hdx

from The Free On-line Dictionary of Computing (8 July 2008)

half-duplex
hdx

   <communications> (hdx, from {telegraphy}) 1. A type of
   communication channel using a single circuit which can carry
   data in either direction but not both directions at once.

   Compare: {simplex}, {full-duplex}.

   2. An obsolete term for {local echo}.

   (2001-07-21)