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diff --git a/contrib/ntp/html/pps.htm b/contrib/ntp/html/pps.htm new file mode 100644 index 0000000..a002c1f --- /dev/null +++ b/contrib/ntp/html/pps.htm @@ -0,0 +1,83 @@ +<HTML><HEAD><TITLE> +Pulse-per-second (PPS) Signal Interfacing +</TITLE></HEAD><BODY><H3> +Pulse-per-second (PPS) Signal Interfacing +</H3><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 20 <FONT FACE=Symbol>m</FONT>s in time and 0.01 PPM in frequency. +The PPS signal can be connected in either of two ways: via the data +leads of a serial port or via the modem control leads. Either way +requires conversion of the PPS signal, usually at TTL levels, 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. + +<P>The data leads interface requires regenerating the PPS pulse and +converting to RS232 signal levels, so that the pulse looks like a +legitimate ASCII character to a serial port. The <TT>tty_clk</TT> line +discipline/streams module inserts a timestamp following this character +in the input data stream. The <A HREF=driver22.htm>PPS Clock +Discipline</A> driver uses this timestamp to determine the time of +arrival of the PPS pulse to within 26 us at 38.4 kbps while eliminating +error due to operating system queues and service times. + +<P>The modem control leads interface requires converting to RS232 levels +and connecting to the data carrier detect (DCD) lead of a serial port. +The <TT>ppsclock</TT> and <TT>ppsapi</TT> streams modules capture a +timestamp upon transition of the DCD signal. Note that the +<TT>ppsclock</TT> module functionality has been subsumed by the new +<TT>ppsapi</TT> interface specification, which is supported by the NTP +daemon. As the latter is expected to become an IETF cross-platform +standard, it should be used in new configurations. The PPS Clock +Discipline driver reads the latest timestamp with a designated system +call or interface routine to determine the time of arrival of the PPS +pulse to within a few microseconds. Alternatively, if provisions have +been made in the kernel for PPS signals, the signal is captured directly +by the kernel serial driver without using the PPS driver. + +<P>The <TT>tty_clk</TT> module is included in the NTP software +distribution, while the <A +HREF=http://www.eecis.udel.edu/~mills/ntp/ntp/ppsclock.tar.Z><TT> +ppsclock</TT></A> module can be obtained via the web at that link or by +anonymous FTP from ftp.udel.edu in the <TT>pub/ntp</TT> directory. Both +the <TT>tty_clk</TT> and <TT>ppsclock</TT> modules are described in the +<A HREF=ldisc.htm>Line Disciplines and Streams Drivers</A> page. +Directions for building the modules themselves are in the +<TT>./kernel</TT> directory. Directions on how to configure +<TT>ntpd</TT> to operate with these modules is described in <A +HREF=build.htm>Building and Installing the Distribution </A>page. + +<P>The PPS driver is operates in conjunction with another reference +clock driver that produces the PPS pulse, 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 furnishes +the coarse timecode used to disambiguate the seconds numbering of the +PPS pulse itself. The NTP daemon mitigates between the radio clock +driver and <TT>PPS</TT> 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>For the utmost time quality, 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 ppsclock module +can be used to interface the PPS signal directly to the kernel for use +as discipline sources for both time and frequency. These sources can be +separately enabled and monitored using the <TT>ntp_adjtime()</TT> system +call described in that page and the <TT>/usr/include/sys/timex.h</TT> +header file. The presence of these kernel provisions is automatically +detected and supporting code compiled. + +<P>In some configurations may have multiple radio clocks, each with PPS +outputs, as well as a kernel provisions for the PPS signal. In order to +provide the highest degree of redundancy and survivability, the kernel +PPS discipline, <TT>tty_clk</TT> module, <TT>ppsclock</TT> module and +kernel modifications may all be in use at the same time, each backing up +the other. The sometimes complicated mitigation rules are described in +the Mitigation Rules and the <TT>prefer</TT> Keyword page. + +<hr><a href=index.htm><img align=left src=pic/home.gif></a><address><a +href=mailto:mills@udel.edu> David L. Mills <mills@udel.edu></a> +</address></a></body></html> |
