This documentation describes version V1.1 (1997/06/23) of the source and has been tested (amongst others) against ntpd3-5.90 on Solaris-1 (SunOS 4.1.3_U1 on an SS1 serving as a router and firewall) and against ntpd3-5.90 on Solaris-2.5 (on a SS1+ and TurboSPARC 170MHz). This code will probably work, and show increased stability, reduced jitter and more efficiency (fewer context switches) with the tty_clk discipline/STREAMS module installed, but this has not been tested. For a to-do list see the comments at the start of the code.
This code has been significantly slimmed down since the V1.0 version, roughly halving the memory footprint of its code and data.
This driver is designed to allow the unit to run from batteries as designed, for something approaching the 2.5 years expected in the usual stand-alone mode, but no battery-life measurements have been taken.
Much of this code is originally from the other refclock driver files with thanks. The code was originally made to work with the clock by Derek Mulcahy, with modifications by Damon Hart-Davis. Thanks also to Lyndon David for some of the specifications of the clock.
There is support for a Tcl/Tk monitor written by Derek Mulcahy that examines the output stats; see the ARC Rugby MSF Receiver page for more details and the code.
Look at the notes at the start of the code for further information; some of the more important details follow.
The driver interrogates the clock at each poll (ie every 64s by default) for a timestamp. The clock responds at the start of the next second (with the start bit of the first byte being on-time). The time is in `local' format, including the daylight savings adjustment when it is in effect. The driver code converts the time back to UTC.
The clock claims to be accurate to within about 20ms of the MSF-broadcast time, and given the low data transmission speed from clock to host, and the fact that the clock is not in continuous sync with MSF, it seems sensible to set the `precision' of this clock to -5 or -4, -4 being used in this code, which builds in a reported dispersion of over 63ms (ie says ``This clock is not very good.''). You can improve the reported precision to -4 (and thus reduce the base dispersion to about 31ms) by setting the fudge flag3 to 1.
Even a busy and slow IP link can yield lower dispersions than this from polls of primary time servers on the Internet, which reinforces the idea that this clock should be used as a backup in case of problems with such an IP link, or in the unfortunate event of failure of more accurate sources such as GPS.
By default this clock reports itself to be at stratum 2 rather than the usual stratum 0 for a refclock, because it is not really suited to be used as other than a backup source. The stratum reported can be changed with the fudge directive to be whatever you like. After careful monitoring of your clock, and appropriate choice of the time1 fudge factor to remove systematic errors in the clock's reported time, you might fudge the clock to stratum 1 to allow a stratum-2 secondary server to sync to it.
The driver code arranges to resync the clock to MSF at intervals of a little less than an hour (deliberately avoiding the same time each hour to avoid any systematic problems with the signal or host). Whilst resyncing, the driver supplements the normal polls for time from the clock with polls for the reception signal quality reported by the clock. If the signal quality is too low (0--2 out of a range of 0--5), we chose not to trust the clock until the next resync (which we bring forward by about half an hour). If we don't catch the resync, and so don't know the signal quality, we do trust the clock (because this would generally be when the signal is very good and a resync happens quickly), but we still bring the next resync forward and reduce the reported precision (and thus increase reported dispersion).
If we force resyncs to MSF too often we will needlessly exhaust the batteries the unit runs from. During clock resync this driver tries to take enough time samples to avoid ntpd losing sync in case this clock is the current peer. By default the clock would only resync to MSF about once per day, which would almost certainly not be acceptable for NTP purposes.
The driver does not force an immediate resync of the clock to MSF when it starts up to avoid excessive battery drain in case ntpd is going to be repeatedly restarted for any reason, and also to allow enough samples of the clock to be taken for ntpd to sync immediately to this clock (and not remain unsynchronised or to sync briefly to another configured peer, only to hop back in a few poll times, causing unnecessary disturbance). This behaviour should not cause problems because the driver will not accept the timestamps from the clock if the status flag delivered with the time code indicates that the last resync attempt was unsuccessful, so the initial timestamps will be close to reality, even if with up to a day's clock drift in the worst case (the clock by default resyncs to MSF once per day).
The clock has a peculiar RS232 arrangement where the transmit lines are powered from the receive lines, presumably to minimise battery drain. This arrangement has two consequences:
Three commands are sent to the clock by this driver. Each command consists of a single letter (of which only the bottom four bits are significant), followed by a CR (ASCII 13). Each character sent to the clock should be followed by a delay to allow the unit to echo the character, and then by a further 10ms. Following the echo of the command string, there may be a response (ie in the cae of the g and o commands below), which in the case of the o command may be delayed by up to 1 second so as the start bit of the first byte of the response can arrive on time. The commands and their responses are:
# hostname(n) means we expect (n) to be the stratum at which hostname runs. #------------------------------------------------------------------------------ # SYNCHRONISATION PARTNERS # ======================== # Our betters... server 127.127.27.0 # ARCRON MSF radio clock(1). # Fudge stratum and other features as required. # ADJUST time1 VALUE FOR YOUR HOST, CLOCK AND LOCATION! fudge 127.127.27.0 stratum 1 time1 0.016 flag3 1 flag4 1 peer 18.104.22.168 # tick(1--2). peer 22.214.171.124 # tock(3), boot/NFS server. # This shouldn't get swept away unless left untouched for a long time. driftfile /var/tmp/ntp.drift #------------------------------------------------------------------------------ # RESTRICTIONS # ============ # By default, don't trust and don't allow modifications. Ignore in fact. restrict default ignore notrust nomodify # Allow others in our subnet to check us out... restrict 126.96.36.199 mask 255.255.255.0 nomodify notrust # Trust our peers for time. Don't trust others in case they are insane. restrict 127.127.27.0 nomodify restrict 188.8.131.52 nomodify restrict 184.108.40.206 nomodify # Allow anything from the local host. restrict 127.0.0.1There are a few #defines in the code that you might wish to play with:
Without this defined (and without ARCRON_MULTIPLE_SAMPLES below) a typical set of offsets reported and used to drive the clock-filter algorithm is (oldest last):
filtoffset= -4.32 -34.82 -0.78 0.89 2.76 4.58 -3.92 -2.17Look at that spike!
With this defined a typical set of offsets is:
filtoffset= -7.06 -7.06 -2.91 -2.91 -2.91 -1.27 -9.54 -6.70with the repeated values being some evidence of outlyers being discarded.
Each resync and result (plus gaining or losing MSF sync) is logged to the system log at level LOG_NOTICE; note that each resync drains the unit's batteries, so the syslog entry seems justified.
Syslog entries are of the form:
May 10 10:15:24 oolong ntpd: ARCRON: unit 0: sending resync command May 10 10:17:32 oolong ntpd: ARCRON: sync finished, signal quality 5: OK, will use clock May 10 11:13:01 oolong ntpd: ARCRON: unit 0: sending resync command May 10 11:14:06 oolong ntpd: ARCRON: sync finished, signal quality -1: UNKNOWN, will use clock anyway May 10 11:41:49 oolong ntpd: ARCRON: unit 0: sending resync command May 10 11:43:57 oolong ntpd: ARCRON: sync finished, signal quality 5: OK, will use clock May 10 12:39:26 oolong ntpd: ARCRON: unit 0: sending resync command May 10 12:41:34 oolong ntpd: ARCRON: sync finished, signal quality 3: OK, will use clock
ARC Rugby MSF Receiver page