about the property "RelativeTo" in DAQmx

[老王]

The Reference says that "The "Relative to" property Specifies the point in the buffer at which to begin a read operation."

"DAQmx_Val_FirstSample" means "Start reading samples relative to the first sample acquired".

and "DAQmx_Val_FirstPretrigSamp" means "Start reading samples relative to the first pretrigger sample. You specify the number of pretrigger samples to acquire when you configure a reference trigger. "

My question is : what's the difference between "the first sample acquired" and "first pretrigger sample"?
In a "reference trigger " condition,are they -- "the first sample acquired" and "first pretrigger sample" mean the same sample(the first sample acquired after receiving the "start" command,which are sent by a command or a starttrigger?)? Or in another word, what are "pre-trigger samples"?

[thayles]

The first sample is acquired when the task is started (either by a software command or a hardware start trigger).

You have pretrigger samples *only* when you have also configured a Reference trigger. A Reference trigger is called that because it establishes the reference point that separates the pretrigger data (acquired before the reference trigger occurred) from the posttrigger data (acquired after the reference trigger occurred.

Once a reference triggered task is started, it operates in a continuous mode until the reference trigger occurs. When the reference trigger occurs, a number of posttrigger points are acquired and the task then stops. After it stops, the location of the earliest (first) pretrigger data point in the buffer is known to DAQmx and DAQmx can return data based on that location in your buffer.

[老王]

Thank you for reply!