Plessy core memory

Chris Zach cz at
Sun Jan 9 18:57:20 CST 2022

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?


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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