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

DEC Flip-Chip Extender

Category: Tools
Written by Administrator

Why extenders?

When repairing computer circuit boards, you must measure their signals.
To measure their signals, you must add some signal clips.
To add signal clips, the boards must be operated outside the card cages.

dec quad extender original

That's why you can not repair DEC PDPs without so called "Extenders".

These come in several widths, identified by the number of 2*18 pin flip chip sockets they provide: "single-slot", "dual-width", "quad-width" and "hex-width". They have regulary DEC module numbers, for example: the quad-width is W987A, the dual-width is W984A. For dimensions see attached PDF.

pdp1134 extended boards

Extenders even can be cascaded ... usually not a good idea!

Where to get them?

Extenders are rare, as nobody gives them away.

But there is at least one company which is still selling them:


Here is a project to build extenders on yourself. Extenders consist of the circuit board and Flip-Chip sockets soldered to them.

dec quad extender diy


Attached below are Gerber files for single, dual, quad and hex-width circuit boards, each in standard DEC form factor, and in a "+5" variant, elongated by 5 cm.

extender pcbs 1246

You can view Gerber files with

These chinese board makers can produce at reasonable prices. For example, for a batch from we had in january 2020 (including shipment to Germany and taxes): Single = 8€, dual = 8.50€, quad = 15€, hex = 24€.

OK, we had quite a volume:

extender pcbs many

The "+5" variants let the sockets look above neighbor cards. This allows to attach clips to the socket wires.
Or you solder on rotated sockets, like in this strange PDP-10? extender:

pdp10 extender rotated sockets - pdp10 extender rotated sockets back


Flip-Chip connector sockets

These black DEC connector blocks are also rare. Note "rare" like in eBay advertisements, but really really rare.

dec flipchip socket

Some of us guys harvested them from dead backplanes. Steps are

  1. unwind all the wire-wrap from their pins (very boring)
  2. cut the PCB with sockets into handy pieces
  3. unsolder socket pins and remove sockets from PCB.

The latter may be easy or impossible, depending on the backplane maker style.

Some 3rd party backplane consist of separate sockets.

backplane dataram

DEC backplanes use sockets in quad or niner-packs.

backplane dec quadblocks

These can only be separated by sawing through the blocks. I successfully used a tile-cutter with diamond blade, as the socket bodies seem to consist of Bakelite, which behaves quite mineralic.

Unsoldering from DEC backplanes once was not too difficult, as PCB holes have some air around.
On some other DATARAM backplane we found the pins to be pressed into the PCB ... no chance here!


There's also a company named "ECS" which sells single-width connectors, see attached datasheet.

We got an offer of about $9 / piece in a quantity of 300.
So you have $56 for a hex extender, and have to find lot of other interested people, not to mention the effort to manage distribution ... lets see if this ever happens!

Download this file (2400-Series125-X-150-Contact-Spacing.pdf)2400-Series125-X-150-Contact-Spacing.pdf[ECS flip chip socket datasheet]636 kB
Download this file (2418-13-30.pdf)2418-13-30.pdf[ECS flip chip socket dimensions]41 kB
Download this file (flipchip-dimensions.pdf)flipchip-dimensions.pdf[DEC board dimensions]210 kB
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Category: Tools
Written by Administrator

The DEC "KM11" is a diagnostic tool for older DEC PDP-11 CPUs and some peripheral devices.

km11 pcb

It's a passive maintenance panel with just 4*7 lamps and 4 switches, which can be plugged into special slots on the host device.

There's no own "KM11" manual, it's use and schematics are part of the device to be debugged.

The best overall description would be at .

KM11 is invaluable in repair, very scarce, but easy to build. So some people reproduced it, I throw in mine project here.

km11 100


km11 101

Main use so far was microstepping 11/40 and 11/05 CPUs.

Especially for that I added pinheaders, so the micro program counter on display can also be monitored with a logic analyzer. And the one "momentary action" switch for generating "micro-step" pulses can be disabled, so pulses can be injected via a variable highspeed square-wave generator.

Attached you find the PCB Gerber files, a 3D print STL for the light template, and paper overlay for 11/40 and 11/05 CPUs.


Download this file ([Gerber files to produce KM11 PCB]127 kB
Download this file (km11-1105.pdf)km11-1105.pdf[KM11 labels for 11/05 CPU]148 kB
Download this file (km11-1140.pdf)km11-1140.pdf[KM11 labels for 11/40 CPU]1265 kB
Download this file (km11-schematic.pdf)km11-schematic.pdf[KM11 schematics]59 kB
Download this file (km11.stl)km11.stl[3D print for KM11 light mask]822 kB

G7273 Diagnostic

Category: Tools
Written by Administrator

Enhancing a G7273 UNIBUS GRANT continuity card

Much too late I designed a special GRANT Continuity G7273 card with some diagnostic features. Lets call it "G7273diag".

When setting up an old UNIBUS PDP-11, you typically have to deal with two topics first:

  1. is power good?
  2. is the GRANT chain closed?

G7273diag adds some comfort here with minimal effort.

SPC slot GRANT Continuity

General purpose UNIBUS backplanes with SPC slots route Interrupt GRANT (BG4,5,6,7) and DMA GRANT (NPG) in a special way:
All five BG* and NPG must be bridged on empty slots, connecting IN and OUT signals.

On the typical quad SPC slot in a hex socket the four BG* are forwarded by plugging in a little G727 GRANT continuity card into the "D"-row.
NPG is forwarded by setting a wire-wrap on the backplane pin side in "C"-row, between pins A1 and B1. I've written about it earlier.

Verifying and setting the BR* forward is simple: if the the SPC slot is empty, the BR* chain is open.
To close it, insert one of these little G727 card (OK, you may loose some blood and skin).

g727 multi

In contrast, working with the NPG chain is incredible difficult:
Neither its easy to find the correct backplane pins, nor can you easily access the slot contacts from the card side.

Thats why DEC complemented the G727 with the G7273 continuity card: it has full size, and closes BG* and NPG simultaneously.
Compared to G727 the G7273 is quite rare.

g7273 dec

While its now easy to close NPG with an G7273 in empty slots, you still must be able to detect the open/close status of those hidden pin-side NPG jumpers.

  • There are SOME cards (namely the omnipresent serial DL11-W M7856), which neither use nor close NPG. These cards can only be plugged into slots with closed NPG jumper.
  • On the other side, controllers doing DMA (all non-trivial ones, including UniBone) must be plugged into slots with OPEN NPG jumper.

So the first thing you do on every SPC backplane is: mark for each slot whether NPG is open or closed.

The G7273 diagnostic card

This "G7273diag" card is just a simple clone of DECs G7273. The BG4, BG5, BG6, BG7 signals as well as NPGIN and NPGOUT are routed to the upper card edge onto pin headers. Also the SPC slot voltages GND, +5V, +15V and -15V are exposed. g7273diag

So you can easily

  • verify whether NPG is closed by beeping NPGIN and NPGOUT
  • close the NPG chain by setting a jumper between NPGIN and NPGOUT
  • verify whether BG* and/or NPG are closed between two slots by inserting two G7372 and beeping the signals between them.
  • verify supply voltages with a voltmeter.


Voltage eye candy

With help of these little chinese volt meters, you can convert G7273diag into a permanent voltage monitor.

g7273diag voltmeter

See it here plugged into an 11/34 between serial DL11-W and a memory card, surrounded by more G7273diags:

g7273diag voltmeter plugged in

These tiny volt meters are small enough to fit into the space reserved for standard DEC card handles.  Key element is a 3D print:

g7273diag voltmeter handle


ebay voltmeter3

Build your own

To reproduce the G7273diag with volt meters, you need 3 components.

The printed circuit board (PCB) can be made by sending the attached Gerber file to one of these cheap chinese makers. I used Don't forget to select "gold fingers / ENIG".

The 3D printed handle is generated from the attached "stl" file.

The volt meters are eBay stuff. My ones were labled "3-Wires Mini DC 0-100V Voltmeter LED Panel 3-Digital Display Voltage Meter UE". The connection is in fact 2-wire, connect "yellow" to "red".

ebay voltmeter1 ebay voltmeter2
Download this file ([G7273diag volt meter handle 3D print]28 kB
Download this file ([Gerber files for G7272diag PCB]35 kB

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


  • 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!


  • QProbe - a QBUS signal adapter

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

    So after UniProbe, I also designed a "QProbe".

    qprobe total

    The card has these features:

    • it acts optionally as bus terminator via changeable resistor packs.
    • it visualizes the QBUS signals. 
    • Two display modes:
      a) ADDR and DATA are demultiplexed, INTR and DMA pulses are elongated to be visible.
      b) "passthrough": LEDs just show state of bus lines.
    • Pin headers provide access to signals via logic analyzer (LA) or multimeter.
    • Patch field to connect LA probes to QBUS signals in a clean way. LA ports with changeable impedance-matching resistor packs.
    • LED drivers and demultiplexer logic realized with two EPM240T100 CPLDs. These can be programmed with own logic via an onboard JTAG header and Intel QUARTUS tool.

    Read on!