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

EIA-423
RS-423

    (Formerly "RS-423") An EIA
   serial line standard which specifies single ended
   communication.  The mechanical connections for this interface
   are specified by EIA-449.  Although it was originally
   intented as a successor of EIA-232 it is not widely used.
   The EIA-232 standard has its limits at 20kbps and 1.5m.
   EIA-423 can have a cable lenght of 1200m, and achieve a data
   rate of 100Kbps.  When no data is being transmitted, the
   serial line is at a logical zero (+3 to +15 Volts).  A logical
   one is represented as a signal level of -15 to -3 Volts.  In
   practise, one often finds signals which switch between
   nominally +4.5 and +0.5 Volts.  Such signals are large by
   modern standards, and because the impedance of the circuits is
   relatively high, the allowable bit rate is modest.  The data
   is preceded by a start bit which is always a logical
   one.  There may be seven or eight bits of data, possibly
   followed by an even or odd parity bit and one or two stop
   bits.  A "break" condition is a continuous logical one on the
   line which is what would be observed if nothing was connected.

   Comparing EIA-422, 423, 449 to RS-232-C
   (http://rad.com/networks/1995/rs232/rs449.htm).  Details
   on RS-232, 422, 423 and 485
   (http://rs485.com/rs485spec.html).

   (2002-10-05)