PDP-8 CPU question - test-drb@ccmp.vtda.org

18 Jun 2005

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Subject: PDP-8 CPU question
   From: "Joe R." <rigdonj at cfl.rr.com>
   Date: Sat, 18 Jun 2005 11:10:18 -0400
     To: cctalk at classiccmp.org 
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 >>>>>>>>>Some snippage
done...<<<<<<<<<< 
Hi Joe,

Welcome to the dark side. ;)  The pdp-8 family is the most widely copied 
archetecture, home brewed and sought after. Its a simple machine that
really does put an underscore on less is more.  It's a great programming
experience and it's IO while very strange by microprocessor standards
is one of the simplest yet powerful. If you can find a bare proto board
building IO for the -8 omnibus machines is straightforward (as easy as S100, 
maybe easier!) with DEC docs to lead the way.

RE: docs, there are some on line already as well as PDP-8 software 
including the various "tapes".

...
    Anyway I've been reading the PDP-8a User's
Manual and trying to figure
out what all I have here and I have a question about the PDP-8 CPUs. DEC
offered two different CPUs for the PDP-8, one is the KK8-A which is a
single M8315 card. The other is the KK8-E which consist of an M8300 and
M8310 cards which are the CPU and an M8320 Bus Loads card and a M8330
Timing Generator card. Can someone explain the differences between the two
and why DEC offered two very different CPUs for the 8?  Can I replace
simply the M8300/8310/8320/8330 cards with the M8315? 
4 quad wide boards with over the top connectors vs one or two hex width.
Functionally they are about the same. the 4 board set however is a tiny 
bit faster.  The 4board set is the older PDP-8e/f/m quad style omnibus
while the hex are the newer 8A style.  

...
   I also have a question about the diagnostic paper
tapes. Does anyone have
a ROM emulator or other way to load the diagnostics without using a PT
reader? Yes, I know there are some SIMPLE diagnostics that can be loaded
from the Programmer's Panal but I concerned about the more extensive
diagnostics. In fact, I'm wondering if anyone has tried burning the
diagnostics into PROMs and installing them in place of the Boot Roms on the
M8317 card. 
You could but only to the extent that you'd then have to copy to core
for execution.  ROM on PDP-8 is a tricky thing as subroutine calls 
(JSR instruction) puts the return address at the first word of the 
subroutine.  Hard to do that in ROM {yes DEC did have a rom/ram 
board to fake that.}

The upside is with 16k or more of core (which on power down never forgets.)
you can load core and copy it around.

...
   Can anyone explain the purpose of the small card that
plugs into the back
of the chassis near the power regulators? It's not mentioned in the User's
Manual and I don't even know it's name. Thom says that it controls the
power and the machine will be completely dead if it fails but that's all
that I know about it. 
Ok that must be a PDP-8a box (hex wide).  Its still omnibus 
(as is the 8e/f/m) but allows for the 8a cpu and other wide cards.
That card if memory serves is the sense amp and for the power supply.

There are a number of sites for the PDP-8 family on the net.  
Huge resource.

Start with http://www.pdp8.net and from there you find links to
a bunch of other sites that are also noteworthy.

Good luck, enjoy.

Allison