freebsd-dev/contrib/ntp/html/pps.htm
2001-08-29 14:35:15 +00:00

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