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diff --git a/html/pps.htm b/html/pps.htm new file mode 100644 index 000000000000..a20dd4d744ea --- /dev/null +++ b/html/pps.htm @@ -0,0 +1,106 @@ +<!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> + |