You can time control loops using software timing or hardware timing. You can also use the Timed Loop structure.
For software timing, the software and operating system determines the rate at which the loop executes. Software timing is not deterministic. Controlling a while loop and using the Wait Until Next ms Multiple VI to handle timing is an example of a software-timed loop. Hardware timing uses the DAQ device internal clock or an external clock to control when a read executes within a loop. The example block diagram shown in Control Loops in the Common Applications section uses hardware timing.
The Timed Loop structure is hardware timed. It is ideal for multirate applications. By default, the Timed Loop uses the 1 kHz clock of the Windows operating system as its timing source. Refer to your LabVIEW Help for more information about the Timed Loop structure.