Generic Reference Driver

Last update: July 4, 2023 16:28 UTC (fd17d3567)

Table of Contents


Address: 127.127.8.u

Reference ID: PARSE


Serial Port: /dev/refclock-u; TTY mode according to clock type

PPS device: /dev/refclockpps-u; alternate PPS device (if not available via the serial port)


The PARSE driver supports 20 different clock types/configurations. PARSE is actually a multi-clock driver.

The actual receiver status is mapped into various synchronization states generally used by receivers. The driver is configured to interpret the time codes of Meinberg DCF77 AM receivers, DCF77 FM receivers, Meinberg GPS16x/17x receivers, Trimble SV6 GPS, ELV DCF7000, Schmid, Wharton 400A and low cost receivers.

The reference clock support in NTP contains the necessary configuration tables for those receivers. In addition to supporting several different clock types and up to 4 devices, the processing of a PPS signal is also provided as a configuration option. The PPS configuration option uses the receiver-generated time stamps for feeding the PPS loopfilter control for much finer clock synchronization.

CAUTION: The PPS configuration option is different from the hardware PPS signal, which is also supported (see below), as it controls the way ntpd is synchronized to the reference clock, while the hardware PPS signal controls the way time offsets are determined.

The use of the PPS option requires receivers with an accuracy of better than 1ms.

Timecode variables listed by ntpq (8)

The ntpq program can read and display several clock variables. These hold the following information:

A qualification of the decoded time code format.
The overall running time and the accumulated times for the clock event states.
Lists the currently active receiver flags. Additional feature flags for the receiver are optionally listed in parentheses.
The local time with the offset to UTC (format HHMM).
The actual time code.

If PPS information is present, additional variables are available:

The difference between the RS-232-derived timestamp and the PPS timestamp.
The PPS timestamp.

Supported Devices

Currently, twenty-four clock types are supported by the PARSE driver and up to four devices (/dev/refclock-0 - /dev/refclock-3) of these clocks may be operational at any one time.

A note on the implementations: These implementations were mainly done without actual access to the hardware, thus not all implementations provide full support. The development was done with the help of many kind souls who had the hardware and kindly lent me their time and patience during the development and debugging cycle. Thus for continued support and quality, direct access to the receivers is a big help. Nevertheless I am not prepared to buy these reference clocks - donations to ( are welcome as long as they work within Europe.

Verified implementations are:

The pictures below have been taken from and are linked to the vendors' web pages.

Actual data formats and setup requirements of the various clocks can be found in NTP PARSE clock data formats.


The reference clock support software carefully monitors the state transitions of the receiver. All state changes and exceptional events (such as loss of time code transmission) are logged via the syslog facility. Every hour a summary of the accumulated times for the clock states is listed via syslog.

PPS support is only available when the receiver is completely synchronized. The receiver is believed to deliver correct time for an additional period of time after losing synchronization, unless a disruption in time code transmission is detected (possible power loss). The trust period is dependent on the receiver oscillator and thus is a function of clock type.

Raw DCF77 pulses can be fed via a level converter to the RXD pin of an RS-232 serial port (pin 3 of a 25-pin connector or pin 2 of a 9-pin connector). The telegrams are decoded and used for synchronization. DCF77 AM receivers can be bought for as little as $25. The accuracy is dependent on the receiver and is somewhere between 2ms (expensive) and 10ms (cheap). Synchronization ceases when reception of the DCF77 signal deteriorates, since no backup oscillator is available as usually found in other reference clock receivers. So it is important to have a good place for the DCF77 antenna. During transmitter shutdowns you are out of luck unless you have other NTP servers with alternate time sources available.

In addition to the PPS loopfilter control, a true PPS hardware signal can be utilized via the PPSAPI interface. PPS pulses are usually fed via a level converter to the DCD pin of an RS-232 serial port (pin 8 of a 25-pin connector or pin 1 of a 9-pin connector). To select PPS support, the mode parameter is the mode value as above plus 128. If 128 is not added to the mode value, PPS will be detected to be available but will not be used.

Hardware PPS support

For PPS to be used, add 128 to the mode parameter.

If the PPS signal is fed in from a device different from the device providing the serial communication (/dev/refclock-{0..3}), this device is configured as /dev/refclockpps-{0..3}. This allows the PPS information to be fed in e.g. via the parallel port (if supported by the underlying operation system) and the date/time telegrams to be handled via the serial port.

Monitor Data

Clock state statistics are written hourly to the syslog service. Online information can be found by examining the clock variables via the ntpq cv command.

Some devices have quite extensive additional information (GPS16x/GPS17x, Trimble). The driver reads out much of the internal GPS data and makes it accessible via clock variables. To find out about additional variable names, query for the clock_var_list variable on a specific clock association as shown below.

First let ntpq display the table of associations:

ntpq> as    
ind assID status  conf reach auth condition  last_event cnt
1   19556  9154   yes   yes  none falsetick   reachable  5      
2   19557  9435   yes   yes  none  candidat  clock expt  3      
3   19558  9714   yes   yes  none  pps.peer   reachable  1

Then switch to raw output. This may be required because of display limitations in ntpq/ntpd - so large lists need to be retrieved in several queries.

ntpq> raw    
Output set to raw

Use the cv command to read the list of clock variables of a selected association:

ntpq> cv 19557 clock_var_list

The long output of the command above looks similar to:

assID=19557 status=0x0000,

Then use the cv command again to list selected clock variables. NOTE: The following command must be entered as a single line:

ntpq> cv 19557 refclock_status,refclock_format,refclock_states,refclock_id,

The output of the command above is wrapped around depending on the screen width and looks similar to:

refclock_format="Meinberg GPS Extended",    refclock_states="*NOMINAL: 21:21:36 (99.99%);
FAULT: 00:00:03 (0.00%);    running time: 21:21:39",    refclock_id="GPS", refclock_iomode="normal",
refclock_driver_version="refclock_parse.c,v 4.77 2006/08/05 07:44:49    kardel RELEASE_20060805_A",
meinberg_gps_status="[0x0000] ",    gps_utc_correction="current correction 14 sec, last correction 
on c7619a00.00000000  Sun, Jan  1 2006  0:00:00.000",    gps_message="/PFU3SOP-4WG14EPU0V1KA",
meinberg_antenna_status="RECONNECTED on 2006-07-18 08:13:20.0000000 (+0000)    UTC CORR, LOCAL TIME, 
reconnect clockoffset +0.0000000 s,   disconnect time 0000-00-00 00:00:00.0000000 (+0000) ", 
gps_tot_51="week 1400 + 3 days + 42300.0000000 sec",    gps_tot_63="week
1400 + 3 days + 42294.0000000 sec",   gps_t0a="week 1400 + 5 days + 71808.0000000 sec",
gps_cfg[1]="[0x9] BLOCK II",   gps_health[1="[0x0] OK;SIGNAL OK",    gps_cfg[2]="[0x0] BLOCK I", 
gps_health[2]="[0x3f] PARITY;MULTIPLE ERRS", gps_cfg[3]="[0x9] BLOCK II", gps_health[3]="[0x0] OK;
SIGNAL OK",    gps_cfg[4]="[0x9] BLOCK II", gps_health[6]="[0x0] OK;SIGNAL OK",    gps_cfg[5]="[0x9] BLOCK II" 

Fudge Factors

time1 time
Specifies the time offset calibration factor, in seconds and fraction. The default value depends on the clock type.
time2 time
If flag1 is 0, time2 specifies the offset of the PPS signal from the actual time (PPS fine tuning).

If flag1 is 1, time2 specifies the number of seconds a receiver with a premium local oscillator can be trusted after losing synchronisation.

stratum stratum
The stratum for this reference clock.
refid refid
The refid for this reference clock.
flag1 { 0 | 1 }
If 0, the fudge factor time2 refers to the PPS offset.

If 1, time2 refers to the TRUST TIME.

flag2 { 0 | 1 }
If flag2 is 1, sample PPS on CLEAR instead of on ASSERT.
flag3 { 0 | 1 }
If flag3 is 1, link kernel PPS tracking to this refclock instance.
flag4 { 0 | 1 }
Delete next leap second instead of adding it. (You’ll need to wait a bit for that to happen.)

Note about auxiliary Sun STREAMS modules (SunOS and Solaris): The timecode of these receivers can be sampled via a STREAMS module in the kernel. (The STREAMS module has been designed for use with Sun systems under SunOS 4.1.x or Solaris 2.3 - 2.8. It can be linked directly into the kernel or loaded via the loadable driver mechanism.) This STREAMS module can be adapted to convert different time code formats. Nowadays the PPSAPI mechanism is usually used.

Making your own PARSE clocks

The parse clock mechanism deviates from the way other NTP reference clocks work. For a short description of how to build parse reference clocks, see making PARSE clocks.

Additional Information

Reference Clock Drivers