PDP-8 CPU question

[Allison]

>
>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

>>>>>>>>>>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