Time triggers can be specified in Host Time or I/O Device Time, depending on the needs of
your application.
•
I/O Device Time
—The time the FieldDAQ device uses internally. This time is
determined by the network configuration and is shared by all IEEE 802.1AS
network-synchronized devices on your subnet.
•
Host Time
—The time on your Windows computer or NI Linux Real-Time controller.
This is usually the current global time and is provided by a local real-time clock or a
network time protocol (NTP) server.
NI-DAQmx automatically translates from Host Time to I/O Device Time as necessary. The
accuracy of this translation depends on the relationship between these times and can reduce the
relative accuracy of time triggers across multiple devices. For maximum accuracy, use an
NI Linux Real-Time controller as the host in a supported topology. However, NI-DAQmx
guarantees that two tasks configured to start at the same host time always start at the same
I/O Device Time in all scenarios, preserving precise synchronization between chassis in this
common use case. Refer to the
Time Triggering
topic in the
NI-DAQmx Help
for more
information on accessing time-based features in the NI-DAQmx API.
Timebases
The following figure shows the FD-11634 clock routing circuitry and timebases.
Figure 20. Clock Routing Circuitry
13.1072 MHz Timebase
12.288 MHz Timebase
12.8 MHz Timebase
Clock
Generator
Onboard
100 MHz
Oscillator
10.24 MHz Timebase
You can use the 13.1072 MHz, 12.8 MHz, 12.288 MHz, and 10.24 MHz timebases to generate
the AI Sample Clock signals. These timebases are generated directly from the onboard clock
generator. By default, NI-DAQmx automatically chooses the most suitable timebase for a
selected data rate. You can also manually select a timebase through the NI-DAQmx API.
Refer to the
FieldDAQ Timing Considerations
and
Master Timebase Synchronization
topics in
the
NI-DAQmx Help
for more information about configuring timing in the NI-DAQmx API.
Synchronization across a Network
The onboard 100 MHz oscillator automatically synchronizes to other network-synchronized
devices that are part of your local IEEE 802.1AS subnet.
The 13.1072 MHz, 12.8 MHz, 12.288 MHz, and 10.24 MHz Timebases are derived from the
oscillator, and are synchronized to it. Therefore, they are also synchronized to other network-
FD-11634 User Guide
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© National Instruments
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