Solution
Both functions are used to block execution for a specified period of time, usually in order to control the rate at which a loop will execute. The difference is that the Wait (ms) function simply blocks until the time specified at milliseconds to wait elapses, but the Wait Until Next ms Multiple function blocks until the current value of the system millisecond clock is evenly divisible by the value specified at millisecond multiple.
The figure below illustrates when execution would be unblocked if both VIs had 100 as their respective inputs and started running at 50ms. The horizontal axis is the value of the system millisecond clock.
You can also observe the difference in execution using the following code examples.
Wait (ms):
The Wait (ms) function will block execution until the time specified at
milliseconds to wait has
elapsed.
- Example 1: Consider a loop with code that takes 5 ms to execute. The loop also includes a Wait (ms) function with 10 ms wired to its milliseconds to wait input. The While Loop takes 10 ms total to execute because the code finishes after 5 ms and then the Wait (ms) function finishes 5 ms later. In this case, the Wait (ms) function executes in parallel with the code.
In this example the loop timing is set by the outer Wait (ms) function.
- Example 2: The loop is the same as in the previous example, but the code now takes 15 ms to execute. The outer Wait (ms) function still has 10 ms wired to its milliseconds to wait input. Because the time wired to the Wait (ms) function is less than the time it takes the code to execute, there is no delay after the code finishes, and the loop moves to the next iteration immediately after 15 ms elapses.
In this example the loop timing is not set by the Wait (ms) function. It is set by the overall execution time of the code within the loop.
Wait Until Next ms Multiple:
You can use the Wait Until Next ms Multiple function to synchronize separate loops to your system's millisecond clock. As the name suggests, it will wait until the next multiple of the number of milliseconds specified at the
milliseconds multiple input before becoming unblocking.
- Example 3: There is a loop with code that takes 100 ms to execute and a Wait Until Next ms Multiple function running in parallel with the code. The Wait Until Next ms Multiple function has 200 ms wired to its millisecond multiple input. The loop executes at every 200 ms multiple of the system's millisecond clock. There is second loop in the same VI that has a Wait Until Next ms Multiple function with 200 ms wired to its millisecond multiple input. The code in the second loop takes 150 ms to execute running in parallel. The two loops will be synchronized to move to their next iterations every time a multiple of 200 ms on the system clock elapses. Using this method, it will be insured that each loop begins subsequent iterations at the same time.