PDP 11/24 - A Step Backwards

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Mar 28 18:07:11 CDT 2022

    > From: "Rob Jarratt"

    > Thanks for the lengthy reply.

Glad to help - or try to.

    > As an aside I have also been trying to find a fault on a Pro 350 which
    > uses the same CPU chipset. I have a pinout but no datasheet.

There doesn't seem to be as lot on the F-11 set. I looked in the DEC
semiconductor handbook, and  it's not there - although perhaps it
had been dropped by the one I looked at (which was mostly uVAX stuff)
as obsolete?

If you look in the KDF11-A and KDF11-U Tech Manuals, there is a chapter on
the F-11 chip set, as used in those cards, and that's better than nothing -
it talks a fair amount about the low level details of how the various chips
operate and interact, etc.

    > I don't think the CPU is working at all. The reason being that there is
    > absolutely no LED activity. Including an LED that is supposed to indicate
    > a clock.

Looking at the KDF11-U prints, I finally found that LED (it's pretty low
level - I was worried that it might be a bit in a register, and driven by
software, but it's not, it's actually driven directly by the the CPU's
internal clock signal; it's on page K1 of the prints, 'Clock, State Decode',
in the very upper left corner). (The source of the CPU's internal clock is
just an RC circuit, in the lower middle of that page, and the trim pot that's
part of it - along the upper edge of the board - can be adjusted to set the
clock speed 'properly', per the note at the back of the prints on the page
which lists the configuration switches. The 2MHz crystal along the upper edge
drives the baud rate generator.)

    > Having hopefully eliminated all the power voltages it left me wondering
    > if there was a fault on the CPU or in the PSU.

If ODT isn't running, the problem is almost certainly in one of those two

    > Having had activity on those LEDs recently it seems most likely that it
    > will be the PSU, particularly as *something* in there blew up.

I'm not so sure. Those boards mostly just want +5V; looking a bit more, the
CPU chips do seem to use +12V. The RS232 drivers will use +/-12V.

I'm afraid that if i) it used to show activity, but no longer does so, and
ii) the main voltages (+5V, +12V) look good, something on the CPU card has
failed. But it will take a bit of digging to i) verify that, and ii) identify
the fault.

    > The only signal that I can identify that seems likely to have this kind
    > of effect is LTC, but I don't know enough about LTC to know if its
    > absence could cause the CPU board not to work at all, although I see
    > above that you think it unlikely.

I have yet to trace how the LTC signal is used in the KDF11-U, but on the
KDF11-A, it not being there is a total NOP. (In fact, in the BA11-N/S type
mounting boxes, there's a 'Clock Enable' switch on the front panel which turns
the LTC signal off - and the machine runs fine with it off - except for the
TOD clock not ticking.) That clock signal - totally different from the main
CPU clock - is only used as an input to what is in effect a peripheral.

    > I had a console attached. There is nothing on the console. When I first
    > got the machine I did get output on the console.

Not a good sign, alas.

If you have a scope of some kind, and want to keep poking, I'd recommend that
you start by seeing if the clock is running, and move forward from there. The
KDF11-U prints are online, as is the KDF11-U Tech Manual. Skim the chapter on
the CPU (4, I think), and then grovel around in the prints for a bit. Don't
try to totally understand it all, just skim through it, so you know roughly
where most things are.


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