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Table Of Contents

Time-Based Triggers

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    Last Modified: March 12, 2018

    FieldDAQ devices feature automatic network-based synchronization with compatible networks and 802.1AS-capable NI Linux Real-Time controllers. The SYNC logo on the device front panel indicates that the device is capable of hardware-based synchronization over a network.

    FieldDAQ devices can be daisy-chained together or connected to external networks that support 802.1AS synchronization, and all device timebases will be automatically synchronized. Refer to Synchronization across a Network for more information about supported topologies and other technical requirements.

    Network-synchronized devices can also take advantage of time-based synchronization features in NI-DAQmx. Certain triggers can be specified in terms of time of day. Time-based triggers and multidevice tasks (spanning multiple network-synchronized FieldDAQ devices) can help simplify programming for large systems.

    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 802.1AS network-synchronized devices on your subnet.
    • Host Time—The time on your 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.


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