A difference between two technologies is described here:
An FXS interface provides power (battery) to FXO equipment and generates ring signals
(e.g. The phone jack on the wall)
An FXO interface receives power (battery) and receives ring signals
(e.g. The phone line port on a telephone set)
Always check the sense of the device whether it is labeled FXS or FXO, as some vendors get it backwards and your device will provide battery when you expect it to behave as a station and receive battery. Be careful!
