( This is older work, and has been superseded by UniProbe.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

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  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.

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 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.

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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.

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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!

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unibus_signals.txt -- UNIBUS signals, flip-chip connectors and signal adapter wires