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Thread: Oh dear... 4 bits extra! PDP11/10

  1. #1

    Smile Oh dear... 4 bits extra! PDP11/10

    Xmas was early this year! But oh dear... this is 16 bit! 4 bits more than the 12 that I'm used too

    PDP11_10 01.jpg

    So pulled out all of the boards and fans to clean the innards of the system.
    I'm always surprised to see how much dust is collected into the slots when
    I use compressed air to clean a machine...

    PDP11_10 02.jpg PDP11_10 03.jpg PDP11_10 04.jpg PDP11_10 05.jpg

    I didn't like the feel of the lock. In my opinion I had to apply too much force and I was afraid
    to damage the lock over time. So I removed the lock and removed al the dried out grease.
    It was bare aluminium from the lock scraping on the bare aluminium of the front panel.
    A few new drips of oil in the lock made the key go in and out normal again. And new grease
    around the lock lets it move with a minimal force again.

    PDP11_10 06.jpg PDP11_10 07.jpg PDP11_10 08.jpg

    And I really like those Boxer fans. You can take them apart, clean them and re-oil them.
    The piece of felt needs to keep a certain amount of oil to relubricate the fan over time while in use.
    But after these years they are al dried out. So now these can run for another few years
    without damaging the bearing.

    PDP11_10 09.jpg

    And there is a well known power supply again. The rubber between the big can capacitors and PCB
    is decaying. And this process seems to eat the PCB tracks over time. I had this in my 8/M too.
    So replaced that with neoprene rubber and cleaned the PCB. And I made the GND and +5V wires redundant.

    PDP11_10 10.jpg PDP11_10 11.jpg PDP11_10 12.jpg

    And no RS232 on this machine! Only current loop or TTL output. So I connected a small converter.
    The receiving part was inverted, so I looped it back trough the MAX232 to invert it again.
    And I saw the serial card can run on external baud rates. So I made a little PCB for that.
    In this way I can set the internal rate at 110 BAUD and I can connect a TTY if I want to.
    And connecting the external PCB makes it easy to connect RS232 stuff on higher BAUD rates.
    We will see how that works in the future. (I would love to run some graphics on this machine with my Tektronix 4010 some day
    if that is even possible with a 56K machine. But that 4010 can run on max 9600 which is to be advised at graphics.)

    PDP11_10 13.jpg PDP11_10 14.jpg

    I found some nice info on the web to do some simple tests on a PDP11. Hints on Testing a Dead PDP.pdf

    Now writing data to the transmit buffer on 177566 gives the corresponding character on the terminal.
    And Reading the 177562 register gives the right ASCII value in the LEDs. So far so good!
    I have done some writing to and reading back from memory, this seems to work just fine. No stuck bits.

    The simple CPU test to write 5007 (clr PC) at address zero keeps the machine in RUN state.
    Also the 777 keeps the machine running.

    But any other test program just halts the machine immediately. Are there some simple
    toggle in CPU tests which I can perform to check what is going wrong?

    Regards, Roland
    Last edited by Roland Huisman; July 11th, 2020 at 12:49 AM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

  2. #2


    You don’t have a M9301 or M9312 board, right? If you have such a board PDP11GUI is a good idea. These bootstraps contain a basic CPU test. With PDP11GUI you can then downloaded whatever you like.

  3. #3


    Hi Mattis,

    Yes I have a M9301 but in unknown state. I have also build a Unibone but this is not tested either.
    I want to start with the plain machine before I add stuff to the machine what might even cause more troubles.
    And since it does not even run an echo program I don't think a monitor program will run... But I did already
    configure the M9301 with restoring W1-W5 and removing W6 and set the switches to the monitor program.

    So these boards are fun for later I guess...
    M9301.jpg UniBone.jpg M7800.jpg M7856.jpg M7846.jpg

    Regards, Roland
    Last edited by Roland Huisman; July 11th, 2020 at 05:44 AM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

  4. #4
    Join Date
    Feb 2011
    Salisbury Maryland


    1000 005000 CLR R0
    1002 12701 MOV #177564 to R1
    1004 177564
    1006 105711 Loop TSTB R1
    1010 100376 BPL Loop
    1012 110061 MOVB R0, 2(R1)
    1014 2
    1016 005200 INC R0
    1020 000137 JMP #1006
    1022 001006
    Itís a loop that increments R1 and prints the results out the con, works on all PDP-11 family.

    16 bit is where its at, 12 bit is somewhat not as good but better then eight!

  5. #5


    Hi Roland,

    I haven't read and processed all the testprogs in the pdf, but most of these do a lot of work and then just HALT. I have no idea how hard a PDP-8 can run, but this entry level PDP-11 can do roughly 200,000 to 300,000 instructions per second and your memory is 8 KW at this moment. Doen in a flash of a second.

    Hope this helps,

  6. #6


    Congratulations on your new 11/10! I have a 11/05 with a M9301 and a Unibone, so it seems we're on a similar path.

    Did you try the console loopback test? When it halts, what address shows on the console?

    When debugging my system, I found it helpful to eliminate variables by pulling the core and memory boards and running very simple test programs out of the register area (177700-177705). For example, if you load the following code at 177700, and then start execution at 177702, you should see a stream of 'v' characters printed on the console:

    177700: 177566 # Address of console xmit buffer
    177701: 0
    177702: 110010 loop: movb r0, (r0)
    177703: 000777 br loop

    Unfortunately I don't have my 11/05 in front of me, so I'm going by memory here.

    Also, in case you're interested, I've built a simple USB adapter for my 11/05 console which supports baud rates up to 38400 and autobaud detection from the host. Details are here:


  7. #7


    A M9301 is easy to check out. Just examine memory at 173000-173776 and 165000-165776. There is a small instruction test that test some basic tests in there. Just start at 173000. You can single step through it and see what it does.

  8. #8


    Very nice Any plans for it?
    Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
    Synthesizers: Roland JX-10/SH-09/MT-32/D-50, Yamaha DX7-II/V50/TX7/TG33/FB-01, Korg MS-20 Mini/ARP Odyssey/DW-8000/X5DR, Ensoniq SQ-80, E-mu Proteus/2, Moog Satellite, Oberheim SEM
    "'Legacy code' often differs from its suggested alternative by actually working and scaling." - Bjarne Stroustrup

  9. #9


    Did some more testing with other programs and got the loop back going now as well.

    @ Mattis, I tried the M9301 but that card is not working well. Bit 4 is always on when I write something
    into memory and read it back. So even without using the bootstrap it corrupts the data.
    I will check out that board later.

    In the meanwhile I cut the NPG wire on the upper slot and put in the Unibone. It seems the Unibone is working.

    I don't have a clue yet about how to configure the Unibone to fill up the memory above
    8KW up to 56KW, and emulate a RL disk. I'm missing a how to test and setup document for newbies
    But I'm sure it is documented. A lot of fun to figure out
    unibone message.jpg

    Quote Originally Posted by commodorejohn View Post
    Very nice Any plans for it?
    Well I had an Unibus machine on my wish list for a while to learn about the bus.
    Maybe make some hardware for it in the future... Who knows

    @Jay, good to know you have the same setup. Did you get it going already?
    @Geert, thanks for the insight!
    @Qbus, thanks for the test program!

    Regards, Roland
    Last edited by Roland Huisman; July 11th, 2020 at 12:32 PM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

  10. #10


    @Jay, good to know you have the same setup. Did you get it going already?
    Whoops, seems I got myself confused. I have an M9312, not a M9301. Similar, but not identical.

    My system is working pretty well, actually. I have my USB console adapter plugged into the BBB in the Unibone. So once the system is powered on, I can ssh into the Unibone and use a terminal program to access the console. Rather conveniently, the Unibone can simulate a power-on reset of the PDP-11, which allows me to get back to the M9312 prompt from pretty much any state without having to be physically present.

    Next up is connecting my DL11-W to one of the serial ports on the BBB, for use as an emulated TU58.



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