1. Synopsis

Name: truetime
Reference ID: TRUE
Serial Port: /dev/trueu; 9600 bps 8N1
Features: tty_clk

2. Deprecation warning

This refclock is deprecated and obsolete. The NTPsec maintainers plan to remove it in a future release. If you have a requirement for it, please make this known to us.

This driver reports only two-digit years, and is thus reliant on the system clock to be near correct before samples will be processed properly. You will not be able to use it to run autonomously, nor will it reliably recover from a trashed or zeroed system clock.

It is likely any surving instances of this hardware will have era-rollover issues when reporting dates. One or more "g" suffixes on your time1 option may be useful as a workaround.

3. Description

This driver supports several models of Kinemetrics/TrueTime timing receivers, including GPS- DC MK III and GPS/TM-TMD GPS Synchronized Clock, XL-DC (a 151-602-210, reported by the driver as a GPS/TM-TMD), GPS-800 TCU (an 805-957 with the RS232 Talker/Listener module), and very likely others in the same model families that use the same timecode formats.

Most of this code is originally from refclock_wwvb.c (now refclock_spectracom.c) with thanks. It has been so mangled that wwvb is not a recognizable ancestor.

Timecode format: ADDD:HH:MM:SSQCL
A - control A (this is stripped before we see it)
Q - Quality indication (see below)
C - Carriage return
L - Line feed

Quality codes indicate possible error of:

GPS-TM/TMD Receiver
    ? +/- 500 milliseconds # +/- 50 milliseconds
    * +/- 5 milliseconds . +/- 1 millisecond
    space less than 1 millisecond

In general, an alarm condition is indicated by ? at A, which occurs during initial synchronization and when received signal is lost for an extended period; unlock condition is indicated by other than <SP> in the quality field.

4. Notes on the TL-3 receiver:

The mini-DIN RS-232 port uses the Apple pinout.

Send the clock ST1 to turn on continuous (1/s) timecodes. You can also enable "mode C" via the front panel. ST0 turns off this mode.

QV will return the firmware revision (and is useful in identifying this clock.)

QW will return its weekly signal log, useful if you’re testing antennas. You may wish to turn the loss interval down from 4 h (04) to 1 h (01), so the receiver declares itself unlocked sooner. When in holdover, drift can be on the order of 10 ms/hr since there is no high quality reference oscillator.

5. Monitor Data

When enabled by the flag4 option, every received timecode is written as-is to the clockstats file.

6. Driver Options

unit number

The driver unit number, defaulting to 0. Used as a distinguishing suffix in the driver device name.

time1 time

Specifies the time offset calibration factor, in seconds and fraction, to be used for the West satellite, with default 0.0.

time2 time

Specifies the time offset calibration factor, in seconds and fraction, to be used for the East satellite, with default 0.0.

stratum number

Specifies the driver stratum, in decimal from 0 to 15, with default 0.

refid string

Specifies the driver reference identifier, an ASCII string from one to four characters, with default TRUE.

flag1 {0 | 1}

Silence the clock side of ntpd, just reading the clock without trying to write to it.

flag2 {0 | 1}

Generate a debug file /tmp/true%d.

flag3 {0 | 1}

Not used by this driver.

flag4 {0 | 1}

Enable verbose clockstats recording if set.


Not used by this driver.


Not used by this driver.

path filename

Overrides the default device path.

ppspath filename

Not used by this driver.

baud number

Overrides the default baud rate.

7. Configuration Example

refclock truetime

8. Additional Information