The USB-6421 has a flexible timing engine.

The following figure summarizes all of the timing options provided by the analog input timing engine.

Figure 27. USB-6421 Analog Input Timing Options


The USB-6421 uses AI Sample Clock (ai/SampleClock) and AI Convert Clock (ai/ConvertClock) to perform interval sampling. As the following figure shows, AI Sample Clock controls the sample period, which is determined by the following equation:

1 / S a m p l e ⁢ P e r i o d ⁢ =   S a m p l e ⁢ R a t e
Figure 28. Interval Sampling


AI Convert Clock controls the Convert Period, which is determined by the following equation:

1 / C o n v e r t ⁢ P e r i o d ⁢ =   C o n v e r t ⁢ R a t e

Post-triggered data acquisition allows you to view only data that is acquired after a trigger event is received. A typical post-triggered DAQ sequence is shown in the following figure. The sample counter is loaded with the specified number of post-trigger samples, in this example, five. The value decrements with each pulse on AI Sample Clock until all desired samples have been acquired.

Figure 29. Post-triggered Data Acquisition Example


Pre-triggered data acquisition allows you to view data that is acquired before the trigger of interest, in addition to data acquired after the trigger. The following figure shows a typical pre-triggered DAQ sequence. AI Start Trigger (ai/StartTrigger) can be either a hardware or software signal. If AI Start Trigger is set up to be a software start trigger, an output pulse appears on the ai/StartTrigger line when the acquisition begins. When the AI Start Trigger pulse occurs, the sample counter is loaded with the number of pre-triggered samples, in this example, four. The value decrements with each pulse on AI Sample Clock. The sample counter is then loaded with the number of post-triggered samples, in this example, three.

Figure 30. Pre-triggered Data Acquisition Example


If an AI Reference Trigger (ai/ReferenceTrigger) pulse occurs before the specified number of pre-trigger samples are acquired, the trigger pulse is ignored. Otherwise, when the AI Reference Trigger pulse occurs, the sample counter value decrements until the specified number of post-trigger samples have been acquired.