Fabritek MP-12 Loader

Kyle Owen kylevowen at gmail.com
Thu Dec 3 14:41:36 CST 2015

On Thu, Dec 3, 2015 at 2:24 PM, Josh Dersch <derschjo at gmail.com> wrote:
> My understanding is that 6015 does modify the accumulator -- from the
> manual
> (
> http://www.mirrorservice.org/sites/www.bitsavers.org/pdf/fabritek/402-1001-00_MP12refMan_1974.pdf
> )
> Function code 5(8) is interpreted as "...if th the addressed device is
> DONE, the next instruction in sequence is skipped, the contents of the
> device buffer are inclusive OR'd with the accumulator, and the result is
> retained in the accumulator..."
> So 6015 reads the next 8-bit quantity from the paper tape and ORs it onto
> AC.

Yes, it might very well. I wrote up a bit of code to test it out and don't
remember that being the case, but then again, it was late and I was
probably not thinking clearly.

Yeah, I was actually just looking at that again and the jump to 7755 is
> rather odd.  I think that's what stumped me a couple of years back and
> since I never got around to building the interface... I think I just
> assumed a couple of bits had gone south on my PROMs, to be honest.

Well, fear not for bad bits, unless three of these units have failing
PROMs. I think the ones in my colleague's unit are from the 1990s.
Impressive they were using these units up until then (or considerably
after, perhaps!).

That's interesting -- on my MP-12, the sync interface brings out (IIRC)
> three lines: clock, data in, and data out.  I'd be interested to know what
> hardware's hooked up to yours to provide all the goodies you have.

Indeed. All of the functions mentioned are all transferred via the
synchronous interface. I'm not sure how much decoding is done in the big
box versus the little box, but if I were a betting man, I'd say it's mostly
done in the big box. The little box has the seven 7-segment displays, a
paper tape reader, some arbitrary outputs, and some buttons. There are
three removable cards to make it all happen, and the DA-15 connector
supplies 120VAC as well as the synchronous data/clock lines to the little
box. There's an effort to reverse engineer the little box, then move up to
the big box. My colleague has already reverse engineered the 7-segment
driver board. It's my goal to get all of this information collected
together somewhere where it can be made available to all.

I think the size and surprising capability of the Fabritek makes it the
most clone-able of the TTL PDP-8s; much of the functionality of a
Straight-8 for instance, without the bulk. If we went with surface-mount
7400-series, as well as battery-backed SRAM, there's a good chance we could
improve upon the functionality (especially front panel operations!) and add
more memory, switchable ROM space, etc. It'll be a pipe dream for now,


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