Controlling Where in the Buffer to Read Samples

Default read behavior depends on if a Reference Trigger is configured. If there is no Reference Trigger, NI-DAQmx reads samples beginning with the first sample acquired with each subsequent read beginning where the previous one left off. If there is a Reference Trigger, NI-DAQmx reads samples beginning with the first pretrigger sample and cannot begin reading until the acquisition has finished. This default behavior can be changed by using the Relative To and Offset attributes/properties.

The place where a read begins is called the Current Read Position. Each time data is read, the Current Read Position is computed based on the settings of the Relative To and Offset attributes/properties. When there is no Reference Trigger, the default for Relative To is Current Read Position. When there is a Reference Trigger, the default for Relative To is First Pretrigger Sample. In either case, the default for Offset is 0. Changing the settings of these two attributes/properties controls where in the buffer data is read.

During a continuous acquisition, for example, you can always read the most recent 1000 points by setting Relative To to Most Recent Sample and Offset to -1000. Even when a Reference Trigger is configured, you can begin reading samples immediately by setting Relative To to First Sample.