Is The M9312 Boot Module Essential?

[Noel Chiappa]

So, I've made what I think is a significant discovery about the -11/34:

    > 1B _is_ necessary, but can be provided anywhere on the bus; most
    > UNIBUS/QBUS CPU [pullups] have it built in

I was wrong. Neither the KD11-E nor the KD11-EA has built-in termination and
pull-ups (those are both done with one set of components). I haven't yet
checked, but it may be the only PDP-11 CPU of which that is true

Without _something_ doing the latter of the above, the UNIBUS won't function
at all. (The UNIBUS signal lines mostly operate as negative-logic wired-OR;
the pull-ups float it high for '0', and any board pulls it low to send a '1'.
No pull-ups, then..)

This is almost certainly the reason that the manual calls for the use of
either an M9301 ROM or M9312 ROM (which include bus termination) at the start
of their UNIBUS, in slot 3 or 4 of the CPU's backplane.

(The M7859 of the KY11-LB doesn't have pull-ups either; so in a system with a
set of KD11-E/EA cards, and a KY11-B, and nothing else, the KY11-B won't be
able to examine UNIBUS locations - even though in a system with _just_ a
KY11-B, and one of M9301/M9302/M9312, and NO KD11-E/EA, the KY11-B _can_
do UNIBUS operations.)

A system with just an M9302, and no M9301/M9312, will _probably_ work, even
though the UNIBUS is only terminated at one end (see my previous post, about
QBUS termination on one end only); the M9302 will provide the pull-ups needed
for the UNIBUS to function (above).


I have also made a number of interesting discoveries about the SACK
turnaround; I'll put them in a reply to Fritz's message.

	Noel