There are 2 different "UNIBUS" connectors on PDP-11 backplanes.
The "Standard UNIBUS" slot typical sits at the edge of backplanes, and is used to connect backplanes via UNIBUS BC11 white flat cable, or hold an terminator.
The "Modified UNIBUS" ("ModUB") often occupies Slot A and B of the hex socket which have an SPC in slots C to F.
It is optimized to hold memory cards and differs from the Standard UNIBUS:
- There are additional voltages for memory boards exposed.
We have -5V and +20V for core, and Battery Backup voltages of -15V, +15V and +5V there. - There is support for a separate parity controller (M7850). We have extra parity lines, and a internal SSYN signal, all that is not used in UniProbe.
- the Bus GRANT lines BG4,5,6,7 and NPG, which are daisy chained from socket to socket, are missing.
This has some advantage, as a card can plug into a ModUB socket without need to open or close GRANT jumpers.
Just plug in "on-the-fly". - the "SACK turnaround" logic of M9302 is not used. Good if you don't have these 8881 and 8837 DEC driver chips.
- as regular terminators remain plugged in, UniProbe on "Modified" sockets are not allowed to have own terminator packs.
This makes UniProbe "neutral" to the bus, you can plug in and out without consequences.
Putting a standard UNIBUS card into a "ModUB" socket may cause major damage, as the >15V voltages are routed to signal wires then.
See the BIG warning label on the M9302 card!
Signal differences between Standard and Modified UNIBUS are (from the 11/34 system manual):
UniProbe can be build for either Standard or (exclusive or!) Modified UNIBUS sockets.
If build for ModUB:
- set the solder jumpers to 2-3 position
- do not populate the M9302 "SACK turnaraound logic" U1,U2,U3
- do not populate the BG4,5,6,7 and NPG LEDs
- replace the 180/390 ohm terminator resistors by high ohm 100k/100k.
This allows for diagnostic of open signal wires: LEDs light with half intensity then. - use a two-row male connector for voltage measuring, the additional supply voltages can be checked at the upper front connector row.
- clear the "Standard UNIBUS" text in the warning label!
SPC or UNIBUS?
Building UniProbe for the 4-row SPC socket would also have been a good idea.
- SPC also carries many voltages AND the bus grant signals simultaneously.
- SPC carries also the clock signal LTC and on some CPU backplanes (11/34) CPU HALT.
- it is 4 slots width, so more space available. A design without SMD chips would be possible.
- is also very popular and available on most (all?) PDP-11 systems somewhere.
However there are situtations where the SPC slot is not available.
- in tape drives like TU56 "DECtape" or TU10, where the controller is built into the tape drive rack and UNIBUS is routed through the separate rack.
- in rack boxes which contain ONLY special controller backplanes (like RK11 for RK05 drives, or RK611 for RK06 disks).
These backplanes always have Standard UNIBUS slots, but never SPC. - finally, DEC used the flat white UNIBUS cable BC11 and the M930 terminator also for non-UNIBUS busses. For example to daisy chain TM10 slave tape drives or RK05 disk drives.
Despite we have completely different signals there, UniProbe's LEDs and logic analyzer connnectors may be useful in these setups too.
Prefering the UNIBUS slot over SPC is in fact a decision for flexibility and against comfort.