VT100.net: Digital VT102 User Guide

Table 6-3 Modem Control Feature Selections
Selection | Description | Usual Application
--- | --- | ---
FDX A | Full-duplex with no EIA modem control (data leads only) | Full-duplex communication with a null modem (direct) connection to the computer or with a modem that does not use modem control signals.
FDX B | Full-duplex with EIA modem control | Full-duplex communication with a modem that uses modem control signals.
FDX C | Asymmetric full-duplex with EIA modem control | Full-duplex communication with a half-duplex modem using the secondary channel.
HDX A | Half-duplex supervisory mode | Half-duplex communication with the secondary channel control lines indicating when to perform line turnarounds.
HDX B | Half-duplex coded control | Half-duplex communication with control characters indicating when to perform line turnarounds.