Arcron MSF Receiver

Last update: May 3, 2023 17:13 UTC (e8077bcf6)

Table of Contents


Address: 127.127.27.u

Reference ID: MSFa / MSF / DCF / WWVB


Serial Port: /dev/arcu; 300 baud, 8-bits, 2-stop, no parity

Features: tty_clk


This driver supports the Arcron MSF, DCF and WWVB receivers. The clock reports its ID as MSFa, MSF, DCF, or WWVB to indicate the time source.

This documentation describes v1.3 (2003/2/21) of the source and has been tested against ntpd 4.1.0 on linux x86. Changes from v1.1 and v1.2 include patches to work with the new ntp4 code, clock support for DCF and WWVB configurable via mode flag, an option to ignore resync request (for those of us at the fringes of the WWVB signal, for instance), averaging of the signal quality poll and several bug fixes, code cleanup and standardizations. In all other respects, the driver works as per v1.1 if a mode is not specified.

To use the alternate modes, the mode flag must be specified. If the mode flag is 0, or unspecified, the original MSF version is assumed. This should assure backwards compatibility and should not break existing setups.

The previous documentation described version V1.1 (1997/06/23) of the source and had 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). That code claimed 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 The code was originally made to work with the clock by Derek Mulcahy, with modifications by Damon Hart-Davis.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. Paul Alfille added support for the WWVB clock. Christopher Price added enhanced support for the MSF, DCF and WWVB clocks.

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). In the default or original MSF mode, 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. In modes 1-3 the driver can be configured for UTC or local time depending on the setting of flag1.

The clock claims to be accurate to within about 20ms of the 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 stratum 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.

In default mode, 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 with up to a day’s clock drift in the worst case (the clock by default resyncs to MSF once per day).

When alternate modes 1-3 are selected, the driver can be configured to ignore the resync requests by setting flag2 to 1. This allows clocks at the fringe of the signal to resync at night when signals are stronger.

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:

This driver, running on standard Sun and x86 hardware, seems to work fine; note the use of the send_slow() routine to queue up command characters to be sent once every two seconds.

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 case 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:

g CR
Request for signal quality. Answer only valid during (late part of) resync to MSF signal. The response consists of two characters as follows:
First Bit Number Description Second Bit Number Description
7 parity 7 parity
6 always 0 6 always 0
5 always 1 5 always 1
4 always 1 4 always 1
3 always 0 3 always 0
2 always 0 2–0 reception signal quality in the range 0–5 (very poor to very good); if in the range 0–2 no successful reception is to be expected. The reported value drops to zero when not resyncing, ie when first returned byte is not 3.
1 always 1
0 0 if no reception attempt at the moment; 1 if reception attempt (ie resync) in progress
h CR
Request to resync to signal. Can take up from about 30s to 360s. Drains batteries so should not be used excessively. After this the clock time and date should be correct and the phase within 20ms of time as transmitted from the source signal (remember to allow for propagation time). By default the clock resyncs once per day in the late evening/early morning (presumably to catch transitions to/from daylight saving time quickly). This driver code, by default, resyncs at least once per hour to minimise clock wander.
o CR
Request timestamp. Start bit of first byte of response is on-time, so may be delayed up to 1 second. Note that the driver will convert time to GMT, if required. The response data is as follows:
  1. hours tens (hours range from 00 to 23)
  1. hours units
  1. minutes tens (minutes range from 00 to 59)
  1. minutes units
  1. seconds tens (seconds presumed to range from 00 to 60 to allow for leap second)
  1. seconds units
  1. day of week 1 (Monday) to 7 (Sunday)
  1. day of month tens (day ranges from 01 to 31)
  1. day of month units
  1. month tens (months range from 01 to 12)
  1. month units
  1. year tens (years range from 00 to 99)
  1. year units
  1. BST/UTC status (Ignored in WWVB version)
Bit Number Description
7 parity
6 always 0
5 always 1
4 always 1
3 MSF: always 0
WWVB: Leap year indicator bit
0 = non-leap year
1 = leap year
2 MSF: 1 if UTC is in effect (reverse of bit 1)
WWVB: Leap second warning bit
1 MSF: 1 if BST is in effect (reverse of bit 2)
WWVB: 0 if ST is in effect
1 if DST is in effect
1 if transition from ST with bit 0 is set to 0
0 MSF: 1 if BST/UTC change pending
WWVB: 0 if ST is in effect
1 if DST is in effect
0 if transition from DST with bit 1 is set to 0
  1. clock status
Bit Number Description
7 parity
6 always 0
5 always 1
4 always 1
3 1 if low battery is detected
2 1 if last resync failed (though officially undefined for the MSF clock, officially defined for WWVB)
1 1 if at least one reception attempt was successful
MSF: since 0230
DCF: since 0300
WWVB: resets if not successful between 0300-0400
0 1 if the clock has valid time—reset to zero when clock is reset (eg at power-up), and set to 1 after first successful resync attempt.

The driver only accepts time from the clock if the bottom three bits of the status byte are 011 or flag2 is set to 1 to ignore resync requests. For the MSF clock, if the UK parliament decides to move us to +0100/+0200 time as opposed to the current +0000/+0100 time, it is not clear what effect that will have on the time broadcast by MSF, and therefore on this driver’s usefulness.

A typical ntp.conf configuration file for this driver might be:

# hostname(n) means we expect (n) to be the stratum at which hostname runs.

# ========================

# Default configuration (Original MSF mode)s...
server mode 333 # ARCRON MSF radio clock
# Fudge stratum and other features as required.
fudge stratum 1 time1 0.016 flag3 1
# WWVB users should change that line to:
server mode 3 # ARCRON WWVB radio clock
fudge stratum 1 time1 0.030 flag1 1 flag3 1

peer # tick(1--2).
peer # tock(3), boot/NFS server.

# This shouldn't get swept away unless left untouched for a long time.
driftfile /var/tmp/ntp.drift

# ============

# 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 mask nomodify notrust

# Trust our peers for time.  Don't trust others in case they are insane.
restrict nomodify
restrict nomodify
restrict nomodify

# Allow anything from the local host.

There are a few #defines in the code that you might wish to play with:

With this defined, the code is relatively trusting of the clock, and assumes that you will have the clock as one of a few time sources, so will bend over backwards to use the time from the clock when available and avoid ntpd dropping sync from the clock where possible. You may wish to undefine this, especially if you have better sources of time or your reception is ropey. However, there are many checks built in even with this flag defined.
When is defined, we regard each character in the returned timecode as at a known delay from the start of the second, and use the smallest (most negative) offset implied by any such character, ie with the smallest kernel-induced display, and use that. This helps to reduce jitter and spikes.
When is defined, we try to do a resync to MSF as soon as possible in the first hour of the morning of the first day of the first and seventh months, ie just after a leap-second insertion or deletion would happen if it is going to. This should help compensate for the fact that this clock does not continuously sample MSF, which compounds the fact that MSF itself gives no warning of an impending leap-second event. This code did not seem functional at the leap-second insertion of 30th June 1997 so is by default disabled.
Currently set to -4, but you may wish to set it to -5 if you are more conservative, or to -6 if you have particularly good experience with the clock and you live on the edge. Note that the flag3 fudge value will improve the reported dispersion one notch if clock signal quality is known good. So maybe just leave this alone.

Monitor Data

Each timecode is written to the clockstats file with a signal quality value appended (0-5 as reported by the clock, or 6 for unknown).

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[615]: ARCRON: unit 0: sending resync command
May 10 10:17:32 oolong ntpd[615]: ARCRON: sync finished, signal quality 5: OK, will use clock
May 10 11:13:01 oolong ntpd[615]: ARCRON: unit 0: sending resync command
May 10 11:14:06 oolong ntpd[615]: ARCRON: sync finished, signal quality -1: UNKNOWN, will use clock anyway
May 10 11:41:49 oolong ntpd[615]: ARCRON: unit 0: sending resync command
May 10 11:43:57 oolong ntpd[615]: ARCRON: sync finished, signal quality 5: OK, will use clock
May 10 12:39:26 oolong ntpd[615]: ARCRON: unit 0: sending resync command
May 10 12:41:34 oolong ntpd[615]: ARCRON: sync finished, signal quality 3: OK, will use clock

Fudge Factors

mode 0 | 1 | 2 | 3

Specifies the clock hardware model. This parameter is optional, it defaults to the original mode of operation.

Supported modes of operation:
0 - Default, Original MSF
1 - Updated MSF
2 - New DCF77
3 - New WWVB
time1 time

Specifies the time offset calibration factor, in seconds and fraction, with default 0.0. On a Sun SparcStation 1 running SunOS 4.1.3_U1, with the receiver in London, a value of 0.020 (20ms) seems to be appropriate.

time2 time

Not currently used by this driver.

stratum number

Specifies the driver stratum, in decimal from 0 to 15, with default 2. It is suggested that the clock not be fudged higher than stratum 1 so that it is used as a backup time source rather than a primary when more accurate sources are available.

refid string

Specifies the driver reference identifier, an ASCII string from one to four characters, with default MSFa. When used in modes 1-3, the driver will report either MSF, DCF, or WWVB respectively.

flag1 0 | 1

(Modes 1-3) If set to 0 (the default), the clock is set to UTC time. If set to 1, the clock is set to localtime.

flag2 0 | 1

(Modes 1-3) If set to 0 (the default), the clock will be forced to resync approximately every hour. If set to 1, the clock will resync per normal operations (approximately midnight).

flag3 0 | 1

If set to 1, better precision is reported (and thus lower dispersion) while clock’s received signal quality is known to be good.

flag4 0 | 1

Not used by this driver.

Additional Information

Reference Clock Drivers
ARC Rugby MSF Receiver