Half duplex is communication in a single direction at a time.
One side sends and the other side receives. When the roles reverse, the sending side now receives and the receiving side now sends.
On ASCII terminals used with the D3 system, half duplex is the mode in which characters entered from a keyboard are displayed on the terminal screen and simultaneously transmitted to the D3 host.
Normally, D3 ports are configured for full duplex also called echo-plex. Every character received is echoed to the terminal. The system expects the terminal to also be in full duplex. If a terminal is inadvertently set to half duplex, each character displays twice for each keystroke.