Is The M9312 Boot Module Essential?

[Rob Jarratt]

I saw this reply later than the previous one. It confirms that I don't
*need* it for booting, but it would be useful.

I suspect some of the other cards that were in the machine might do the
necessary termination stuff.

Thanks

Rob

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Noel Chiappa via
> cctalk
> Sent: 19 February 2022 09:18
> To: cctalk at classiccmp.org
> Cc: jnc at mercury.lcs.mit.edu
> Subject: Re: Is The M9312 Boot Module Essential?
> 
>     > 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