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The Restoration of the PDP-12 Minicomputer (PAGE 5)

IT'S ALIVE! -- 2007 09 18

PDP-12 First CPU Power Up S M L XL

Well, there you have it. Red LEDs for most of the front panel, a few (not lit on this picture) on the right hand side are green.


Did I mention no smoke? :-)

PDP-12 Damaged Option Cover S M L XL

I did notice that one of the option covers seems to be suffering from flaking paint :-( Strangely (and thankfully!) none of the other covers has the same problem. [A note added later: see Side Panel Restoration for the saga.]

So, what to do now? Well, there are a bunch of things that I could do in the immediate short term:
  • Fix the squeaky top fan; this is very annoying.
  • Investigate the front panel (there are about a dozen LEDs that I have not seen light) -- a working front panel is absolutely key.
  • Do basic instruction tests with the "DO" button (this is way cool; you set up an opcode in the LEFT switches, and hit DO, and the machine executes that opcode!)
  • Investigate why bit 8 is stuck in a lot of places. [DONE 20070919]
  • Investigate the strange counting sequence involving bit 8. [DONE 20070919]
  • Create a new faceplate for the option cover. [IN PROGRESS 20081101]

That should keep me busy for the next few days... :-)

I wanted to do something interesting, so I simply disconnected the AC to the squeaky fan.

Let's look at the strange counting sequence... Executing 7001 (IAC) using the DO button (because I can) yields a counting pattern of:

Bit Number 11 01234567 89 01 (alias) AB -------- -- -- 00000000 00 00 00000000 00 01 00000000 00 10 00000000 00 11 00000000 01 00 00000000 01 01 00000000 01 10 00000000 01 11 00000000 10 00 00000000 10 01 00000000 10 10 00000000 10 11 00000001 01 00 * 00000001 01 01 00000001 01 10 ... 00000001 10 10 00000001 10 11 00000010 01 00 *

Here we see a bogus carry out of bit 8 (alias "A") (at the bolded / starred lines) when bit 8 is 1. Note that there was no carry when "AB" was 01 going to 10...

Swapping the M221 at HJ19 (bits 8 and 9) with the one at HJ20 (bits 10 and 11) gives:

Bit Number 11 0123456789 01 (alias) AB ---------- -- 0000000000 00 0000000000 01 0000000001 00 * 0000000001 01 0000000010 00 * 0000000010 01 0000000011 00 * 0000000011 01 0000000100 00 *

instead. Here we see a bogus carry out of bit 10 (alias "A") every time. Similar symptoms, different bit positions. The "A" and "B" aliases are the relative bits on the card, just so we're comparing apples and apples. It's conceivable that this is a carry problem, and that the symptoms aren't identical because there's always a carry input when the card is at bit positions 10 and 11 (that's how they generate the "increment" function).

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