Pulse-per-second (PPS) Signal Interfacing

From 52f0477edd81105d995864898a64a601b67d66d8 Mon Sep 17 00:00:00 2001 From: roberto Date: Tue, 20 Jul 2004 15:19:51 +0000 Subject: Merge conflicts. Lots of added files, some removed and quite a large number of renames :( --- contrib/ntp/html/pps.htm | 106 ----------------------------------------------- 1 file changed, 106 deletions(-) delete mode 100644 contrib/ntp/html/pps.htm (limited to 'contrib/ntp/html/pps.htm') diff --git a/contrib/ntp/html/pps.htm b/contrib/ntp/html/pps.htm deleted file mode 100644 index a20dd4d..0000000 --- a/contrib/ntp/html/pps.htm +++ /dev/null @@ -1,106 +0,0 @@ - - - - -Pulse-per-second (PPS) Signal Interfacing - - -

Pulse-per-second (PPS) Signal Interfacing

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from Alice's -Adventures in Wonderland, Lewis Carroll - -

Alice is trying to find the PPS signal connector.
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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 ms 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 Gadget Box PPS Level -Converter and CHU Modem page. Note that NTP no longer supports -connection via the data leads of a serial port.

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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 ppsclock and -tty_clk 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.

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The PPSAPI inerface requires a -/usr/include/sys/ppstime.h 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 -nanokernel.tar.gz 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 ppstime.h file for -each architecture can be found in the subdirectory of the same -name.

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In the preferred mode of operation, PPS signals are processed by -the PPS Clock Discipline 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 -pps configuration command has been obsoleted by this -driver.

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The PPS driver operates in conjunction with a preferred peer, as -described in the Mitigation Rules and the -prefer Keyword page. One of the drivers described in the -Reference Clock Drivers 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.

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Some Unix system kernels support a PPS signal directly, as -described in the A Kernel Model for Precision -Timekeeping 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.

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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.

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David L. Mills -<mills@udel.edu>

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