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ntp_clock(5)		      File Formats Manual		  ntp_clock(5)

       ntp_clock - Reference Clock Options

       The  NTP	 Version  4  daemon supports some three dozen different radio,
       satellite and modem reference clocks plus a special  pseudo-clock  used
       for  backup  or	when  no  other	 clock	source	is available. Detailed
       descriptions of individual device drivers and options can be  found  in
       the  Reference  Clock Drivers page. Additional information can be found
       in the pages linked there, including the Debugging Hints for  Reference
       Clock Drivers and How To Write a Reference Clock Driver pages. In addi-
       tion, support for a PPS signal is available as described in  Pulse-per-
       second (PPS) Signal Interfacing page.

       A  reference  clock will generally (though not always) be a radio time-
       code receiver which is synchronized to a source of standard  time  such
       as  the	services offered by the NRC in Canada and NIST and USNO in the
       US. The interface between the computer and  the	timecode  receiver  is
       device  dependent,  but	is usually a serial port. A device driver spe-
       cific to each reference clock must be selected and compiled in the dis-
       tribution;  however,  most common radio, satellite and modem clocks are
       included by default. Note that an  attempt  to  configure  a  reference
       clock  when  the	 driver has not been compiled or the hardware port has
       not been appropriately configured results in a scalding remark  to  the
       system log file, but is otherwise non hazardous.

       For  the	 purposes  of configuration, ntpd treats reference clocks in a
       manner analogous to normal NTP peers as	much  as  possible.  Reference
       clocks  are  identified	by  a  syntactically  correct  but  invalid IP
       address, in order to distinguish them from normal NTP peers.  Reference
       clock  addresses	 are  of  the  form 127.127.t.u, where t is an integer
       denoting the clock type and u indicates the unit number	in  the	 range
       0-3. While it may seem overkill, it is in fact sometimes useful to con-
       figure multiple reference clocks of the same type, in  which  case  the
       unit numbers must be unique.

       The  server  command  is used to configure a reference clock, where the
       address argument in that command is the clock address. The key, version
       and  ttl	 options  are  not  used for reference clock support. The mode
       option is added for reference clock support, as	described  below.  The
       prefer  option can be useful to persuade the server to cherish a refer-
       ence clock with somewhat more enthusiasm than other reference clocks or
       peers.  Further	information on this option can be found in the Mitiga-
       tion Rules and the prefer Keyword page. The minpoll and maxpoll options
       have  meaning only for selected clock drivers. See the individual clock
       driver document pages for additional information.

       The fudge command is used to provide additional information  for	 indi-
       vidual  clock drivers and normally follows immediately after the server
       command. The address argument specifies the clock  address.  The	 refid
       and  stratum  options  control can be used to override the defaults for
       the device. There are two optional device-dependent  time  offsets  and
       four flags that can be included in the fudge command as well.

       The  stratum  number of a reference clock is by default zero. Since the
       ntpd daemon adds one to the stratum of  each  peer,  a  primary	server
       ordinarily  displays  an	 external  stratum of one. In order to provide
       engineered backups, it is often useful to specify the  reference	 clock
       stratum	as greater than zero. The stratum option is used for this pur-
       pose. Also, in cases involving both a reference clock and a  pulse-per-
       second  (PPS)  discipline signal, it is useful to specify the reference
       clock identifier as other than the default, depending  on  the  driver.
       The  refid  option  is used for this purpose. Except where noted, these
       options apply to all clock drivers.

       server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]
	       This command can be used to configure reference clocks in  spe-
	       cial ways. The options are interpreted as follows:

	       prefer  Marks  the  reference  clock  as	 preferred.  All other
		       things being equal, this host will be chosen  for  syn-
		       chronization  among a set of correctly operating hosts.
		       See the Mitigation Rules and the	 prefer	 Keyword  page
		       for further information.

	       mode int
		       Specifies  a  mode  number  which  is  interpreted in a
		       device-specific fashion. For  instance,	it  selects  a
		       dialing	protocol  in the ACTS driver and a device sub-
		       type in the parse drivers.

	       minpoll int

	       maxpoll int
		       These options specify the minimum and  maximum  polling
		       interval	 for  reference	 clock	messages  in  seconds,
		       interpreted as  dual  logarithms	 (2  ^	x).  For  most
		       directly	 connected  reference clocks, both minpoll and
		       maxpoll default to 6 (2^16 = 64 s). For modem reference
		       clocks, minpoll defaults to 10 (2^10 = 1024 s = 17.1 m)
		       and maxpoll defaults to 14 (2^14 = 16384 s  =  4.5  h).
		       The  allowable  range is 4 (16 s) to 17 (36.4 h) inclu-

       fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid  string]
       [flag1 0|1] [flag2 0|1] [flag3 0|1] [flag4 0|1]
	       This  command can be used to configure reference clocks in spe-
	       cial ways. It must immediately follow the server command	 which
	       configures  the driver. Note that the same capability is possi-
	       ble at run time using the ntpdc program. The options are inter-
	       preted as follows:

	       time1 sec
		       Specifies  a  constant  to  be added to the time offset
		       produced by the driver, a fixed-point decimal number in
		       seconds.	 This  is  used	 as  a calibration constant to
		       adjust the nominal time offset of a particular clock to
		       agree  with  an	external standard, such as a precision
		       PPS signal. It also provides a way to correct a system-
		       atic error or bias due to serial port or operating sys-
		       tem latencies,  different  cable	 lengths  or  receiver
		       internal	 delay. The specified offset is in addition to
		       the propagation delay provided by other means, such  as
		       internal	 DIPswitches. Where a calibration for an indi-
		       vidual system and driver is available,  an  approximate
		       correction  is noted in the driver documentation pages.
		       Note: in order to facilitate calibration when more than
		       one  radio  clock or PPS signal is supported, a special
		       calibration feature is available. It takes the form  of
		       an argument to the enable command described in the Mis-
		       cellaneous Options page and operates  as	 described  in
		       the Reference Clock Drivers page.

	       time2 secs
		       Specifies  a  fixed-point  decimal  number  in seconds,
		       which is interpreted in a driver-dependent way. See the
		       descriptions of specific drivers in the reference clock
		       drivers page.

	       stratum int
		       Specifies the stratum number assigned to the driver, an
		       integer	between	 0  and	 15. This number overrides the
		       default	stratum	 number	 ordinarily  assigned  by  the
		       driver itself, usually zero.

	       refid string
		       Specifies  an  ASCII string of from one to four charac-
		       ters which defines the reference identifier used by the
		       driver.	This  string  overrides the default identifier
		       ordinarily assigned by the driver itself.

	       flag1 flag2 flag3 flag4
		       These four flags are used  for  customizing  the	 clock
		       driver. The interpretation of these values, and whether
		       they are used at all, is a function of  the  particular
		       clock  driver.  However, by convention flag4 is used to
		       enable recording monitoring data to the clockstats file
		       configured  with	 the filegen command. Further informa-
		       tion on the filegen command can be found in  the	 Moni-
		       toring Options page.


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