
\S simplex:  duplex:  half-duplex \S

In order for you to communicate with the computer system, information must
be passed back and forth between your terminal and the system.  Data
transmission lines can pass information in one direction or both
directions.  Simplex lines transmit information in one direction only.  You
must have two simplex lines in order to have communication between your
terminal and the system and back again.  Half-duplex lines can transmit
information in both directions, but only in one direction at a time.  You
only need one half-duplex line between your terminal and the system.  To
transmit information in both directions simultaneously, you can have two
half-duplex lines transmitting in opposite directions.  Full-duplex lines
transmit information in two directions simultaneously.  You only need one
full-duplex line to have simultaneous communication between your terminal
and the computer system.  UNIX systems expect that data transmissions can
occur in both directions at once.

