> On Mar 29, 2025, at 12:33 PM, shadoooo via cctalk <cctalk@classiccmp.org>
> wrote:
>
> Hello,
> I'm searching information about all existing variants of DEC Unibus in
> Dual/Quad/Hex flavors.
> I read the "UnibusSpec1979.pdf" on bitsavers, which reports a "Standard
> Unibus" pinout in the last pages.
> However in several backplanes "Small Peripheral Controller", "Modified Unibus
> Device" and "Extended Unibus" are supported.
> Maybe also other unlisted Unibus variants do exist (e.g VAX 11/730)?
> I also found the gunkies.org WIKI very helpful, however it is still quite
> difficult to compare the pinout differences (dummy proof).
Try this one:
https://hampage.hu/dr/unibus.html
>
> Where could I find a specific DEC documentation about the more recent
> variants, similar to the 1979 specs, but referred to SPC, MUD, EUB, ect?
>
> Big doubts:
> - why DEC, having defined the dual Standard pinout, had then to implement the
> quad SPC backplanes?
The “dual Standard pinout” for slots A/B was generally used for a BC11-A Unibus
cable,
with a Unibus terminator in the very last slot. The Unibus cable carried
signals (not power!) between backplanes.
Quad SPC is common to (at least) Modified Unibus(MUD) and Standard Unibus. The
differences
between the two backplane specs are in slots A/B only, with slots C-F the same
(AFAIK.)
Also some slots of the of the VAX730 backplane.
> - why DEC, having defined quad backplanes, had then to implement the hex
> (standard + SPC) or (MUD + SPC) or EUB?
Probably due to changes in cabinets and packaging. The 11/20 was unique in
having a quad
backplane mounted from side to side in a BA-11 (and upside-down too!) Most of
the subsequent
Unibus pdp11s had hex backplanes mounted either vertically, top to bottom,
(11/40, 11/45 and
11/70,) horizonatlly, front to back, (some 11/05s, 11/34, 11/35, 11/44 etc.),
another
orientation was vertically to one side, front to back in 5.25” cabinets. EUB
was unique to
the 11/24 and 11/44 and only for memory boards.
[https://gunkies.org/wiki/Extended_UNIBUS]
>
>
> I mean: given that in AB all Unibus signals are present (from
> specifications), what is the need for CDEF?
The ability to use larger printed circuit boards; it gets you higher density
and avoids the need to run (as many) interconnects off the board and thru the
backplane. Remember that 50-60 years ago was the era of 14,16-pin DIP packages
for small-scale and medium-scale
integration; larger packages existed of course, but they were the exception.
You still
needed lots of wires to interconnect functionality between chip packages. Using
a hex
board instead of single or dual boards allowed many of those “wires” to be
traces on a PCB, rather than run through the backplane or on an over-the-top
jumper. (See the 11/34a.)
> Provided that several signals are duplicated in hex pinout, the backplane
> will connect homologue signals together,
> or AB bus will always be separated from CDEF bus?
>
> My aim is to design a reprogrammable digital logic board which could be
> employed in any system,
> using 18bits address or also 22bits (i.e. for 11/24).
See the Unibone, to ensure you’re not reinventing the wheel. It also includes a
Unibus description that might be helpful. For something simpler take a look at
the M1710 Unibus Interface Foundation Module. Brochure:
https://vt100.net/manx/details/1,22302
https://retrocmp.com/projects/unibone
—Milo
