Source : Webster's Revised Unabridged Dictionary (1913)

Diplex \Di"plex\, a. [Pref. di- + -plex, as in duplex.] (Teleg.)
   Pertaining to the sending of two messages in the same
   direction at the same time. Diplex and contraplex are the two
   varieties of duplex.

Source : Free On-Line Dictionary of Computing

diplex
     
         (From {telegraphy}) Two simultaneous
        transmissions in one direction.
     
        Compare: {duplex}.
     
        (2000-03-30)