Almost OT: Pushbutton switch latching

[der Mouse]

> 8 74xx30's (8 input NAND) with each output feeding one of the inputs
> of the other stages.  The remaining input of each gate is connected
> to a pushbutton [to ground].  Basically an 8-stable
> flip-flap-floop...-flop.

Nice idea, but it doesn't work.  Draw out three NANDs cross-coupled as
you described.  Mark the logic states for a stable state with one
output low.  Now consider what happens when each of the inputs goes
low.  The only one that does anything is the one feeding the gate whose
output is low.

To put it another way, an input going low doesn't pull the
distinguished state to that gate; it pushes it off that gate.  With two
two-input NANDs, there's only one place for it to go, so it's
well-defined; with more, it could stabilize anywhere (or,
theoretically, oscillate forever if the gates have identical
propagation delays).

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