i'd probably use a one-shot triggered by a rising edge of A and a one-shot
triggered by a falling edge of B and AND the two one-shot outputs.

the duration of the one-shot pulses depends on what you mean with "at the
same time". in logic circuits there is no "at the same time", there's only
a "within a time x" (sometimes max, sometimes min).

At 11:34 11/16/2001 -0500, M. Adam Davis wrote:
>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.

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