You can make a circuit which will output a pulse of very short duration
when A changes L-H.  You can then use this pulse to open a gate which
allows another circuit to make a pulse if B is going low during the
short pulse from A.  If A never gives the pulse, then B won't trigger.
 How closely the signals must match would be determined by how short the
pulse from A is.

It's not the optimal solution, but it is a solution.

-Adam

James Williams wrote:

>A  _____/^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>B  ^^^^^\________/^^^^^^^\_____________
>
>I can't seem to come up with some DISCRETE logic that will detect when A
>goes from low to high at the SAME time as B goes from high to low.
>
>Any ideas, without a pic.  Sorry pics are not fast enough.  I know that I
>need to store previous state, then latch current state of either rising edge
>of A or falling edge of B, and then compare the two.  However i am not
>curtain of how to do it.  I mean if A is used to clock B, then B would be at
>2.5v when A clks, which gives false indication.
>
>Any ideas greately appriciated.
>
>Thanks,
>
>James
>
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