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

Telocator Alphanumeric Protocol
IXO
TAP

    (TAP, or "IXO", "PET") A protocol
   for submitting requests to a pager service.  IXO/TAP is an
   ASCII-based, half-duplex protocol that allows the
   submission of a numeric or alphanumeric message.

   Examples, protocol description, clarifications
   (ftp://mirror.lcs.mit.edu/telecom-archives/technical).

   See also RFC 1568.

   (1996-04-07)