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

V.28

    An ITU-T physical interface
   standard for serial data communications equipment,
   formally titled "Electrical Characteristics for Unbalanced
   Double-Current Interchange Circuits".  The combination of V.28
   and V.24 is equivalent to the EIA's RS-232 and uses the
   same 25-pin connectors.

   (2001-04-06)