Quoted from Quench:
If the CPU locks up, there is potential for the displays to suffer from digit burn due to a single digit at each display being stuck bright on. The oneshot delay prevents this from occurring by making sure the displays stay blanked when no active blanking signal is being generated.
Ah, so it's actually the inverse. It's a one shot that makes it so the blanking can only ever be off for so long.
Quoted from jimy_speedt:
Why is the blanking necessary to turn on the transistor ? In that case, how does this blanking signal work, thanks again
To be clear, this is only in reference to williams machines, which don't have a zero crossing, and I assume isn't relevant to your original questioning as it's completely unrelated to the way a bally board works.
In a williams, the goal of their 'blanking' is that, if the CPU isn't running, then all outputs should be disabled (solenoids, lamps, displays), so that nothing locks on and burns up. Their blanking signal is generated from a periodic keep-alive the MPU sends while it's operating. If that keep-alive is ever missing, the blanking signal activates via a timer and disables evreything.