One of the switches on the front panel is used to determine where the PD-PP is going to get its input from. The input can come from the front panel itself or can come from the parallel port. The choice is done using a 2:1 24-bit multiplexor. Since the parallel port can only output 8 bits at a time, the bits are latched into 3 8-bit latches that correspond to the 16 switches and 8 pushbuttons. Notice that conceptually, a switch button can be either on or off (corresponding to high and low), a push button goes high then low again. Our code took care of that by multiple writing to the port. The 3 control bits are connected to a 3:8 decoder. The decoder activates one of the 3 8-bit latches.