Source : Free On-Line Dictionary of Computing

simplex printer
     
         A term applied by Western Union Telegraph
        Company to {teletypewriters} that are not part of a
        {multiplex} system.  They usually provided for alternate
        transmission in both directions.  If working {simplex} or
        {half-duplex}, what was keyed in at the keyboard would be
        typed out at the printing portion.  If working {full-duplex},
        sending would be blind as the printing portion was being used
        only for reception.
     
        (2000-04-02)