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May 152014

PDP-11/34 cabinets

We rescued two PDP-11/34 computers and their associated equipment from a storage unit in the Bronx and have been working on getting them running again. The computing system included multiple RK05 hard drives, two RL02 decpack drives, a TU11 tape drive and tons of media, including “digitized monkey brains“. Read on for more details and the exciting boot sequence.

PDP-11/34 Unibus wirewrap PDP-11/34 Unibus slots

The PDP-11 uses a wire-wrapped Unibus backplane with functions split across many cards. The top four are the CPU/FPU and cache, the next is 64 KB of memory, then a few “bus grant cards”, the console controller, some others, and finally the disk controller.

PDP-11/34 service position

Adding or removing cards required adjusting the wirewrap on the bottom of the backplane as well as installing bus continuity cards, so the rack slides are built to allow the CPU chassis to be positioned at 45 or 90 degrees for easy access.

PDP-11/34 Front panel

The boot sequence had to be toggled in on the earlier models, but the /34 has an octal keypad so you can type the addresses and values rather than flipping binary switches. One of ours has a sticker with the boot sequence printed on it and we’ve singled stepped through it to verify that it works.

PDP-11/34 bootrom / unibus terminator

However, we’re lucky that this machine has a M9312 “bootstrap / Unibus terminator” board, which has a several small bootroms for different devices like the RK05 decpack drive, RL02 harddrive or TU11 tape drive and also includes a serial console interface. This card allows the the machine to be booted with push button convenience using the VT100 terminals that we’ve restored.

When faced with the bootup of an unfamiliar OS from the 1970s (we’re all Unix kids), “DIR” seems to be the most likely command. For a full walkthrough of the RT-11SJ boot sequence, check out or just ask for HELP.

RT-11 y2k bug
UPDATE: somewhat as expected, RT-11SJ is not y2k compliant. 31-DEC-99 23:59:59 rolls over to 0-H99.

  21 Responses to “PDP-11/34 says hello from 1978”

Comments (21)
  1. So effing cool.
    Great job, peeps!

  2. Could I see it if I happen to be in NYC next week?

  3. Wow. This is a blast from the past for me. I used to program these. PDP 11/23, 11/34 and 11/70 and also the equivalent of today’s Sys Administrator.

    The good old days 🙂

    • I remember those. I was involved in real-time data capture from radar. We used RSX11-M (maybe there were slashes in the name somewhere RSX/11-M?). I thought they were great machines with an elegant instruction set.

  4. We have a PDP-11/34 online from years 🙂 😉

  5. PIP dst = src Peripheral Interchange Program
    PIP /help (if I remember correctly)

    • Yep you remember correctly. It was always confusing initially when making the transition to the DOS copy command. With PIP the destination was the first parameter! And DOS dropped the = parameter.

      PIP B:=A:*.COM

      Copy all files with a “.COM” extension on disk “A” in the current user area code to disk “B” in the current user area code.

    • They also had PIP on CP/M… so moving from micros to minis was easier in the 80s. 😉

  6. Nice job. My first job out of college was programming in Fortran and assembly on one of these. Once when working through adding a hard drive, DEC support had me clean the edge connectors with a pencil eraser.

    • Excellent. I managed an 11/34 at Penn State. Had one of my own for awhile, but now have an 11/84 with an RA92. Still fun to mess with. Good luck on those.

  7. re: RT-11 and Y2K, you might want to poke around in the alt.sys.pdp-11 archives over at Jerome Fine has done a lot of work on that score and frequently (well, for alt.sys.pdp-11 anyway) complains that no one seems to be interested.

    Oh. I see from that you’re running V03B. There may be no hope for that version.

  8. Wow! Advanced technology, eh?

    No kidding – I used a PDP-8A as a test equipment controller for many years. I would have given anything to upgrade to a PDP-11.

    As for Y2K compliance, the PDP-8 used a 12-bit word to store the date, of which 3 bits were used for the year. So the year would roll over every 8 years.

  9. I worked with many a PDP-11 including the /34 in college. At night, I would boot Version 6 UNIX.

  10. I used to own a few pdp 11’s and vax 11’s – My memory is super scratchy but I’m sure I can help with restoration if you all need it. I’m very patient with wire wrap, for example. I once received an 11/785 with the entire backplane wire wrap slashed by some college dolt.. Rewired the whole thing. Quite theraputic, especially when it booted up just fine 😀

  11. Went to Navy school on these and the 11/40s back in 78! LAter moved up to the 11/23 uVax and uVax II!! 🙂 They still used some 34s in Saudi until we removed them about 2006!!!

  12. 3 years in high school, about 9 years of work on PDP’s and VAX’s. RSTS/E, the original DUNGEO and ADVENT text adventure games, TECO ($$ escape-escape), VT-EDIT, VT-52’s, VT-100’s, LA-34’s, LA-36’s, and doing COBOL on cards. My account was 128,21 on my high school PDP.

    I tried buying one of the newer 11/83’s about 10 years ago, but the guy wanted too much. Would be fun to have one though.

  13. Brilliant machines, learnt everything about computers on one of these 🙂

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