PDP 11/24 - A Step Backwards

[Noel Chiappa]

Finally found time to get to this one...

    > From: Rob Jarratt

    > However, there is a puzzle. On the CPU I found that the track from the
    > pull up resistor to E70 has been cut.

I don't know about the "pull up resistor" part, but I have several KDF11-U's,
and _all_ of them have the trace on the bottom of the card connected to pin 1
of E70 cut, in the exact same place (about 1/8" from the pin). This suggests
that it's not a local mod (as you suggest below), but an 'official' DEC ECO.

    > This would suggest that E70 pin 2 is floating, which I think means that
    > K2 BUF ACLO H is also floating

The input (pin 1) will be floating, but not the output (pin 2); TTL doesn't
work that way, I think. I may have this wrong, but I think open TTL inputs
float high, so BUF ACLO should be low. I looked, and I don't see any other
traces (e.g. on the top side) going to pin 1. So I'm a bit puzzled that DEC
allowed that input to float, as open inputs can lead to erratic operaton;
they're usually tied high, or to ground.

    > K2 BUS ACLO L however has been patched to E52 pin 4, which is the
    > output of a gate on sheet K6. Can't say I understand why.

Me neither; that's an unused (on the prints) driver in the DS8641 (center
bottom of the page) - although that gate seems to have been fully wired up
(wires to pins 4, 5 and 6) as part of the ECO.

There is an ECO list on pg. 167 of the -11/24 prints (2 pages after K14),
but I don't see an E52 in it anywhere.


The puzzle here, if E70p1 is cut, and the output is low, is why the CPU clock
isn't running. The -15V on BUS ACLO shouldn't have taken out any other
semiconductors; it's not attached to anything else.

(It will have run C6, on the lower right of the card, the wrong way, but i) I
think that's a non-polarized item - and 100V rated, per the prints, and ii) I
don't think that goes anywhere else, even if it's not.)

So what's stopping the clock from running, then?

	Noel