isolator drive line contains a 1.5 kOhm resistor to limit the
current through the opto-isolator.
Therefore the input pins can be connected directly to a ground
pin to cause an active input.
The voltage level at the external input pin should not be allowed
to go below ground or above +6 V with respect to ground.
Figure 4-16: Non-isolated mode
Figure 4-17: Pin configuration of the General Inputs
connector