0000: signal is always invalid
0001: input positive signal from
SI1
0002: input positive signal from
SI2
0003: input positive signal from
SI3
0004: input positive signal from
SI4
0010: signal is always valid
0011: input negative signal from
SI1
0012: input negative signal from
SI2
0013: input negative signal from
SI3
0014: input negative signal from
SI4