PDP11/55 sells on Ebay for 5K$ - was it really the fastest 11?

Eric Smith eric at brouhaha.com
Mon Apr 18 14:25:41 CDT 2005

Heinz wrote:
> Isn't a 55 just a 45 with bipolar instead of core? or was that a 50?

Guy wrote:
> Actually, the 11/45 came in 3 variations originally:
>       * core
>       * upto 64KW of MOS on the fastbus
>       * upto 64KW of Bipolar on the fastbus
> The 11/50 was identical to the 11/45 and came standard with either MOS
> or Bipolar on the fastbus.
> The 11/55 is sort of half way between the 11/45 and 11/70 CPU-wise.
> Most of the CPU boards are from the 11/45, but there are a number that
> came from the 11/70 (including the FPU boards).  It is a distinct CPU
> from the 11/45 (as designated by the KB11D).

But it's more complicated than that, because the 11/45 production was
switched to the KB11-D CPU as well.

It's basically safe to say that the distinctions between the 11/45,
11/50, and 11/55 were mostly marketing.  The 11/50 and 11/55 were
apparently always shipped with at least some Fastbus memory, and
the 11/55 always had the KB11-D CPU (vs. KB11-A), but other than
that many combinations were possible.

The main difference between the KB11-A and KB11-D is that the FPP
interface of the latter is sychronous.  (The same as the difference
between the KB11-B and KB11-C processors in the 11/70.)  The
microword is four bits wider to support that.


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