00: Set the signal to be 
invalid all the time. 
01: Input positive signal 
from SI1 terminal. 
02: Input positive signal 
from SI2 terminal. 
03: Input positive signal 
from SI3 terminal. 
04: Input positive signal 
from SI4 terminal. 
10: Set the signal to always 
be "valid". 
11: Inverse signal is input 
from SI1 terminal. 
12: Inverse signal is input