On the circuit I built SLOW just permanently connects the 2 and 6 PINS together so the system runs as a 1Hz ASTABLE. When not in SLOW mode the STEP button just allows one or more astable vibrations while held down.
When you pointed out why that did not work I realised that SLOW is just a permanent short of STEP. See the amended diagram above - I changed the EARTH connection to be a wire to the top of the CAP on pin 6. This now works as I would expect generating a steady stream on SLOW or a single pulse using STEP when SLOW is nit selected.
The RUN/HALT side of the 7474 makes R (the HALTFLAG) to be dependent on the D7 value when NADS is asserted.
I.e.
Code:
RUN/HALT NAND1 NAND2 HALTFLAG
D7 H/L /NADS H on NADS so halts each cycle
N/C H /NADS H/L depends on the 555 pulse generator
GND L /NADS H - so no halts on the processor