Some information on the Motorola Oncore VP used in the HP Z3801A http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/Motorola_Oncore/Oncore_Interface_Brian_Kirby.pdf http://www.realhamradio.com/tac32-interface.htm http://www.realhamradio.com/GPS_Frequency_Standard.htm