2001-08-29 14:35:15 +00:00
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<title>Pulse-per-second (PPS) Signal Interfacing</title>
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<h3>Pulse-per-second (PPS) Signal Interfacing</h3>
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1999-12-09 13:01:21 +00:00
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2001-08-29 14:35:15 +00:00
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<img align="left" src="pic/alice32.gif" alt="gif"><a href=
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"http://www.eecis.udel.edu/~mills/pictures.htm">from <i>Alice's
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Adventures in Wonderland</i>, Lewis Carroll</a>
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1999-12-09 13:01:21 +00:00
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2001-08-29 14:35:15 +00:00
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<p>Alice is trying to find the PPS signal connector.<br clear=
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"left">
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</p>
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1999-12-09 13:01:21 +00:00
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2001-08-29 14:35:15 +00:00
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<hr>
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<p>Some radio clocks and related timekeeping gear have a
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pulse-per-second (PPS) signal that can be used to discipline the
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local clock oscillator to a high degree of precision, typically to
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the order less than 10 <font face="Symbol">m</font>s in time and
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0.01 parts-per-million (PPM) in frequency. The PPS signal can be
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connected in either of two ways: via the data carrier detector
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(DCD) pin of a serial port or via the acknowledge (ACK) pin of a
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parallel port, depending on the hardware and operating system.
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Connection via a serial port may require signal conversion and
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regeneration to RS232 levels, which can be done using a circuit
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such as described in the <a href="gadget.htm">Gadget Box PPS Level
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Converter and CHU Modem</a> page. Note that NTP no longer supports
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connection via the data leads of a serial port.</p>
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1999-12-09 13:01:21 +00:00
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2001-08-29 14:35:15 +00:00
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<p>Both the serial and parallel port connection require operating
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system support, which is available in only a few operating systems,
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including Linux, FreeBSD and latest Solaris beginning with 2.7.
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Support on an experimental basis is available for several older
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systems, including SunOS, Digital Ultrix and HP-UX, and in current
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Digital Tru64 (Alpha). The PPS application program interface
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defined in RFC-2783 (PPSAPI) is the only interface currently
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supported. Older PPS interfaces based on the <tt>ppsclock</tt> and
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<tt>tty_clk</tt> streams modules are no longer supported. As the
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PPSAPI is expected to become an IETF cross-platform standard, it
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should be used by new applications.</p>
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1999-12-09 13:01:21 +00:00
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2001-08-29 14:35:15 +00:00
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<p>The PPSAPI inerface requires a <tt>
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/usr/include/sys/ppstime.h</tt> header file. This file is included
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in Linux and FreeBSD distributions, but not in other distributions
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or standard workstation products at this time. Header files for
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other systems, including Solaris, can be found in the <tt>
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nanokernel.tar.gz</tt> distribution, which can be found via the
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Collaboration Resources link at www.ntp.org. The top level
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directory contains a number of subdirectories for each
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architecture, including Solaris. The <tt>ppstime.h</tt> file for
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each architecture can be found in the subdirectory of the same
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name.</p>
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1999-12-09 13:01:21 +00:00
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2001-08-29 14:35:15 +00:00
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<p>In the preferred mode of operation, PPS signals are processed by
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the <a href="driver22.htm">PPS Clock Discipline</a> driver and
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other clock drivers which might be involved need not know or care
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about them. In some cases where there is no other driver, time
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might be obtained from remote NTP servers via the network and local
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PPS signals, for instance from a calibrated cesium oscillator, used
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to stabilize the frequency and remove network jitter. Note that the
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<tt>pps</tt> configuration command has been obsoleted by this
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driver.</p>
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1999-12-09 13:01:21 +00:00
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2001-08-29 14:35:15 +00:00
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<p>The PPS driver operates in conjunction with a preferred peer, as
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described in the <a href="prefer.htm">Mitigation Rules and the <tt>
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prefer</tt> Keyword</a> page. One of the drivers described in the
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<a href="refclock.htm">Reference Clock Drivers</a> page or another
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NTP server furnishes the coarse timing and disambiguates the
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seconds numbering of the PPS signal itself. The NTP daemon
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mitigates between the clock driver or NTP server and the PPS driver
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as described in that page in order to provide the most accurate
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time, while respecting the various types of equipment failures that
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could happen.</p>
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<p>Some Unix system kernels support a PPS signal directly, as
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described in the <a href="kern.htm">A Kernel Model for Precision
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Timekeeping</a> page. Specifically, the PPS driver can be used to
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direct the PPS signal to the kernel for use as a discipline source
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for both time and frequency. The presence of the kernel support is
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automatically detected during the NTP build process and supporting
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code automatically compiled. Note that the PPS driver does not
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normally enable the PPS kernel code, since performance is generally
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better without it. However, this code can be enabled by a driver
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fudge flag if necessary.</p>
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<p>Some configurations may include multiple radio clocks with
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individual PPS outputs. In some PPSAPI designs multiple PPS signals
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can be connected to multiple instances of the PPS driver. In such
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cases the NTP mitigation and grooming algorithms operate with all
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the radio timecodes and PPS signals to develop the highest degree
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of redundancy and survivability.</p>
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<hr>
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<a href="index.htm"><img align="left" src="pic/home.gif" alt=
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"gif"></a><br>
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<br>
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<address><a href="mailto:mills@udel.edu">David L. Mills
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<mills@udel.edu></a></address>
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</body>
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</html>
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1999-12-09 13:01:21 +00:00
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