Instructions

UM-0085-B09 DT80 Range User Manual Page 271
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The NTP server address (host address) may be specified as a numeric IP address (e.g. 10.2.30.212) or as a host
name (e.g.
ntp3.petacorp.com). Using a host name requires that the DT80 have access to a working DNS
(domain name system) server.
NTP Options
The above three profile settings are the minimum that need to be set in order to enable NTP updates. There are,
however, a number of settings that may be adjusted to suit particular applications.
 Time Request Interval
By default, the DT80 will perform a time request every 3599 seconds. Selecting a value of "not quite one hour" helps
ensure that the DT80's requests do not ever end up always occurring "on the hour". NTP servers tend to be busiest at
this time and this can cause reduced time accuracy for requests made at these times.
The request interval can be changed to any value from 30 minutes to 7 days, using the following profile:
PROFILE NTP BACKGROUND_PERIOD=time
where time is a time value such as
50M (50 minutes) or 97H (97 hours)
 Correction Limits
When the DT80 obtains a time value from the NTP server, it will do one of the following, based on the magnitude of the
required time adjustment:
• do nothing (the DT80 time is already "close enough")
• begin a gradual time adjustment (speed up or slow down the DT80 clock rate until it reaches the correct time)
• immediately set the DT80 time to the NTP server value
• do nothing (the time difference is very great – possibly the NTP server is faulty)
The following profiles set the thresholds between these behaviours:
•
PROFILE NTP MIN_CORRECTION=time specifies the smallest time adjustment that will be applied. By
default this is
50T, so if the DT80's time is within 50ms of the server time then nothing will be done.
•
PROFILE NTP MAX_SLEW_CORRECTION=time specifies the largest adjustment that will be performed
using the "gradual" adjustment method. By default this is 3 seconds (
3S). Setting this larger will reduce the
chance of time jumps in logged data, at the expense of possibly allowing larger time deviations from the true
time.
•
PROFILE NTP MAX_JUMP_CORRECTION=time specifies the largest adjustment that will be performed
using the "jump" adjustment method. If the time difference is greater than this then the DT80 will reject it and do
nothing. By default this setting is 24 hours (24H). This value allows a logger that has changed time zone to
receive its initial time update from NTP rather than having to be set manually. However you may wish to reduce
this value in order to prevent the DT80 clock being changed when the server time difference is unexpectedly
large – possibly due to a faulty server. Set to 0 to disable "jump" corrections altogether.
If all power to the DT80 is lost, including the internal lithium battery, then the DT80's clock will be reset to 1-Jan-1989. As
a special case, therefore, if the DT80 finds that its date is set to a value earlier than 2000 then it will apply any NTP
update, even though the "max jump correction" limit is exceeded.
 Gradual Adjustment Rate
By default, during a gradual time adjustment the DT80 clock will run 10% faster or slower. This rate can be adjusted
between 1% and 50% using the following profile:
PROFILE NTP SLEW_RATE=percent
Lower rates will give a more gradual time adjustment, which may be preferable if your data is being logged at a fast rate,
at the expense of the adjustment process taking longer.
 Communications Timing
When the DT80 sends a time request, it expects a timely reply. A late reply (e.g. due to network congestion) may be
worse than no reply at all, given that the purpose of the message is to accurately set the DT80 time. The default
communications timeout for NTP is 2 seconds (
2S). This may be made shorter or longer using:
PROFILE NTP TIMEOUT=time
 Sleep Wakeup Timing
Each time the DT80 wakes from sleep, it will attempt to perform an NTP time update. However, it can take a little time for
the network to become operational following wake – for example a DHCP query may need to be done. By default
therefore, the DT80 will, after waking up, wait 3 seconds before attempting an NTP request. This may be adjusted up or
down using the following profile:
PROFILE NTP BACKGROUND_WAKEUP_DELAY=time