Using the Half-Duplex Contention/Flip-Flop Protocol
The change-direction (CD) indicator is used with both of the following
protocols:
- Half-duplex contention protocol, which is a normal-flow send/receive mode in which either half-session can send normal-flow requests at the beginning of the session or after sending or receiving the last request of a chain.
- Half-duplex flip-flop protocol, which is a normal-flow send/receive mode in which one half-session sets the CD indicator in the response header (RH) on an end-of-chain to enable the other half-session to begin sending.
A CD indicator tells the receiver that transmitting can begin.
For example, if the SLU initiates a transaction, the SLU begins by sending the messages that completely describe the transaction. On the last message, the SLU sets the CD indicator to tell the PLU that it can begin transmitting a reply. If the PLU needs additional information to complete the transaction, it sends an inquiry and sets the CD indicator. The dialog proceeds in this half-duplex mode until the transaction is complete. During a half-duplex dialog, the SLU can use the SIG command to tell the PLU to stop sending data and to change the direction of the data flow.