All of these test are done. Conclusion: if I remove the wire to SC1 and
reset the 4013, it works fine, just as you described. In fact, I spent a
few hours sending pulses from an Arduino (microsecond ones) to simulate
SC1. I even managed to see the pulse from *Q* on the oscilloscope. But once
the wire between the 4013 and the SC1 pin is used, nothing works. I even
tried two different 1802s and they work fine (when the DMA IN pin is set
low, SC1 instantly goes high). The only possible reason I can think of
(since both the 1802 and 4013 works perfectly) is that one of the diodes
are either broken or placed the wrong way around. An other thing is that I
use a SPST button (input) instead of the SPDT. It still works (for some
weird reason no SPDT buttons are rare and expensive in the UK). I'll check
this tomorrow. The other mystery is that why the 4023 never sets the HP
displays to enable mode. The displays are stuck (latched) and even though
they work perfectly, the NAND gate (4023) never sets the enable pins low. I
wonder if that has to anything with the previous problem? Anyway, at least
I know where the problem is, even though I don't know what it is. It's
certainly a good start! Thanks for all of your effort and indispensable
help!
Greg
Debugging trial: remove the connection from SC1 to the diode at the 4013
reset (10).
Now, when the input switch is released, nQ & nDMAIN should go low and STAY
low as there is nothing to reset the 4013 (until you flip the load mode
switch).
It follows SC1 should go high and stay high or repeatedly pulse high as
the 1802 is being held in the DMA state (which I think you already tested
by forcing nDMAIN low).