INT FD - TFPCX - INSTALLATION CHECK                                             
	AH = function (also see separate entries below)
Program: TFPCX is an interface between modem and terminal program for packet-
	  radio communications
InstallCheck:	test for the string "N5NX" three bytes beyond the interrupt
	  handler
Range:	INT FD is the default, but may be changed, so the full installation
	 check consists of scanning for the signature
Note:	TFPCX returns AX=FFFFh on any unsupported function call
SeeAlso: AH=01h,AH=03h,AH=FEh