The bus used for DEC's LSI-11 microcomputer implementation of the PDP-11 was originally and prosaically referred to as the LSI-11 bus, but thereafter it was universally referred to as the Q-bus.

Does this name have any meaning? And if so, what was it?

  • 2
    Looking through the material on bitsavers, the name change from "LSI-11 bus" to "Q-bus" must have happened between 1983 and 1985, if that helps anyone else in digging up an answer.
    – dirkt
    Apr 7, 2019 at 7:38
  • That timeframe suggests the introduction of the MicroVAX, which is plausibly a reason for not calling it the "LSI-11 bus" any more. The uVAX I description says "extended LSI-11 bus (also called Q22 bus)". Apr 7, 2019 at 20:12
  • But I am still be curious why they called it "Q". But it's a good point; maybe MicroVAX docs can provide hints.
    – dirkt
    Apr 7, 2019 at 20:16
  • I also noticed the Q22 name, but I think that means "Q-bus with extra signals".
    – dirkt
    Apr 7, 2019 at 20:17
  • @dirkt - I too am still curious as to 'why'; and yes, Q22 distinguishes the variant from Q16 (=original LSI-11 bus) and Q18 (=PDP-11/23 bus). Apr 7, 2019 at 20:50

1 Answer 1


The LSI-11 Bus Specification has a section on history and folklore, and states

The original backplane to implement the LSI-11 Bus was designed to accommodate Digital's quad and dual height modules. It became known as the "Q-Bus" to reflect the quad board form factor. Both LSI-11 Bus and Q-BUS are Digital trademarks, but are not registered trademarks. Several bus architecture features of the LSI-11 Bus are protected by claims in Digital Unibus patent numbers 3,710,324 and 3,815,099. Some publications may also use the term SUB-UNIBUS, although this is not recommended.

When the LSI-11 Bus was expanded to provide 22 address lines, the terms Q22-Bus and extended LSI-11 Bus were adopted to specify backplanes [of] that bus [with] the four additional lines.


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