Plessy core memory

Chris Zach cz at
Sun Jan 9 19:17:56 CST 2022

Worst case I guess I could put a broken MS11-PL board in and set it for 
16k start. It's got bad memory chips on it but I think the first 16k 
were good. And as a bonus it can work in both a +15 *AND* a +12 volt 
Unibus (yeah, the 11/24 used +12 on the +15 lines. No idea what was 
wrong with DEC)

Oh yeah, the first 4 slots in an 11/24 had A/B as NOT MUD but were EUB 
(extended Unibus with 22 bit addressing for the KT24 MMU). The CDEF 
slots were SPC, so you had to have a 7273 in each of them or a special 
memory card. Don't know what would happen if you plugged a RL11 or other 
hex height card into one of those slots, probably blow everything up.

I think I prefer Q Bus. Although they managed to make that a mess with 
the Q/CD slots as well....


On 1/9/2022 8:00 PM, Nigel Johnson Ham via cctalk wrote:
> I'm sorry, on the memories, I went to look at my old notebook form that 
> era, but it seems some pages have fallen out over time :-)
> I think a friend has an 11/05 or 10, I will ask him for its 
> configuration and see if that will help.
> cheers,
> Nigel
> Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU
> Amateur Radio, the origin of the open-source concept!
> Skype:  TILBURY2591nw.johnson at
> On 2022-01-09 19:57, Chris Zach via cctalk wrote:
>> Probably. It's a 1973 version, the DD11-B is a MUD backplane, but it 
>> seems to only have power plugs for +5,+15,-15. Looking at the MUD 
>> specs I see there are backplane pins assigned for +20 and -5 so I 
>> might be able to cobble something together.
>> That's one of the fun things about Unibus: There were so many 
>> different types that there was no guarantee that anything would work 
>> in anything. I know this one worked with an RX11 because that's what I 
>> used 30 years ago. Would it work with an RX21? Maybe. Maybe not.
>> Note: I did check the DD11 and sure enough all of the DMA grant wires 
>> are factory jumpered. I did remove the wire from one of them so I can 
>> use either a RL11 or an RX21 but in the meantime I have a G7273 in 
>> there along with the knucklebusters in the other slots)
>> Getting this MM11-DP to work might take a fair bit of thought. I'm 
>> guessing that the DD11-F is significantly different from the DD11-B?
>> And the big question: What memories (if any) will work in a DD11-B 
>> equipped pdp11/10?
>> C
>> On 1/9/2022 5:48 PM, Nigel Johnson Ham via cctalk wrote:
>>> I would assume that your 11/10 has the suffix -NC.  There were two 
>>> versions, the -NC used -15V four wire memory, and the later -SC used 
>>> +20V three-wire memory.
>>> The +20V was provided by a MUD slot (Modified Unibus Device) and 
>>> there was usually a big red sticker in it warning to not plug a 
>>> module in that was not wired for it.
>>> I believe it was standard in all models after the second version of 
>>> the 11/05 -/10
>>> I once tried to fix an 11/34 off-contract that somebody had done 
>>> exactly that to- I gave up when I found that the +20V had been routed 
>>> to some unobtainable ROMs on the CPU board.
>>> cheers,
>>> Nigel
>>> Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU
>>> Amateur Radio, the origin of the open-source concept!
>>> Skype: TILBURY2591nw.johnson at
>>> On 2022-01-09 17:04, Chris Zach via cctalk wrote:
>>>> I'm working on my pdp11/10 getting it back together. One problem I 
>>>> think I have is that the secondary memory (a Plessy 700101-100) may 
>>>> be shorting the -15 line for some reason. Working on it, but does 
>>>> anyone have a manual or anything like that for this kind of memory 
>>>> board?
>>>> Alternately, what kind of Unibus 16k memory board exists to get a 
>>>> 11/10 from 16kw to 32kw of memory? Apparently I can't use a MM11-B 
>>>> as it requires +20 and -5, both of which are not provided by the 
>>>> power supply or sourced on a DD11-B backplane module. The 11/10 has 
>>>> +15, -15, and +5.
>>>> On a related note, where did +20 come from for Unibus and which 
>>>> systems even supported it? Was it an 11/45,11/70 thing?
>>>> Thoughts?
>>>> C
>>>> (Yes, I could build a regulator to take the 30 volts between +15 and 
>>>> -15 and create an independent 20 volts. Maybe. Likewise I could 
>>>> generate -5 from the -15 and a 7815 regulator. Maybe.)

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