On Sat, Mar 29, 2025 at 1:41 PM shadoooo via cctalk <[email protected]> wrote: > 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)?
Yes. The 11/730 is a little different because below the CPU boards, the first 5 slots can take 1MB memory cards or peripheral cards (each of those slots has a single memory select line that goes back to the memory controller, so a) the cards must be 1MB each, and b) there are only 5 select lines coming out of the PAL. Our original COMBOARD wasn't 100% conformant to the Unibus spec - it works in DD11 backplanes just fine but does _not_ work in an 11/730 without dozens of cuts and bodge wires. > Big doubts: > - why DEC, having defined the dual Standard pinout, had then to > implement the quad SPC backplanes? > - why DEC, having defined quad backplanes, had then to implement the hex > (standard + SPC) or (MUD + SPC) or EUB? > I mean: given that in AB all Unibus signals are present (from > specifications), what is the need for CDEF? > 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? The "why" for a good portion of that was how the Unibus evolved from 1970. In the 11/20, A/B slots were the true Unibus signals, meant for BC11 cables and M930 terminators. Many (most?) original peripherals were implemented not as single cards, but as system units (a block of 4 or 9 slots with Unibus in and out on first and last A/B but with unspecified wiring in between) including the console serial port before the SPC M7800 DL11. For system unit peripherals, there was frequently a common scheme with an M105 and other cards to implement the Unibus parts of the peripheral that resulted in a typical wiring arrangement of certain signals on CDEF that were just wired over to Unibus signals from A/B. With all the needed signals on CDEF and with Unibus in/out and power connectors in some B and even sometimes a KM11 diagnostic board, that left room for quad SPCs in any open slot. Starting with the 11/05, which had to fit in a single backplane, the amount of integration rose to where hex peripherals (and memory) were possible. As mentioned, EUB slots exist to support 22-bit memory addressing for Unibus memory (11/24). short answer - the Unibus evolved as integration density increased. -ethan
