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

Rockwell Protocol Interface
RPI

   (RPI) A cost-cutting feature of some modems allowing data
   compression and error correction (e.g. ITU-T V.42bis,
   V.42) to be provided in software instead of hardware.

   Usually an RPI modem comes with RPI-aware software (e.g. the
   low-end RPI models of Supra come with the COMit which
   supports RPI, providing MNP 2,4,5,7, V.42 and V.42bis).  RPI
   is not supported by many commercial packages nor by current
   releases of popular shareware communication programs
   (Telix v3.22 and Telemate v4.12).  ProComm Plus for
   Windows 2.0 will support RPI.

   Currently Rockwell produce two classes of RPI chip set.  The
   original is capable of 2400 bit/s data, 9600 bit/s class
   1-only fax.  The newer one is capable of 14400 bit/s data/fax.
   Currently there are no RPI chipset from Rockwell supporting
   speeds higher than 14400 bit/s.

   (1994-07-01)