DEC G9047 QBUS Grant and voltage monitor

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Enhancing a G9047 QBUS GRANT continuity card

In analogy to the UNIBUS grant and voltage monitor G7273diag I designed a special QBUS GRANT Continuity G9047 card with some diagnostic features. Lets call it "G9047diag".

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

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

G9047diag adds some comfort here with minimal effort.

GRANT Continuity

QBUS backplanes - like UNIBUS - route the Interrupt GRANT "IAK" signal and the DMA GRANT "DMG" in a special way:
IAK and DMG must be bridged on empty slots, connecting IN and OUT signals.

The typical solution is not to leave empty slots between cards, but to arrange all card nearest to the CPU. If you have holes in backplane usage, you must fill them with the G9047 QBUS GRANT continuity card. This card just connects IAKI to IAKO and DMGI to DMGO

g9047 dec

In practice, handling of QBUS systems is much easier then UNIBUS: QBUS system typically do not span multiple backplanes, there are much fewer slot types, there's only one Interrupt GRANT IAK for the four IRQ4..7, and the DMA GRANT signal DMG is not hidden on the backplane backside, as its UNIBUS counterpart NPG.

Nonetheless: as on UNIBUS, it may be a challenge to close the GRANT chain. In QBUS systems there only standardized slots, I don't know of any device-specific special backplane. But  "standard" means: many QBUS varieties. In every A/B row there's QBUS, but we have 18bit and 22bit QBUS, or 18bit wired to 22bit manually. C/D rows are used as standard QBUS slots too, or are bussed together for custom applications, or partly prewired for special local CPU memory extension. Another speciality are the "zig-zag" backplanes like H9276 with counter-intuitive QBUS slot ordering. And then there are the microVAXes.

See this excellent site for more info.


The G9047 diagnostic card

This "G9047diag" card is just a simple clone of DECs G9047. The INTR grant "IAK" IN and OUT signals as well as the DMA grant "DMG" IN and OUT are routed to the upper card edge onto pin headers. Also the mandatory +5V, +12V, as well as optional -12V and battery voltages are exposed.

g9047diag upwards

So you can easily

  • verify whether DMGI/O and/or IAKI/O are closed between two slots by inserting two G9047 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 G9047diag into a permanent voltage monitor.

g9047 dvms

See it here plugged into an 11/23 card cage with H9276 backplane. H9276 has QBUS only in A/B and special CD in all rows, so no G9047 in the C/D area here.

g9047diag pdp11

In DEC QBUS systems, only TWO DVMs for +5V and +12V are used. DEC cards that need other voltages generate them locally. However the QBUS standard defines many more voltages,

-12V and some battery backups. G9047 has sodler points for all these, so you can decide on yourself which voltage to display on the 3rd DVM.

g9047diag solder points



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

dvm 3d stl

ebay voltmeter3

Build your own

To reproduce the G9047diag 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 (g7273-g9047.stl)g7273-g9047.stl[Voltmeter body 3d print]190 kB
Download this file ([PCB production files]47 kB