PDP-11/05 (was: PDP-11/05 microcode dump?)

Tom Uban uban at ubanproductions.com
Sun Jun 13 15:44:08 CDT 2021

I am working on the first of my two 11/05s. Interestingly, it has the early version M7261E Control
Logic & Microprogram board and the later version M7260 Data Paths board (with circular baud rate
selector switch) as described in:


>From the description there, it seems like an older/newer combination, but maybe that was common. I
would not have guessed that the four possible combinations would all work together, but maybe they do?

I have a couple different drawing sets for the 11/05 and while some have the matching M7260
schematic, only the GT40 drawings (I found on bitsavers) has the M7261E schematic:


The GT40 drawings has the PROM listings and related, so I am hoping that they match what is in the
two boards.

Presently, the machine sometimes runs relatively well and other times it does not. It does have bit
1 stuck ON in memory, but that should be a relatively simple task to diagnose as it is not
intermittent. When the machine is "working" I am able to deposit 0777 at 0100 and run. When running
this simple program, I've experimented with flexing the boards and such, so it doesn't seem like an
obvious poor connection, but that remains to be seen.

The machine is a configuration #2 model (as described in the "gunkies" site) and my initial messing
with KM11 boards, reveals that I can step the microcode with a KM11 in either the #1 or #2 position,
but when two KM11s are installed at the same time, they do not function properly together. Is this
expected or do I have an issue there too?

Thanks much to those who have provided details and documents on the web, they have already been of
great value and will most certainly continue to be a resource in the future.

More updates in the future...


On 5/6/16 5:32 PM, Noel Chiappa wrote:
>     > From: Mattis Lind
>     > Thanks Noel for sorting this out.
> Eh, de nada. But thank you.
>     >> I wonder if the ucode in the two versions is identical? The uROM chip
>     >> numbers should give it, (if they are the same on both versions, albeit
>     >> in different locations on the board), but I have yet to check. Does
>     >> anyone happen to know?
> OK, so the situation here is pretty complicated. To start with / make things
> worse, that CPU uses lots of PROMs. Lots and lots and lots and lots of PROMs.
> For the data paths board (M7260), both major versions appear to contain the
> same PROMs (going by the DEC part numbers), but the chip location (Exx)
> numbers are all different.
> For the control board (M7261), the C, E ('early' version) and F ('late'
> version) etch revisions each contain mostly the same PROMs, but apparently
> with slight differences between the sets of PROMs in each (as reflected in
> different DEC part numbers). For details see:
>   http://gunkies.org/wiki/PDP-11/05#Control_PROMs
> to which I have just added all the gory details.
> As to getting the contents of all of them dumped in machine-readable form -
> oi vey!
>     >> on the earlier version (prints for that version are in the GT40 prints
>     >> online
> It turns out that I have hard-copy prints for the "C" etch revision of the
> M7261, which do not yet appear to be online; the GT40 prints have the "E"
> etch revision.
> I will scan the pages for that revision of the board, and put them up 'soon'.
> (I'm not doing the whole print set, it's about 1" thick, and most of them are
> for other things anyway, like MM11-L memory, etc.)
> 	Noel

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