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 jlcpcb.com. 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

gerber.zip -- Gerber files for G7272diag PCB

g7273diag.stl.zip -- G7273diag volt meter handle 3D print