# Half duplex

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.

## See also

- [echo command](https://d3codex.com/tcl/echo-command/)
- [echo statement](https://d3codex.com/pickbasic-flashbasic/echo-statement/)
- [Full duplex](https://d3codex.com/definitions/full-duplex/)

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Source: https://d3codex.com/definitions/half-duplex/ - part of the D3Codex reference.