The Free On-line Dictionary of Computing (30 December 2018):

half-duplex
hdx

    (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)