Is The M9312 Boot Module Essential?

[Noel Chiappa]

    > From: Rob Jarratt

    > is the M9312 essential to ever get this machine to boot up an operating
    > system?

Interesting question. I don't have my -11/24 running yet, so this reply is
theoretical, not tried in practice (and as we all know, the difference
between theory and practice is even larger in practice than it is in theory),
but here goes.

The M9312 basically provides two things: 1) UNIBUS termination, and 2)
boostrap ROM.

To further subdivide the former, it provides 1A) analog termination (i.e. a
resistance at the end of a transmission line that prevents reflections of
signals passing down the otherwise un-terminated transmission lines of the
bus), 1B) pullups (so those transmission lines normally float at roughly 3V,
unless actively driven by one of the boards plugged into the bus) and 1C)
'SACK turnaround' (a start-up 'safety check' where an un-requested - and thus
'un-grabbed' by any device - bus grant from the CPU on start-up is 'turned
around' by the terminator; this verifies that the grant lines are un-broken
between the CPU and the terminator - e.g. by someone forgetting to plug in a
grant jumper).

1A is not _absolutely_ necessary; this can be seen in small QBUS systems (the
QBUS is, at the analog level, sort of identical to the UNIBUS; this an be
seen in the use of the same transceiver chips, such as 8641's, on both) which
can get away without 1A in small configurations. Whether it's needed on your
-11/24 is hard to predict, theoretically; the easiest thing is to just try
it and see. Note: it may 'work' without it, but not be as _reliable_ as with
it.

1B _is_ necessary, but can be provided anywhere on the bus; most UNIBUS/QBUS
CPUs have it built in, and so does the KDF11-U of the -11/24: see pg.  of
MP01028.

1C is required by _some_ UNIBUS CPUs (ISTR that the -11/04 won't run without
it), but the KDF11's in general don't; e.g. the -11/23 definitely runs
without it. The KDF11-U might have outboard circuitry to require it, but I'm
too lazy to grovel over the prints to see. Easiest to just try it and see.


For 2, it all depends on what you're booting from. E.g. the RK11 has a simple
enough bootstrap that you can just enter it manually (although it gets old
after a while - I remember re-'programming' (think 'soldering iron' :-) a
castoff BM-792 someone gave us for our -11/40 so I wouldn't have to).

But if you're loading it over the console serial line, e.g. with PDP11GUI,
you don't need any ROM bootstrap - the built in console ODT will be enough.
You can also load a bootstrap that way; I was booting off the QSIC RK11 with
a boostrap loaded over the console serial line; that was faster than the
bootstrap in the BDV11. This requires finding - or writing - a bootstrap,
which for later DEC mass storage controllers is not trivial.

YMMV.


TLDR version - probably not!

	Noel