?

To use the Unibone wrap at least:
AC LO
DC LO
NPG

These signals were routed trough the Unibus in-out, but not trough the backplane itself. And I would also advice to place a G8000 and wrap the +8V on the backplane. Then you can use a M7800 in RS232 mode as well. (A G8000 clone design is on my github)

In both of my machines I have a RK11 controller and a DD11A with modifications mentioned above. That leaves space for one MM11E or F memory module. But even with three MM11E/F modules, you have a very limited amount of memory. But it is very nice to have at least one MM11E/F module in it. So I hope mfeberhard finds one.

btw... mfeberhard, are you also the designer of the 1702A programmer?

Regards, Roland

 

[mfeberhard]

>>>btw... mfeberhard, are you also the designer of the 1702A programmer?

Yes. Also the ME27000 and a few other such things