PDP 11/24 - A Step Backwards

Brent Hilpert bhilpert at shaw.ca
Tue Mar 29 00:27:19 CDT 2022

On 2022-Mar-28, at 4:07 PM, Noel Chiappa via cctalk wrote:
>> 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.)

But the LED and CPU clock are not driven directly by that RC oscillator - there's a bunch of logic in-between the oscillator and the LED / CPU clock.

[RC clock] => K1 OSC H/L
	--> [4-bit counter w parallel load] => K1 MCLK H/L
		--> LED
		--> [driver] => K1 CHIP CLK H (fonz CPU clock)

The 4-bit counter looks to be generating some additional phases, but it's also controlled by a bunch of other signals.
One of those signals is K6 BUF DCLO L which can hold the counter in reset, i.e. disable the Master/CPU clock (and LED).
K6 BUF DCLO L is derived on-board from K2 P FAIL H, which is derived from K2 BUS ACLO L which is input from BF1-in-funky-hex-box which I presume is a bus connector pin.

Tony mentioned checking ACLO.
Even if ACLO is good, there's a whack of logic on the CPU board - including two monostables - just to get from ACLO to DCLO to enable the CPU clock, as well as those other signals controlling the phase counter.

ref: pdfPg.152,etc of http://www.bitsavers.org/pdf/dec/pdp11/1124/MP01018_1124schem_Aug80.pdf

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