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The simple explanation of full-duplex mode is that it is like the telephone, where both parties can talk at the same time. In contrast, a pair of walkie-talkies operates in half-duplex mode because only one party can talk at a time.

A full-duplex data transmission is one in which data flows in both directions at the same time. Two separate channels are required for full duplex-either two separate wire pairs or two multiplexed channels. In contrast, a half-duplex connection allows data to flow in only one direction at a time.

A typical serial connection (RS-232) between a computer and some other device like a modem or printer is pictured in Figure F-19. Note that this is the minimal wire configuration, and that the transmit and receive lines share the ground wire. An RS-232 cable will typically consist of additional wire for transmitting control signals.