Cheap board guy

[Noel Chiappa]

    > From: Mark G. Thomas

    > 4x M8192 - KDJ11 (AA or AB?) -- two work, two fail POST

And alas, we don't seem to have any prints for that card (although we do have
what amounts to a tech manual, so maybe we can create a set, with a certain
amount of tracing with an ohmmeter), so at the moment, at least, fixing them
isn't so easy.


    > I was hoping I could boot XXDP or RT11 from an RX33/RQDX3. The
    > RX33/RQDX3 works in my 11/53

Well, that's a very good sign...

    > Since none of this has a bootstrap, I run the bootstrap from ROM
    > provided by a Dilog SCSI card here, but typing "DU" or "DU0" at the
    > prompt spins the floppy ever-so-briefly, then kicks out an error about
    > no boot media found. Suggestions? Maybe I should try other bootstrap?

Definitely; the code on the Dilog card might not support that controller
properly (even though it seems to recognize "DU").


    > I'm a little confused about what should work and what should not work,
    > with just the 18 bit qbus.

If you have less than 256KB of memory (so Q22 processors won't wrap around,
when trying to size memory, and think there's memory there above 256K -
although Q18 memory probably will stop responding at 248KB, anyway), pretty
much everything _should_ work, I would think. The high address lines being
put out by the processor, DMA devices, etc should just have no effect.
Although the details get tricky...

E.g. if you don't have BDAL18-21 for a Q22 memory card, what will its bus
interface do when faced with those lines, which aren't driven in any way -
_especially_ not pulled up by terminators? Some DEC memory cards (e.g.
MSV11-L, M8059) have jumpers to run in either Q18 or Q22 mode, to work around
this.

    > Do I need to wire wrap the additional address lines to be able to do
    > anything with these KDJ11 CPUs?

No, if you have less than 256KB of memory, the high bits should just be
ignored (I think - I haven't actually tried this, to be absolutely sure).

    > Does anyone have good instructions for this modification -- I'll
    > probably want to do it. Do I just add the additional address lines, or
    > are there other considerations?

I have modified an H9273 backplane (Q18) to H9276 (Q22), and it works fine;
all I did was bus all the BDAL18-21 pins together: pretty easy, as it's a
Q/CD backplane, not a Q/Q - just run a wire down, and solder it to each pin
as it goes (those backplanes don't have the pins stick out far enough for
wire wrap).

Q/Q will be only slightly more complicated (since you have to bus down one
side, then run the signals up and across to the top of the other side, and
then bus them in turn - do it this way, to avoid creating a branch in the bus
which will encourage reflections); I have done this mod on a Q18/Q18
backplane (a Sigma Q18/Q18), but have yet to actually try it.

The only complication might come with termination/pull-ups. Not all
backplanes have these built in (e.g. the DEC H9273/H9276 don't). But you
might not need them - IIRC both the 11/23 and 11/73 have on-board termination
which will pull the lines up. But if you _do_ need them... best bet, unless
you want to start soldering resistors to the backplane, is a terminator board
with Q22 pullups. That's a whole separate discussion which I will leave for
the moment... :-)

	Noel