Complete the following steps before you start acquiring data.

1.Specify in the trigger slope input whether to trigger the acquisition on the rising or falling edge of the analog trigger signal.

2.Enter the trigger channel to use for connecting the analog triggering signal.

3.Specify the trigger level on the triggering signal needed to begin acquisition.

After you specify the channel of the triggering signal, LabVIEW waits until the slope and trigger level conditions are met before starting a buffered acquisition. If you use channel names configured in the DAQ Channel Wizard, trigger level is treated as being relative to the physical units specified for the channel in the DAQ Channel Wizard. Otherwise, trigger level is treated as volts.

The Acquire N Scans Analog Hardware Trig VI example, available in the examples\daq\anlogin\anlogin.llb, holds the data in a memory buffer until the device completes data acquisition. The number of data points you want to acquire must be small enough to fit in memory. This VI views and processes the information only after the acquisition. Refer to the Acquire & Proc N Scans-Trig VI in the examples\daq\anlogin\ anlogin.llb if you need to view and process information during the acquisition. Refer to the example Acquire N-Multi-Analog Hardware Trig VI in the examples\daq\anlogin\anlogin.llb if you expect multiple analog trigger signals that will start multiple acquisitions.

Software Triggering

With software triggering, you can simulate an analog trigger using software. This form of triggering is often used in situations where hardware triggers are not available. Another name for software triggering signals, specifically analog signals, is conditional retrieval. With conditional retrieval, you set up your DAQ device to collect data, but the device does not return any data to LabVIEW unless the data meets your retrieval conditions. LabVIEW scans the input data and performs a comparison with the conditions, but does not store the data until it meets your specifications. Figure 6-24 shows a timeline of events that typically occur when you perform conditional retrieval.

The read/search position pointer traverses the buffer until it finds the scan location where the data has met the retrieval conditions. Offset indicates the scan location from which the VI begins reading data relative to the read/search position. A negative offset indicates that you need pretrigger

data (data prior to the retrieval conditions). If offset is greater than 0, you need posttrigger data (data after retrieval conditions).

When trigger conditions are met (at Scan 4)

Figure 6-24. Timeline of Conditional Retrieval