Some utilities to help you managing your hardware. Some are self-written by the authors, some are linked to the web.

PDP11MONLOADER - code download into PDP-11's over serial monitor

Category: Tools
Written by Administrator

Code download?

Most PDP-11's have a serial console monitor. This is a simple line based user interface which allows to write, read and execute code.

This is important

  • if you need to boot device which the Boot ROM is missing. Then you have to enter the boot loader code over monitor.
  • if you want to run diagnostics, and have no working disk or tape devices yet.



Many many people have written auxillary programs to automate code download. Here comes my contribution: "pdp11monloader". 

It is a command line program, written in C for Linux platforms. On MS-Windows it can be compiled for CYGWIN.

Main features:

  • serial port, baudrate and data format settable
  • can work with QBUS-PDP-11's (ODT monitor) and with UNIBUS machines (M9312 / M9301 console emulator).
  • can load code from MACRO-11 listing files, standard paper tape images and plain textfiles with octal <address> - <value> pairs in each line.
  • can start execution of downloaded code
  • can stay in primitive tty-like terminal emulator to let you immediately work with the running code.



Download here: The "demo_files" contains some example code formats.

More info by built in help:




An UNIBUS signal adapter

Category: Tools
Written by Administrator

( This is older work, and has been superseded by UniProbe. )

unibus signal adapter-09


Problem: How to access UNIBUS signals?

Often when you debug your PDP-11, you wish you could measure the signal lines of the UNIBUS. These signals appear in every slot of the backplane, but are usually buried deep inside the card cage:

unibus signal adapter-15 unibus signal adapter-14

Attaching your signal probes to the wire-wrap pins on the backplane's backside seems possible, but is impratical: finding the right pins can take hours!


So you have to make an adapter for routing the signals out of the machine. Here's how you can build one:

Solution: Building an UNIBUS signal adapter!

In PDP-11's many backplane types are used, general purpose ones and such for special controller or memory assemblies. On almost all of these backplane types the UNIBUS signals are concentrated to two standard FlipChip sockets at both ends of the backplane. The UNIBUS has 56 data signals and some GND and Vcc, see the  pdp11bus handbook (1979)" for reference. The standard UNIBUS slots are used to plug in bus terminator cards, or bus cables to connect backplanes in a chain.

Requirement 1: your adapter should use these standard slots, but without blocking it for further use by the system.

Another problem is that the boards are packed very tight together. The board-to board distance is only 10 mm, there is no vertical space for test points on the boards.

Requirement 2: every board you plug into the PDP-11 card cage may be just 10 mm high. You need to route the testpoints outside the case.

A design to meet these requirements has to look like this:

unibus signal adapter-01

There are three separate components (the small terminator is not part of the design):

unibus signal adapter-02

  1. A two-unit width modified extender board (lets call it "MEB") to get the signals out of the machine. You still can attach terminators or bus cables to the original slot  (in the picture a 11/05 terminator is plugged in).
  2. A bread board to sort out the cables, so you can attach a logic analyzer or an oscilloscope. Lets call it the "signal adapter board", or "SAB".
  3. Two normal 40 wire flat cable to connect SAB and MEB. This even gives you the option to plug other boards than the SAB to the MEB. The cables should be not too long , I think 30cm are enough.

The modified extender board "MEB"

In total the two FlipChip sockets of a standard UNBUS slot have 2x2x18 pins, you need to route 72 signals. I reworked a commercial DEC extender board from Douglas Electronics.

unibus signal adapter-13

 These are the modifications:

  1. Two flat cable were crimped to two male 2x20 connectors. We need just 2x36 pins, put 40 wire cable is a standard,
  2. The two connectors were hot-glued to the board.
  3. Holes for 36 wires were drilled through the board (1.5 mm diameter).
  4. Half the wires were soldered on the connector side, the other to the backside.
  5. Paper labels with UNIBUS signals names were taped onto all four connector rows.

unibus signal adapter-05

In result, the MEB remains slim enough to fit into the card cage:

unibus signal adapter-04 rotated unibus signal adapter-12

The MEB can be used as regular extender board again when the flat cables are disconnected.

The signal adapter board "SAB"

The signal adapter board contains test points for the UNIBUS signals. They are nicely labeled and arranged into "Data Transfer Section", "Priority Arbitration Section" and "Initialization Section" signals. Every signals gets two pins, so you can attached both a logic analyzer probe and an oscilloscope.

unibus signal adapter-07

There's only a lot of wire-wrapping on the backside.

Later I added LEDs for the control signals, and decoded A17-A13 into a single "IOpage" signal. This saves logic analyzer probes. The LED drivers are CMOS type with very high impedance, but the IOpage decoder adds one 74LS TTL load to the address lines, I'm not too happy with that. However, all driver chips can be plugged out of their sockets, making the board totally passive.

unibus signal adapter-03-leds

See SACK lighting, because it's being asserted? Yes, this photo was made with an open GRANT chain!

The only hard thing was to get all the wires sorted. Find my net list document in the attachement.


You can also make a SAB with QBUS test points and plug it to the MEB.

I didn't made one yet because

  • I like the big old UNIBUS machines more.
  • QBUS periphery seems much more reliable than UNIBUS boards. Maybe this is because  they're usually younger.

Working with the adapter

Read on here about working with the adapter on my 11/05!

unibus signal adapter-10


Download this file (unibus_signals.txt)unibus_signals.txt[UNIBUS signals, flip-chip connectors and signal adapter wires]8 kB

Hints on testing a dead PDP-11

Category: Tools
Written by Administrator

The web page "Hints on Testing a Dead PDP-11" lists many common reasons for PDP-11 failures, and how to fix them.

Premium content, thanks for it!


  • PDP11GUI

    PDP11GUI is an integrated development environment (IDE) for PDP-11’s, running under MS Windows.

    You can write programms in assembler and load them onto the PDP-11, run programs or single step them, disassemble code, load, dump and display memory and inspect registers.

    You can use it in conjunction with SimH to learn about PDP-11’s, or you can attach it to a real PDP-11 as an handy hardware investigation and diagnostic tool.

    First I’ve written PDP11GUI for some diagnostic tasks on my 11/44, then it developed its own momentum and kept me busy for nearly a year. And it’s still growing.

    PDP11GUI is distributed as a self-installer at GitGub .

    Before installation, backup all files you eventually modified!

    Then proceed through installation instructions and the tutorial.



  • SimH with telnet console access

    A modification of SimH to access it's command prompt over telnet.

  • MACRO11 cross assembler
    Info about different version of MACRO11 for MS-Windows.
  • UNIBUS diagnostic procedures
  • PDP-11 Diagnostic Database

    DEC made diagnostic procedures for all of their hardware, here we concentrate on the PDP-11 universe.

    Many of these diagnostics are on the web today, but widely scattered around. 

    They have several designations, come on several media, are made for different devices, run under several supervisor monitors and are separated from their documentiation.

    So here is a try to pull all pieces together and order them into one system:

    The PDP-11 Diagnostics Database

  • TU58FS - file sharing with a DECtape II emulator

    This is a tool to share files between a DEC PDP-11 and a modern computer.

    The software emulates an TU58 DECtape II, which is controlled by the PDP over serial RS232 line.

    The "tape content" appears as plain files in a shared directory, and can be accessed on both sides. So you can easily exchange files between PDP-11 and your host, just by copying to and from the shared directory, even per drag'n drop.

    XXDP and RT-11 filesystem are implemented.

    Requirements on PDP-11 side are minimal: only a 2nd serial port is needed. Not even TU58 "DD" Boot ROMs are necessary, as tu58fs contains an option to download a boot loader over ODT and M9312 console.

    There is full support for "oversized" TU58 tapes: a TU58 tape image can hold up to 32MB, breaking the 256KB barrier.

    This allows to run the full XXDP diagnostic file set over a single RS232 connection. No need to create special XXDP cartridges, or to wear your RL02 drive while reparing & testing.

    Or you can run a full RT-11 distribution to a host directory, modify sources with your host editor and compile on your PDP-11 simultaneously.


  • UniProbe - an UNIBUS signal adapter

    When repairing DEC PDP-11s there's always the need to see whats happening on the main system bus. On older 11s this is the UNIBUS.

    After several try-and-error cycles, I think I now have the perfect signal adapter for UNIBUS ... calling it "UniProbe". In KiCAD 3D rendering it looks like this:

    uniprobe3d total

    UniProbe has these features:

    • can be built to plug into Standard UNIBUS or Modified UNIBUS slots.
    • can be built to be a PDP-11/34 M9302 or a simple M930 terminator.
    • LEDs for all 56 UNIBUS signals on the card edge.
    • Logic analyzer probe plugs and a patch board area for measuring the signals.

    Read on!