Цитата:
Well, suppose we try to "nudge" a pin with just a few milliamps. If it's an input, the voltage on the pin will will track the voltage we use to "nudge" the pin, as an input draws very little current.
On the other hand, if the pin is an output, the voltage on it will move very little, as (check the datasheet) an output pin can sink or source 20 milliamps with no problem.
To implement this scheme, all we need do is add another 74LS393 counter to give us the 16 possible inputs and take the outputs of the 6 uppermost bits of the 16 and use them to drive the programmable pins, each through a 1 Kohm resistor, which will limit the drive current to no more than 4 milliamps.