diff options
Diffstat (limited to 'html/refclock.htm')
| -rw-r--r-- | html/refclock.htm | 202 |
1 files changed, 0 insertions, 202 deletions
diff --git a/html/refclock.htm b/html/refclock.htm deleted file mode 100644 index df4af3a9dfe3..000000000000 --- a/html/refclock.htm +++ /dev/null @@ -1,202 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"> -<html> -<head> - - <meta name="generator" content="HTML Tidy, see www.w3.org"> - <title>Reference Clock Drivers</title> -</head> - <body> - -<h3>Reference Clock Drivers</h3> - <img align="left" src="pic/stack1a.jpg" alt="gif"> -Master Time Facility at the <a - href="http://www.eecis.udel.edu/%7Emills/lab.htm"> UDel Internet Research -Laboratory</a>: <br clear="left"> - -<hr> -<p>Support for most of the commonly available radio and modem reference clocks -is included in the default configuration of the NTP daemon for Unix <tt>ntpd</tt>. -Individual clocks can be activated by configuration file commands, specifically -the <tt> server</tt> and <tt>fudge</tt> commands described in the <a - href="ntpd.htm"><tt>ntpd</tt> program manual page</a>. The following discussion -presents Information on how to select and configure the device drivers in -a running Unix system.</p> - -<p>Many radio reference clocks can be set to display local time as adjusted -for timezone and daylight saving mode. For use with NTP the clock must be -set for Coordinated Universal Time (UTC) only. Ordinarily, these adjustments -are performed by the kernel, so the fact that the clock runs on UTC will -be transparent to the user.</p> - -<p>Radio and modem clocks by convention have addresses in the form 127.127.<i>t.u</i>, -where <i>t</i> is the clock type and <i>u</i> 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 <tt>/dev/device<i>u</i></tt> to the particular hardware device involved, -where <i><tt>u</tt></i> correspond to the unit number above.</p> - -<p>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.</p> - -<p>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 <a href="audio.htm">Audio Drivers</a> page.</p> - -<p>The local clock driver is also a special case. A server configured with -this driver can operate as a primary server to synchronize other clients -when no other external synchronization sources are available. If the server -is connected directly or indirectly to the public Internet, there is some -danger that it can adversely affect the operation of unrelated clients. Carefully -read the <a href="driver1.htm">Undisciplined Local Clock</a> page and respect -the stratum limit.</p> - -<p>The local clock driver also supports an external synchronization source -such as a high resolution counter disciplined by a GPS receiver, for example. -Further information is on the <a href="extern.htm">External Clock Discipline -and the Local Clock Driver</a> page.</p> - -<h4>Driver Calibration</h4> - -<p>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 <a href="qth.htm">Stations, Frequencies and Geographic -Coordinates</a> page. Receiver coordinates can be obtained or estimated from -various sources. The actual calculations are beyond the scope of this document.</p> - -<p>When more than one clock driver is supported, it is often the case that -each shows small systematic offset differences relative to the rest. To reduce -the effects of jitter when switching from one driver to the another, it is -useful to calibrate the drivers to a common ensemble offset. The <tt>enable -calibrate</tt> configuration command in the <a href="miscopt.htm">Miscellaneous -Options</a> page is useful for this purpose. The calibration function can -also be enabled and disabled using the <tt>ntpdc</tt> program utility.</p> - -<p>Most clock drivers use the <tt>time1</tt> value specified in the <tt>fudge</tt> -configuration command to provide the calibration correction when this cannot -be provided by the clock or interface. When the calibration function is enabled, -the <tt>time1</tt> value is automatically adjusted to match the offset of -the remote server or local clock driver selected for synchronization. Ordinarily, -the NTP selection algorithm chooses the best from among all sources, usually -the best radio clock determined on the basis of stratum, synchronization -distance and jitter. The calibration function adjusts the <tt>time1</tt> -values for all clock drivers except this source so that their indicated offsets -tend to zero. If the selected source is the kernel PPS discipline, the <tt>fudge -time1</tt> values for all clock drivers are adjusted.</p> - -<p>The adjustment function is an exponential average designed to improve -accuracy, so the function takes some time to converge. The recommended procedure -is to enable the function, let it run for an hour or so, then edit the configuration -file using the <tt> time1</tt> values displayed by the <tt>ntpq</tt> utility -and <tt> clockvar</tt> command. Finally, disable the calibration function -to avoid possible future disruptions due to misbehaving clocks or drivers.</p> - -<h4>Performance Enhancements</h4> - -<p>In general, performance can be improved, especially when more than one -clock driver is supported, to use the prefer peer function described in the -<a href="prefer.htm">Mitigation Rules and the <tt> prefer</tt> Keyword</a> -page. The prefer peer is ordinarily designated the remote peer or local clock -driver which provides the best quality time. All other things equal, only -the prefer peer source is used to discipline the system clock and jitter-producing -"clockhopping" between sources is avoided. This is valuable when more than -one clock driver is present and especially valuable when the PPS clock driver -(type 22) is used. Support for PPS signals is summarized in the <a - href="pps.htm">Pulse-per-second (PPS) Signal Interfacing</a> page.</p> - -<p>Where the highest performance is required, generally better than one millisecond, -additional hardware and/or software functions may be required. Kernel modifications -for precision time are described in the <a href="kern.htm">A Kernel Model -for Precision Timekeeping</a> page. Special line discipline and streams modules -for use in capturing precision timestamps are described in the <a - href="ldisc.htm">Line Disciplines and Streams Drivers</a> page.</p> - -<h4>Comprehensive List of Clock Drivers</h4> - -<p>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 <a - href="copyright.htm">Copyright Notice</a> page.</p> - -<p><a href="driver1.htm">Type 1</a> Undisciplined Local Clock (<tt>LOCAL</tt>)<br> - <a href="driver2.htm">Type 2</a> Trak 8820 GPS Receiver (<tt>GPS_TRAK</tt>)<br> - <a href="driver3.htm">Type 3</a> PSTI/Traconex 1020 WWV/WWVH Receiver (<tt>WWV_PST</tt>)<br> - <a href="driver4.htm">Type 4</a> Spectracom WWVB and GPS Receivers (<tt>WWVB_SPEC</tt>)<br> - <a href="driver5.htm">Type 5</a> TrueTime GPS/GOES/OMEGA Receivers (<tt>TRUETIME</tt>)<br> - <a href="driver6.htm">Type 6</a> IRIG Audio Decoder (<tt>IRIG_AUDIO</tt>)<br> - <a href="driver7.htm">Type 7</a> Radio CHU Audio Demodulator/Decoder (<tt>CHU</tt>)<br> - <a href="driver8.htm">Type 8</a> Generic Reference Driver (<tt>PARSE</tt>)<br> - <a href="driver9.htm">Type 9</a> Magnavox MX4200 GPS Receiver (<tt>GPS_MX4200</tt>)<br> - <a href="driver10.htm">Type 10</a> Austron 2200A/2201A GPS Receivers (<tt>GPS_AS2201</tt>)<br> - <a href="driver11.htm">Type 11</a> Arbiter 1088A/B GPS Receiver (<tt>GPS_ARBITER</tt>)<br> - <a href="driver12.htm">Type 12</a> KSI/Odetics TPRO/S IRIG Interface (<tt>IRIG_TPRO</tt>)<br> - Type 13 Leitch CSD 5300 Master Clock Controller (<tt>ATOM_LEITCH</tt>)<br> - Type 14 EES M201 MSF Receiver (<tt>MSF_EES</tt>)<br> - <a href="driver5.htm">Type 15</a> * TrueTime generic receivers<br> - <a href="driver16">Type 16</a> Bancomm GPS/IRIG Receiver (<tt>GPS_BANCOMM</tt>)<br> - Type 17 Datum Precision Time System (<tt>GPS_DATUM</tt>)<br> - <a href="driver18.htm">Type 18</a> NIST Modem Time Service (<tt>ACTS_NIST</tt>)<br> - <a href="driver19.htm">Type 19</a> Heath WWV/WWVH Receiver (<tt>WWV_HEATH</tt>)<br> - <a href="driver20.htm">Type 20</a> Generic NMEA GPS Receiver (<tt>NMEA</tt>)<br> - Type 21 TrueTime GPS-VME Interface (<tt>GPS_VME</tt>)<br> - <a href="driver22.htm">Type 22</a> PPS Clock Discipline (<tt>PPS</tt>)<br> - <a href="driver23.htm">Type 23</a> PTB Modem Time Service (<tt>ACTS_PTB</tt>)<br> - <a href="driver24.htm">Type 24</a> USNO Modem Time Service (<tt>ACTS_USNO</tt>)<br> - <a href="driver5.htm">Type 25</a> * TrueTime generic receivers<br> - <a href="driver26.htm">Type 26</a> Hewlett Packard 58503A GPS Receiver (<tt>GPS_HP</tt>)<br> - <a href="driver27.htm">Type 27</a> Arcron MSF Receiver (<tt>MSF_ARCRON</tt>)<br> - <a href="driver28.htm">Type 28</a> Shared Memory Driver (<tt>SHM</tt>)<br> - <a href="driver29.htm">Type 29</a> Trimble Navigation Palisade GPS (<tt>GPS_PALISADE</tt>)<br> - <a href="driver30.htm">Type 30</a> Motorola UT Oncore GPS (<tt>GPS_ONCORE</tt>)<br> - Type 31 Rockwell Jupiter GPS (<tt>GPS_JUPITER</tt>)<br> - <a href="driver32.htm">Type 32</a> Chrono-log K-series WWVB receiver (<tt>CHRONOLOG</tt>)<br> - <a href="driver33.htm">Type 33</a> Dumb Clock (<tt>DUMBCLOCK</tt>)<br> - <a href="driver34.htm">Type 34</a> Ultralink WWVB Receivers (<tt>ULINK</tt>)<br> - <a href="driver35.htm">Type 35</a> Conrad Parallel Port Radio Clock (<tt>PCF</tt>)<br> - <a href="driver36.htm">Type 36</a> Radio WWV/H Audio Demodulator/Decoder -(<tt>WWV</tt>)<br> - <a href="driver37.htm">Type 37</a> Forum Graphic GPS Dating station (<tt>FG</tt>)<br> - <a href="driver38.htm">Type 38</a> hopf GPS/DCF77 6021/komp for Serial Line -(<tt>HOPF_S</tt>)<br> - <a href="driver39.htm">Type 39</a> hopf GPS/DCF77 6039 for PCI-Bus (<tt>HOPF_P</tt>)<br> - <a href="driver40.htm">Type 40</a> JJY Receivers (<tt>JJY</tt>)<br> -<a href="driver44.htm">Type 44</a> NeoClock4X DCF77 / TDF receiver<br> - </p> - -<p>* 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.</p> - -<p>Additional Information</p> - -<p><a href="prefer.htm">Mitigation Rules and the <tt>prefer</tt> Keyword</a><br> - <a href="rdebug.htm">Debugging Hints for Reference Clock Drivers</a><br> - <a href="kern.htm">A Kernel Model for Precision Timekeeping</a><br> - <a href="ldisc.htm">Line Disciplines and Streams Drivers</a><br> - <a href="audio.htm">Reference Clock Audio Drivers</a><br> - <a href="pps.htm">Pulse-per-second (PPS) Signal Interfacing</a><br> - <a href="howto.htm">How To Write a Reference Clock Driver</a></p> - -<hr> <a href="index.htm"><img align="left" src="pic/home.gif" alt="gif"> -</a> -<address><a href="mailto:mills@udel.edu">David L. Mills <mills@udel.edu></a></address> - <br> -</body> -</html> |