current object will be marked, or, the system will skip the current object and handle next object.
Only four bits (0, 1, 2, 3) are available now. Each bit has three states. The grayed state means
skip this signal. The unchecked state means that if we want to mark this object then the signal of
this bit must be low level voltage (TTL compatibel). The checked state stands for the high level
voltage (TTL compatibel) needed.
: expect the high level voltage
: expect the low level voltage
: skip the signal
Figure 2-16 Input Port