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

Jay Logue jay at toaster.com
Mon Jun 14 12:08:46 CDT 2021

No need to convert current loop, as the outputs from the 11/05 console 
are (roughly) TTL compatible.  All you need is a level shifter such as a 
MAX232 and an inverter (although this will only give you 2400 baud max).

I built a simple console to USB adapter using a Teensy that works quite 
well if you're interfacing with a terminal program running on a modern 
computer.  With this you can run up to 38400 baud. Details are here: 


On 6/14/2021 9:14 AM, Tom Uban wrote:
> I'm not to the point of connecting serial yet, but I did see this page which will help me turn the
> current loop to rs232 when the time comes:
> http://retrocmp.com/how-tos/interfacing-to-a-pdp-1105
> --tom
> On 6/14/21 10:37 AM, Jay Logue via cctalk wrote:
>> I also have an 11/05 with the early CPU boards that exhibited stuck bits on arrival.  Turned out
>> to be bad transistors in the inhibit circuits on the G110.  Pretty easy fix once I tracked it
>> down. So far I've found the GT40 print set to be a fairly accurate, at least for the boards I have.
>> I'll be curious to learn how your serial console works.  Mine had a manufacturing defect that had
>> to be corrected before input would work.
>> --Jay
>> On 6/13/2021 1:44 PM, Tom Uban via cctalk wrote:
>>> 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:
>>> http://gunkies.org/wiki/KD11-B_CPU
>>>   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:
>>> http://www.bitsavers.org/pdf/dec/graphics/VT11/GT40_Graphic_Terminal_Engineering_Drawings_Feb73.pdf
>>> 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...
>>> --tom
>>> 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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