28 Mar
2022
28 Mar
'22
10:27 p.m.
On 2022-Mar-28, at 4:07 PM, Noel Chiappa via cctalk wrote:
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
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.)