107 lines
4.8 KiB
HTML
107 lines
4.8 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
|
|
<html>
|
|
<head>
|
|
<meta name="generator" content="HTML Tidy, see www.w3.org">
|
|
<title>Pulse-per-second (PPS) Signal Interfacing</title>
|
|
</head>
|
|
<body>
|
|
<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
|
|
<mills@udel.edu></a></address>
|
|
</body>
|
|
</html>
|
|
|