The counter error includes abnormal input error (AI), digital filter
error (FE) and disconnection alarm error (ALM).
Abnormal input error
The abnormal input error is the case when A-phase and B-phase changed at
the same time.
Digital filter error
The digital filter error occurs when high-speed
signal (A phase, B phase) input is faster than digital filter setting.
Disconnection alarm error
The disconnection alarm error is the case of detecting disconnection
for line receiver input. It is only valid when the terminal resistor has
been set to ON.