PXI_Clk10 provides frequency but not date/time information. When an NI-XNET session is
created, XNET initializes the date/time information for the local clock using host time.
Network Time
In addition to the local time keeper, the PXIe-8521 can maintain network time (IEEE
802.1AS) for each port. When Ethernet frames are received, each packet is timestamped with
network time as well as with local time.
When a port acts as a master in an electronic control unit (ECU) network, the network time is
initialized from host time and is synchronized to local time.
When a port acts as a slave in an ECU network, local time and network time may eventually
drift relative to each other. The date/time information for network time is obtained from the
ECU that acts as the grandmaster clock.
Both local and network time can be adjusted using the NI-XNET API.
Host Time
Host time is the clock of the operating system on which the NI-XNET driver is running. The
host time can obtain time/date information using a real time clock (RTC) or a network time
protocol (NTP) server.
Although host time provides accurate date/time information, the accuracy and resolution of its
clock can often be in tens of milliseconds. In contrast, the PXIe-8521 provides resolution for
local time and network time in nanoseconds. Although local time and network time use host
time to initialize their date/time information, they do not use the same physical clock as host
time. Therefore, both local time and network time may eventually drift relative to host time.
PXI Triggers
Each of the eight PXI triggers on the PXIe-8521 can be simultaneously timestamped by the
local time keeper and the network time keeper for each port. PXI triggers can be used to
synchronize the PXIe-8521 time keepers with trigger events on other PXI modules.
PXIe-8521 User Manual
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© National Instruments
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