This is a short overview for the reference clocks currently supported
by xntp V3. (Ultimate wisdom can be obtained from xntpd/refclock_*.c
this file was derived from that information - unfortunately some comments
in the files tend to get stale - so use with caution)

Refclock address        Type
127.127.0.x             no clock (fails to configure)
127.127.1.x             local clock - use local clock as reference
127.127.2.x             no clock (fails to configure)
127.127.3.x             PSTI 1010/1020 WWV Clock
127.127.4.x             SPECTRACOM WWVB receiver 8170 and Netclock/2
127.127.5.x             Kinimetric Truetime 468-DC GOES receiver
127.127.6.x             IRIG audio decode (Sun & modified BSD audio driver)
127.127.7.x             CHU Timecode (via normal receiver & Bell 103 modem)
127.127.8.x             PARSE (generic driver for a bunch of DCF/GPS clocks
                              can be extended for other clocks too)
       8.0-3           Meinberg PZF535/TCXO
       8.4-7           Meinberg PZF535/OCXO
       8.8-11          Meinberg DCF U/A 31
       8.12-15         ELV DCF7000
       8.16-19         Walter Schmid DCF receiver (Kit)
       8.20-23         Conrad DCF77 receiver module + level converter (Kit)
       8.24-27         TimeBrick (limited availability ask
                                  [email protected])
       8.28-31         Meinberg GPS166
       8.32-35         Trimble SV6 GPS receiver
127.127.9.x             MX4200 GPS receiver
127.127.10.x            Austron 2201A GPS Timing Receiver
127.127.11.x            Kinemetrics Truetime OM-DC OMEGA Receiver
127.127.12.x            KSI/Odetecs TPRO-S IRIG-B / TPRO-SAT GPS
127.127.13.x            Leitch: CSD 5300 Master Clock System Driver
127.127.14.x            MSFEES
127/127.15.x            TrueTime GPS/TM-TMD
127.127.16.x            Bancomm GPS/IRIG Ticktock
127.127.17.x            Datum Programmable Time System