A half-duplex circuit allows two-way communication, but only in one direction at a time; that is, the two parties to the connection must take turns transmitting and receiving data. The remote system waits for your system to complete its transmission before it can transmit back to your system. Some versions of this protocol, for better interaction between systems, provide a “turnaround character,” which tells the other system: “I am done; your turn now.”
Half-duplex connections are more common over electrical links. Since electricity won’t flow unless you have a complete loop of wire, you need two pieces of wire between the two systems to form the loop. The first wire is used to transmit; the second wire is referred to as a common ground. Thus, the flow of electricity can be reversed over the transmitting wire, thereby reversing the path of communication. Electricity cannot flow in both directions simultaneously, so the link is half-duplex.