User manual

Section 11: LXI Class B Triggering (IEEE-1588) Series 3700 System Switch/Multimeter Reference Manual
11-2 3700S-901-01 Rev. C / July 2008
At periodic intervals, the master clock synchronizes to all slave clocks through
time-stamped messages over the PTP port. This allows IEEE-1588 to maintain
time synchronization between multiple devices on a network.
Program the synchronization interval in the Series 3700 using
ptp.syncinterval (on page 13-229). The default synchronization interval is
two seconds. Increasing the synchronization interval to values of more than two
seconds increases the amount of time that it takes devices on the LAN to
synchronize. If you change the synchronization interval, you must restart the
clock of the Series 3700 by cycling its power.
Read the current time delay and offset between any slave device and its master
on the LAN using ptp.ds.current (on page 13-223). Synchronization of time
stamps between IEEE-1588 enabled devices to within 150ns can take as long
as 2 minutes.
Correlating PTP to Coordinated Universal Time (UTC)
To ensure synchronization across networked devices, you must be aware of the
time protocol utilized by those other devices on the network.
The most widely accepted time scale is Coordinated Universal Time (UTC); in
many places, it is considered standard time. UTC is nearly the same time as
Greenwich Mean Time (GMT), another very familiar time scale, and for the
purposes of the Series 3700, UTC and GMT are the same. Local time is offset
from UTC according to time zones; additional offsets can occur due to Daylight
Savings Time adjustments.
UTC suffers from discontinuities because of non-periodic adjustments known as
“leap seconds”. These adjustments present problems because they can make
events that occurred at different periods of time appear to occur at the same
time. PTP is a time standard that does not have any discontinuities and has no
adjustments for local time (that is, it is not time-zone aware). PTP is presented
as the number of seconds since January 1, 1970.
The Series 3700 offers two versions of time for most IEEE-1588-related
commands, .seconds and .ptpseconds, representing UTC and PTP
respectively. IEEE-1588 requires that devices are synchronized using UTC or
PTP time, not local time. The Series 3700 does not distinguish UTC, PTP, and
local time; it is not time-zone aware. You must be aware of this when
synchronizing with devices that are time-zone aware.
When IEEE-1588 selects a time-zone aware device to be the master clock, the
Series 3700 accepts the time of that clock. This time may not agree with the
local time of the Series 3700, especially when a network spans multiple time
zones. If you schedule events on the Series 3700 to occur according to your
local time, events will not occur at the time you expect.