### Reference Clock Drivers

Support for most of the commonly available radio and modem reference clocks is included in the default configuration of the NTP daemon for Unix `ntpd`. Individual clocks can be activated by configuration file commands, specifically the `server` and `fudge` commands described in the `ntpd` program manual page. The following discussion presents Information on how to select and configure the device drivers in a running Unix system.

Radio and modem clocks by convention have addresses in the form 127.127.t.u, where t is the clock type and u is a unit number in the range 0-3 used to distinguish multiple instances of clocks of the same type. Most of these clocks require support in the form of a serial port or special bus peripheral, but some can work directly from the audio codec found in some workstations. The particular device is normally specified by adding a soft link `/dev/deviceu` to the particular hardware device involved, where `u` correspond to the unit number above.

Most clock drivers communicate with the reference clock using a serial port, usually at 9600 bps. There are several application program interfaces (API) used in the various Unix and NT systems, most of which can be detected at configuration time. Thus, it is important that the NTP daemon and utilities be compiled on the target system or clone. In some cases special features are available, such as timestamping in the kernel or pulse-per-second (PPS) interface. In most cases these features can be detected at configuration time as well; however, the kernel may have to be recompiled in order for them to work.

The audio drivers are a special case. These include support for the NIST time/frequency stations WWV and WWVH, the Canadian time/frequency station CHU and generic IRIG signals. Currently, support for the Solaris and SunOS audio API is included in the distribution. It is left to the volunteer corps to extend this support to other systems. Further information on hookup, debugging and monitoring is given in the Audio Drivers page.

Some drivers depending on longwave and shortwave radio services need to know the radio propagation time from the transmitter to the receiver, which can amount to some tens of milliseconds. This must be calculated for each specific receiver location and requires the geographic coordinates of both the transmitter and receiver. The transmitter coordinates for various radio services are given in the Stations, Frequencies and Geographic Coordinates page. Receiver coordinates can be obtained or estimated from various sources. The actual calculations are beyond the scope of this document.

Following is a list showing the type and title of each driver currently implemented. The compile-time identifier for each is shown in parentheses. Click on a selected type for specific description and configuration documentation, including the clock address, reference ID, driver ID, device name and serial line speed, and features (line disciplines, etc.). For those drivers without specific documentation, please contact the author listed in the Copyright Notice page.

Type 1 Undisciplined Local Clock (`LOCAL`)
Type 2 Trak 8820 GPS Receiver (`GPS_TRAK`)
Type 3 PSTI/Traconex 1020 WWV/WWVH Receiver (`WWV_PST`)
Type 4 Spectracom WWVB and GPS Receivers (`WWVB_SPEC`)
Type 5 TrueTime GPS/GOES/OMEGA Receivers (`TRUETIME`)
Type 6 IRIG Audio Decoder (`IRIG_AUDIO`)
Type 7 Radio CHU Audio Demodulator/Decoder (`CHU`)
Type 8 Generic Reference Driver (`PARSE`)
Type 9 Magnavox MX4200 GPS Receiver (`GPS_MX4200`)
Type 10 Austron 2200A/2201A GPS Receivers (`GPS_AS2201`)
Type 11 Arbiter 1088A/B GPS Receiver (`GPS_ARBITER`)
Type 12 KSI/Odetics TPRO/S IRIG Interface (`IRIG_TPRO`)
Type 13 Leitch CSD 5300 Master Clock Controller (`ATOM_LEITCH`)
Type 14 EES M201 MSF Receiver (`MSF_EES`)
Type 15 * TrueTime generic receivers
Type 16 Bancomm GPS/IRIG Receiver (`GPS_BANCOMM`)
Type 17 Datum Precision Time System (`GPS_DATUM`)
Type 18 NIST Modem Time Service (`ACTS_NIST`)
Type 19 Heath WWV/WWVH Receiver (`WWV_HEATH`)
Type 20 Generic NMEA GPS Receiver (`NMEA`)
Type 21 TrueTime GPS-VME Interface (`GPS_VME`)
Type 22 PPS Clock Discipline (`PPS`)
Type 23 PTB Modem Time Service (`ACTS_PTB`)
Type 24 USNO Modem Time Service (`ACTS_USNO`)
Type 25 * TrueTime generic receivers
Type 26 Hewlett Packard 58503A GPS Receiver (`GPS_HP`)
Type 27 Arcron MSF Receiver (`MSF_ARCRON`)
Type 28 Shared Memory Driver (`SHM`)
Type 29 Trimble Navigation Palisade GPS (`GPS_PALISADE`)
Type 30 Motorola UT Oncore GPS (`GPS_ONCORE`)
Type 31 Rockwell Jupiter GPS (`GPS_JUPITER`)
Type 34 Ultralink WWVB Receivers
Type 35 Conrad Parallel Port Radio Clock (`PCF`)
Type 36 Radio WWV/H Audio Demodulator/Decoder(`WWV`)
Type 37 Forum Graphic GPS Dating station (`FG`)

* All TrueTime receivers are now supported by one driver, type 5. Types 15 and 25 will be retained only for a limited time and may be reassigned in future.

Additional Information

Mitigation Rules and the `prefer` Keyword
Debugging Hints for Reference Clock Drivers
Line Disciplines and Streams Drivers
Reference Clock Audio Drivers
Pulse-per-second (PPS) Signal Interfacing
How To Write a Reference Clock Driver
The Network Time Protocol (NTP) Distribution

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