203 lines
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HTML
203 lines
12 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<title>Reference Clock Drivers</title>
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<h3>Reference Clock Drivers</h3>
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<img align="left" src="pic/stack1a.jpg" alt="gif">
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Master Time Facility at the <a
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href="http://www.eecis.udel.edu/%7Emills/lab.htm"> UDel Internet Research
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Laboratory</a>: <br clear="left">
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<hr>
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<p>Support for most of the commonly available radio and modem reference clocks
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is included in the default configuration of the NTP daemon for Unix <tt>ntpd</tt>.
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Individual clocks can be activated by configuration file commands, specifically
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the <tt> server</tt> and <tt>fudge</tt> commands described in the <a
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href="ntpd.htm"><tt>ntpd</tt> program manual page</a>. The following discussion
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presents Information on how to select and configure the device drivers in
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a running Unix system.</p>
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<p>Many radio reference clocks can be set to display local time as adjusted
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for timezone and daylight saving mode. For use with NTP the clock must be
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set for Coordinated Universal Time (UTC) only. Ordinarily, these adjustments
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are performed by the kernel, so the fact that the clock runs on UTC will
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be transparent to the user.</p>
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<p>Radio and modem clocks by convention have addresses in the form 127.127.<i>t.u</i>,
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where <i>t</i> is the clock type and <i>u</i> is a unit number in the range
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0-3 used to distinguish multiple instances of clocks of the same type. Most
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of these clocks require support in the form of a serial port or special bus
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peripheral, but some can work directly from the audio codec found in some
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workstations. The particular device is normally specified by adding a soft
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link <tt>/dev/device<i>u</i></tt> to the particular hardware device involved,
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where <i><tt>u</tt></i> correspond to the unit number above.</p>
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<p>Most clock drivers communicate with the reference clock using a serial
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port, usually at 9600 bps. There are several application program interfaces
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(API) used in the various Unix and NT systems, most of which can be detected
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at configuration time. Thus, it is important that the NTP daemon and utilities
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be compiled on the target system or clone. In some cases special features
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are available, such as timestamping in the kernel or pulse-per-second (PPS)
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interface. In most cases these features can be detected at configuration
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time as well; however, the kernel may have to be recompiled in order for
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them to work.</p>
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<p>The audio drivers are a special case. These include support for the NIST
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time/frequency stations WWV and WWVH, the Canadian time/frequency station
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CHU and generic IRIG signals. Currently, support for the Solaris and SunOS
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audio API is included in the distribution. It is left to the volunteer corps
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to extend this support to other systems. Further information on hookup, debugging
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and monitoring is given in the <a href="audio.htm">Audio Drivers</a> page.</p>
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<p>The local clock driver is also a special case. A server configured with
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this driver can operate as a primary server to synchronize other clients
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when no other external synchronization sources are available. If the server
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is connected directly or indirectly to the public Internet, there is some
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danger that it can adversely affect the operation of unrelated clients. Carefully
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read the <a href="driver1.htm">Undisciplined Local Clock</a> page and respect
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the stratum limit.</p>
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<p>The local clock driver also supports an external synchronization source
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such as a high resolution counter disciplined by a GPS receiver, for example.
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Further information is on the <a href="extern.htm">External Clock Discipline
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and the Local Clock Driver</a> page.</p>
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<h4>Driver Calibration</h4>
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<p>Some drivers depending on longwave and shortwave radio services need to
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know the radio propagation time from the transmitter to the receiver, which
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can amount to some tens of milliseconds. This must be calculated for each
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specific receiver location and requires the geographic coordinates of both
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the transmitter and receiver. The transmitter coordinates for various radio
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services are given in the <a href="qth.htm">Stations, Frequencies and Geographic
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Coordinates</a> page. Receiver coordinates can be obtained or estimated from
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various sources. The actual calculations are beyond the scope of this document.</p>
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<p>When more than one clock driver is supported, it is often the case that
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each shows small systematic offset differences relative to the rest. To reduce
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the effects of jitter when switching from one driver to the another, it is
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useful to calibrate the drivers to a common ensemble offset. The <tt>enable
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calibrate</tt> configuration command in the <a href="miscopt.htm">Miscellaneous
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Options</a> page is useful for this purpose. The calibration function can
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also be enabled and disabled using the <tt>ntpdc</tt> program utility.</p>
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<p>Most clock drivers use the <tt>time1</tt> value specified in the <tt>fudge</tt>
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configuration command to provide the calibration correction when this cannot
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be provided by the clock or interface. When the calibration function is enabled,
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the <tt>time1</tt> value is automatically adjusted to match the offset of
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the remote server or local clock driver selected for synchronization. Ordinarily,
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the NTP selection algorithm chooses the best from among all sources, usually
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the best radio clock determined on the basis of stratum, synchronization
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distance and jitter. The calibration function adjusts the <tt>time1</tt>
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values for all clock drivers except this source so that their indicated offsets
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tend to zero. If the selected source is the kernel PPS discipline, the <tt>fudge
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time1</tt> values for all clock drivers are adjusted.</p>
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<p>The adjustment function is an exponential average designed to improve
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accuracy, so the function takes some time to converge. The recommended procedure
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is to enable the function, let it run for an hour or so, then edit the configuration
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file using the <tt> time1</tt> values displayed by the <tt>ntpq</tt> utility
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and <tt> clockvar</tt> command. Finally, disable the calibration function
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to avoid possible future disruptions due to misbehaving clocks or drivers.</p>
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<h4>Performance Enhancements</h4>
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<p>In general, performance can be improved, especially when more than one
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clock driver is supported, to use the prefer peer function described in the
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<a href="prefer.htm">Mitigation Rules and the <tt> prefer</tt> Keyword</a>
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page. The prefer peer is ordinarily designated the remote peer or local clock
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driver which provides the best quality time. All other things equal, only
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the prefer peer source is used to discipline the system clock and jitter-producing
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"clockhopping" between sources is avoided. This is valuable when more than
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one clock driver is present and especially valuable when the PPS clock driver
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(type 22) is used. Support for PPS signals is summarized in the <a
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href="pps.htm">Pulse-per-second (PPS) Signal Interfacing</a> page.</p>
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<p>Where the highest performance is required, generally better than one millisecond,
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additional hardware and/or software functions may be required. Kernel modifications
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for precision time are described in the <a href="kern.htm">A Kernel Model
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for Precision Timekeeping</a> page. Special line discipline and streams modules
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for use in capturing precision timestamps are described in the <a
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href="ldisc.htm">Line Disciplines and Streams Drivers</a> page.</p>
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<h4>Comprehensive List of Clock Drivers</h4>
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<p>Following is a list showing the type and title of each driver currently
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implemented. The compile-time identifier for each is shown in parentheses.
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Click on a selected type for specific description and configuration documentation,
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including the clock address, reference ID, driver ID, device name and serial
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line speed, and features (line disciplines, etc.). For those drivers without
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specific documentation, please contact the author listed in the <a
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href="copyright.htm">Copyright Notice</a> page.</p>
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<p><a href="driver1.htm">Type 1</a> Undisciplined Local Clock (<tt>LOCAL</tt>)<br>
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<a href="driver2.htm">Type 2</a> Trak 8820 GPS Receiver (<tt>GPS_TRAK</tt>)<br>
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<a href="driver3.htm">Type 3</a> PSTI/Traconex 1020 WWV/WWVH Receiver (<tt>WWV_PST</tt>)<br>
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<a href="driver4.htm">Type 4</a> Spectracom WWVB and GPS Receivers (<tt>WWVB_SPEC</tt>)<br>
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<a href="driver5.htm">Type 5</a> TrueTime GPS/GOES/OMEGA Receivers (<tt>TRUETIME</tt>)<br>
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<a href="driver6.htm">Type 6</a> IRIG Audio Decoder (<tt>IRIG_AUDIO</tt>)<br>
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<a href="driver7.htm">Type 7</a> Radio CHU Audio Demodulator/Decoder (<tt>CHU</tt>)<br>
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<a href="driver8.htm">Type 8</a> Generic Reference Driver (<tt>PARSE</tt>)<br>
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<a href="driver9.htm">Type 9</a> Magnavox MX4200 GPS Receiver (<tt>GPS_MX4200</tt>)<br>
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<a href="driver10.htm">Type 10</a> Austron 2200A/2201A GPS Receivers (<tt>GPS_AS2201</tt>)<br>
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<a href="driver11.htm">Type 11</a> Arbiter 1088A/B GPS Receiver (<tt>GPS_ARBITER</tt>)<br>
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<a href="driver12.htm">Type 12</a> KSI/Odetics TPRO/S IRIG Interface (<tt>IRIG_TPRO</tt>)<br>
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Type 13 Leitch CSD 5300 Master Clock Controller (<tt>ATOM_LEITCH</tt>)<br>
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Type 14 EES M201 MSF Receiver (<tt>MSF_EES</tt>)<br>
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<a href="driver5.htm">Type 15</a> * TrueTime generic receivers<br>
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<a href="driver16">Type 16</a> Bancomm GPS/IRIG Receiver (<tt>GPS_BANCOMM</tt>)<br>
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Type 17 Datum Precision Time System (<tt>GPS_DATUM</tt>)<br>
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<a href="driver18.htm">Type 18</a> NIST Modem Time Service (<tt>ACTS_NIST</tt>)<br>
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<a href="driver19.htm">Type 19</a> Heath WWV/WWVH Receiver (<tt>WWV_HEATH</tt>)<br>
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<a href="driver20.htm">Type 20</a> Generic NMEA GPS Receiver (<tt>NMEA</tt>)<br>
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Type 21 TrueTime GPS-VME Interface (<tt>GPS_VME</tt>)<br>
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<a href="driver22.htm">Type 22</a> PPS Clock Discipline (<tt>PPS</tt>)<br>
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<a href="driver23.htm">Type 23</a> PTB Modem Time Service (<tt>ACTS_PTB</tt>)<br>
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<a href="driver24.htm">Type 24</a> USNO Modem Time Service (<tt>ACTS_USNO</tt>)<br>
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<a href="driver5.htm">Type 25</a> * TrueTime generic receivers<br>
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<a href="driver26.htm">Type 26</a> Hewlett Packard 58503A GPS Receiver (<tt>GPS_HP</tt>)<br>
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<a href="driver27.htm">Type 27</a> Arcron MSF Receiver (<tt>MSF_ARCRON</tt>)<br>
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<a href="driver28.htm">Type 28</a> Shared Memory Driver (<tt>SHM</tt>)<br>
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<a href="driver29.htm">Type 29</a> Trimble Navigation Palisade GPS (<tt>GPS_PALISADE</tt>)<br>
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<a href="driver30.htm">Type 30</a> Motorola UT Oncore GPS (<tt>GPS_ONCORE</tt>)<br>
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Type 31 Rockwell Jupiter GPS (<tt>GPS_JUPITER</tt>)<br>
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<a href="driver32.htm">Type 32</a> Chrono-log K-series WWVB receiver (<tt>CHRONOLOG</tt>)<br>
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<a href="driver33.htm">Type 33</a> Dumb Clock (<tt>DUMBCLOCK</tt>)<br>
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<a href="driver34.htm">Type 34</a> Ultralink WWVB Receivers (<tt>ULINK</tt>)<br>
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<a href="driver35.htm">Type 35</a> Conrad Parallel Port Radio Clock (<tt>PCF</tt>)<br>
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<a href="driver36.htm">Type 36</a> Radio WWV/H Audio Demodulator/Decoder
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(<tt>WWV</tt>)<br>
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<a href="driver37.htm">Type 37</a> Forum Graphic GPS Dating station (<tt>FG</tt>)<br>
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<a href="driver38.htm">Type 38</a> hopf GPS/DCF77 6021/komp for Serial Line
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(<tt>HOPF_S</tt>)<br>
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<a href="driver39.htm">Type 39</a> hopf GPS/DCF77 6039 for PCI-Bus (<tt>HOPF_P</tt>)<br>
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<a href="driver40.htm">Type 40</a> JJY Receivers (<tt>JJY</tt>)<br>
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<a href="driver44.htm">Type 44</a> NeoClock4X DCF77 / TDF receiver<br>
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</p>
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<p>* All TrueTime receivers are now supported by one driver, type 5. Types
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15 and 25 will be retained only for a limited time and may be reassigned
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in future.</p>
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<p>Additional Information</p>
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<p><a href="prefer.htm">Mitigation Rules and the <tt>prefer</tt> Keyword</a><br>
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<a href="rdebug.htm">Debugging Hints for Reference Clock Drivers</a><br>
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<a href="kern.htm">A Kernel Model for Precision Timekeeping</a><br>
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<a href="ldisc.htm">Line Disciplines and Streams Drivers</a><br>
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<a href="audio.htm">Reference Clock Audio Drivers</a><br>
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<a href="pps.htm">Pulse-per-second (PPS) Signal Interfacing</a><br>
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<a href="howto.htm">How To Write a Reference Clock Driver</a></p>
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<hr> <a href="index.htm"><img align="left" src="pic/home.gif" alt="gif">
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</a>
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<address><a href="mailto:mills@udel.edu">David L. Mills <mills@udel.edu></a></address>
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<br>
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</body>
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</html>
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