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