Reference Clock Options

Reference Clock Support

A reference clock will generally (though not always) be a radio timecode 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 U.S. The interface between the computer and the timecode receiver is device dependent, but is usually a serial port. A device driver specific to each reference clock must be selected and compiled in the distribution; 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 included 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. While it may seem overkill, it is in fact sometimes useful to configure 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 reference clock with somewhat more enthusiasm than other reference clocks or peers. Further information on this option can be found in the Mitigation 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 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 stratum 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 purpose. 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.

Reference Clock Commands

server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]

This command can be used to configure reference clocks in special 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 synchronization 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 subtype in the parse drivers.

 

minpoll int

maxpoll int

These options specify the minimum and maximum polling interval for reference clock messages, in seconds to the power of two. For most directly connected reference clocks, both minpoll and maxpoll default to 6 (64 s). For modem reference clocks, minpoll defaults to 10 (17.1 m) and maxpoll defaults to 14 (4.5 h). The allowable range is 4 (16 s) to 17 (36.4 h) inclusive.

 

fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string] [mode int] [flag1 0|1] [flag2 0|1] [flag3 0|1] [flag4 0|1]

This command can be used to configure reference clocks in special ways. It must immediately follow the server command which configures the driver. Note that the same capability is possible at run time using the ntpdc program. The options are interpreted 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 systematic error or bias due to serial port 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 individual system and driver is available, an approximate correction is noted in the driver documentation pages.

 

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 characters which defines the reference identifier used by the driver. This string overrides the default identifier ordinarily assigned by the driver itself.

 

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 subtype in the parse drivers.

 

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 verbose monitoring data to the clockstats file configured with the filegen command. Further information on the filegen command can be found in the Monitoring Options page.

 

pps device

Specifies the name of the serial port device to which the PPS signal is connected. Note, this command replaces use of fudge flag3, which was used for the same purpose in NTPv3. Note also that this command should preceed the server and fudge command for the same device.