If you can do it, fine. However, even if you just sense one condition, e.g. input[7..0]=reference[7..0], and then start saving the data until the buffer is full, you still have to be able to do that at twice the highest rate at which the unit under test changes its signals.

If he can do that with an 8 MHz AVR, then he can do it with his PC's parallel port and doesn't need any external hardware at all, aside from the cable.

RE