Half-duplex protocols do not allow LUs to exchange information concurrently. Only one LU at a time can send or receive data. One LU is designated as the sender and the other LU as the receiver. The receiver can request permission to become the sender and can take over that role under certain error conditions, but in general, the receiver cannot send data until the sender surrenders that right.

Half-duplex conversations are well-suited to tightly coupled transactions, such as sync point or inquiry and reply transactions.