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A timing source controls the execution of a timed structure. You can select from two types of timing sources—internal or external. Internal timing sources refer to the built-in timing sources that you select using the configuration for the Input node of a timed structure. External timing sources refer to timing sources that you create using the Create Timing Source VI and the DAQmx - Data Acquisition VIs.

Internal Timing Sources

Internal timing sources for controlling a timed structure include the 1 kHz clock of the operating system and the 1 MHz clock of a supported real-time (RT) target. Use the Loop Timing Source or Sequence Timing Source section of the Configure Timed Loop, Configure Timed Sequence, or Configure Timed Loop with Frames dialog boxes to select an internal timing source.

• 1 kHz Clock—By default, a timed structure uses the 1 kHz clock of the operating system as the timing source. Using the 1kHz clock, a timed structure can execute an iteration once every 1 millisecond. All LabVIEW platforms that can run a timed structure support the 1 kHz timing source.
• 1 MHz Clock—Supported targets can use the 1 MHz clock of the target processor to control a timed structure. Using the 1 MHz clock, a timed structure can execute an iteration once every 1 microsecond. If the target does not include a supported processor, the 1 MHz clock is not available for selection.
• 1 kHz Clock <reset at structure start>—A timing source similar to the 1 kHz clock that resets to 0 after every iteration of a timed structure.
• 1 MHz Clock <reset at structure start>—A timing source similar to the 1 MHz clock that resets to 0 after every iteration of a timed structure.

External Timing Sources

You can create external timing sources for controlling a timed structure with NI-DAQmx 7.2 or later. Use the Create Timing Source VI to programmatically select an external timing source. You also can use several types of NI-DAQmx timing sources, including frequency, digital edge counters, digital change detection, and signals from task sources, to control timed structures. Use the DAQmx - Data Acquisition VIs to create the following types of NI-DAQmx timing sources to control a timed structure.

• Frequency—Creates a timing source that causes a timed structure to execute at a constant frequency.
• Digital Edge Counter—Creates a timing source that causes a timed structure to execute on rising or falling edges of a digital signal.
• Digital Change Detection—Creates a timing source that causes a timed structure to execute on rising or falling edges of one or more digital lines.
• Signal from Task—Creates a timing source that uses the signal you specify to determine when a timed structure executes.

Refer to the NI-DAQmx Help, available by selecting Start»All Programs»National Instruments»NI-DAQ»NI-DAQmx Help, for information about using NI-DAQmx VIs and functions to control timed structures.

You can use a secondary timing source to control the execution of the frames of a timed structure. For example, you can use a 1 kHz clock to control the iterations of a Timed Loop and use a 1 MHz clock to control the timing of the frames in every iteration.