Clear-Com HX System Frames - Manual (Page 50)

Non-isolated mode
To cause an input to detect an active signal in non-isolated mode, the
current must flow from the relevant input pin.
The external device should draw no current to cause an inactive input and
at least 5 mA to cause an active input. The opto-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 18: Non-isolated mode
Figure 19: Pin configuration of the General Inputs connector
50 Eclipse HX-Delta User Guide