 Communication may be "simplex" (in one direction only, with no provision for the receiving device to send information back to the transmitting device), "full duplex" (both devices send and receive at the same time) or "half duplex" (devices take turns transmitting and receiving).The idle, no data state is high-voltage, or powered.This is a historic legacy from telegraphy, in which the line is held high to show that the line and transmitter are not damaged.