The conditional retrieval cluster of the AI Read VI specifies the analog signal conditions of retrieval, as shown in Figure 6-25.

Figure 6-25. The AI Read VI Conditional Retrieval Cluster

Tip The actual data acquisition is started by running your VI. The conditional retrieval just controls how data already being acquired is returned.

When acquiring data with conditional retrieval, you typically store the data in a memory buffer, similar to hardware triggering applications. After you start running the VI, the data is placed in the buffer. Once the retrieval conditions have been met, the AI Read VI searches the buffer for the desired information. As with hardware analog triggering, you specify the analog channel of the triggering signal by specifying its channel index, an index number corresponding to the relative order of a single channel in a channel list. You also specify the slope (rising or falling) and the level of the trigger signal.

Note The channel index might not be equal to the channel value. You can use the Channel to Index VI to get the channel index for a channel. You can find this VI on the

Data Acquisition»Calibration and Configuration palette.

The AI Read VI begins searching for the retrieval conditions in the buffer at the read/search position, another input of the AI Read VI. The offset,

a value of the conditional retrieval input cluster, is where you specify the scan locations from which the VI begins reading data relative to the read/search position. A negative offset indicates data prior to the retrieval condition pretrigger data, and a positive offset indicates data after the retrieval condition posttrigger data. The skip count input allows you to specify the number of times the trigger conditions are met and skipped before data is returned. The hysteresis input controls the range used to meet the retrieval conditions. It is useful when your signal has noise that might inadvertently trigger your acquisition. Once the slope and level conditions on channel index have been found, the read/search position indicates the location where the retrieval conditions were met.