Note If you are using channel names configured in the DAQ Channel Wizard, the signal level is treated as being relative to the physical units specified for the channel. For example, if you configure a channel called temperature to have a physical unit of Deg. C, the value you specify for the trigger signal level is relative to Deg. C. If you are not using channel names, the signal level is treated as volts.

In Figure 6-22, the analog trigger is set to start the data acquisition on the rising slope of the signal, when the signal reaches 3.2.

Figure 6-22. Diagram of an Analog Trigger

Figure 6-23 illustrates analog triggering for post-triggered data acquisition using a timeline. You configure your DAQ hardware in LabVIEW to begin taking data when the incoming signal is on the rising slope and when the amplitude reaches 3.2. Your DAQ device begins capturing data when the specified analog trigger conditions are met.

DAQ Device waits until analog trigger conditions are met.

Then …

Figure 6-23. Analog Triggering with Your DAQ Device

Analog Triggering Examples

Refer to the Acquire N Scans Analog Hardware Trig VI in the examples\ daq\anlogin\anlogin.llb for a common example of analog triggering in LabVIEW. This VI uses the Intermediate VIs to perform buffered acquisition, where data is stored in a memory buffer during acquisition. After the acquisition completes, the VI retrieves all the data from the memory buffer and displays it.

You must tell your device the conditions on which to start acquiring data.

In LabVIEW, you can acquire data both before and after an analog trigger signal. If the pretrigger scans is greater than 0, your device acquires data before the triggering conditions. It then subtracts the pretrigger scans value from the number of scans to acquire value to determine the number of scans to collect after the triggering conditions are met. If pretrigger scans is 0, then the number of scans to acquire is acquired after the triggering conditions are met.