I recently found an interesting manual:

    EK-HS571-TM  HSC50/70 Hardware Technical Manual

This gives quite a thorough coverage of the theory of operation of
the HSC50/70 hardware.  It's marked something to the effect of that
it's for DEC support personnel only, so I'm not sure how it escaped.
It references other related manuals which I *don't* have, including

    EK-HS572-TM  HSC50/70 Software Technical Manual
    EK-HS574-TM  HSC50/70 Chip Description Technical Manual

(both of which are also for DEC support personnel only)

    MP-01422  HSC50 Field Print Set
    MP-01426  HSC70 Field Print Set

Does anyone have copies of these or other HSC technical documents, or
any idea where they could be found?  HP has of course never heard of
them.  I *still* haven't found any way to order Field Maintenance Print
Sets, or anyone that will admit that such things exist!  :-(

The HSC hardware is pretty amazing.  The CI port processor module and each
of the SDI/STI data channel modules have not one but TWO microcoded
processors.  On each card there are two independent microcode sequencers,
each using three 2911s, and sharing a common microcode store and 2901-based
data path.  There are three independent busses in the HSC, the processor
bus (between the I/O control processor and its memory), the control path
bus, and the data path bus.  Each bus has its own arbitration system,
address, and data lines.