The OPX interface card is available in two variations:
The CO side used an FXS interface card. It is powered from an express pair, or from a power source at the CO side (a jumper setting is required for this option). The card interfaces with the CO and provides an off-hook circuit, line polarity detection, ring detection, and battery disconnect detection.
The Station side uses an FXO interface card. This card is powered from the backplane supply of -48 Vdc. It provides battery feed with polarity reversal, ringing, and battery disconnect to the station side equipment.