PDP-8/L not incrementing properly

From: O. Sharp <ohh_at_drizzle.com>
Date: Sun Nov 9 22:52:59 2003

Sebastian Br?ckner wrote, in part:

> I am not not yet a PDP professional - I don't have a clue what most of
> the modules are doing. I started to read the Maintenance Manuals but I
> do not quite know where to start. Maybe you can give me a hint:
> [...snipppp]
> CLA and STA work, i can set all bits in AC. IAC (increment) and all the
> rotate instructions however always set AC to 1 completely ignoring the
> previous value.
> Where should I start looking for the problem? What information do I have
> to gather to narrow down the source of the problem?

Do you have Volume 2 of the maintenance manual, or generally speaking a
copy of the schematics? If so, and you know the nomenclature for the
basic gates (AND, OR, NAND, etc.) and flip-flops, you can have a lot of
fun tracing the various signals through the processor and gradually
learn how it processes the various commands in hardware. (Appendix A
of Volume 2 gives a foothold for getting started.) When you're new to
it, tracing how the logic works can make your head hurt at first <g!>;
but as you get more accustomed to it it'll put you on a much more
solid footing for troubleshooting. (It also can become positively
enjoyable. At least, _I_ think so. <g>)

That said... I'd try other commands that affect the AC (TAD would be a
good candidate) to see if they did anything out of the ordinary. Hope-
fully they won't <g>, and that would narrow down the search. If the
problem is confining itself to rotates, it would be worth trying each
of the four rotates (RTL, RAL, RAR, RTR) to see if all of them are
malfunctioning or just a subset of them. The rotate signals are all
generated on the same card - the M617 at E31, if the 8/L and 8/I
share the same backplane layout - so that might be a good starting-
point to look for trouble. (Look for the page "Shift & Carry Gate
Control" in the schematics. The rotate signals go from there to the
"Major Register Gating" page, where they choose the rotate-data to
be gated into the AC. It's really quite simple and elegant how they
take the information from the major-register adders and just select
the version that provides the rotation desired... Okay, okay, I'm
waxing poetic. My apologies.)

Hope this helps!

                     -O.-
Received on Sun Nov 09 2003 - 22:52:59 GMT