(no subject)

Ethan Dicks ethan.dicks at gmail.com
Mon Jul 30 23:08:39 CDT 2007


On 7/30/07, woodelf <bfranchuk at jetnet.ab.ca> wrote:
> Devon Stopps wrote:
> > I believe you're thinking of the book "The Art of Digital
> > Electronics"...I don't remember who wrote it.

Close... "The Art of Digital Design" by Prosser and Winkel (I'm home
and staring at the book).

> >  I too have been
> > considering building an 8/i compatible, and wouldn't mind some insight.

Well... owning an -8/i, my only complaint of it is how
difficult/expensive it is to expand the memory.  Peripherals aren't
bad, but it takes 8K internally (I think I have all the parts I need
to take mine from 4K to 8K), but the external 24K is rare and
expensive.

I own one or more of nearly every style of -8 (lacking -8/f, -8/m,
WT78), but even so, a TTL and/or an AMD29xx-based -8 is still
fascinating to me.  I already happen to have a small stack of '181
ALUs, and need to locate a few '83s to keep my pile of TTL -8s
working.  The F20 uses the '183 adder, so hopefully I can find some at
the same place that has '83s.

>From reading through the F20 design and, obviously, using a pair of
modern "by 8" SRAMs instead of the now-ancient 6147 4Kx1 SRAM, I think
I already have 95% of the needed parts in my parts bins.  FWIW, I
don't see as much value in building a TTL -8 design that doesn't have
a memory extension register, since I could just build an IM6100 board
if I wanted a 4K TTY-only PDP-8.  I'd want to expand the design to
have at least 8K, so why not go on to 32K.  At least with 8K, OS/8 is
possible, but why implement one memory extension bit when 3 bits just
isn't that much harder.

> Switches are not cheep!!!

I disagree.  I have tubs of C&K switches with various sizes and shapes
of paddles.  What isn't cheap is *new* switches with *particular*
toggles/handles.  Those are easily $5 each.  Surplus "match only
themselves" can be had for $0.50 at Hamfests or places like BG Micro.

I've built Elf toggle panels for $5.  A PDP-8 needn't cost more than
$15 unless you want very specific switches to match an existing
design.

-ethan



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