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Transitioning the State Backwards

Last Modified: September 15, 2017

When a task is implicitly transitioned backwards, it returns to the state of the task prior to the last operation that resulted in a forward state transition. For example, if the task was in the Verified state and you called the Start Task function/VI to start the task, the task is reserved, committed, and started, transitioning to the Reserved state and to the Committed state before transitioning to the Running state. When you invoke the Stop Task function/VI, the task is not just stopped and transitioned from the Running state to the Committed state. If this were the case, the result is unexpected because the task still has its resources reserved despite the fact that you never explicitly reserved them. Instead, the task is stopped, uncommitted, and unreserved, returning to the Verified state, its state immediately before you performed the last operation that resulted in the state transition, calling the Start Task function/VI.

As another example, suppose the task is in the Reserved state, and you call the Read function/VI to perform a finite measurement. This results in the task implicitly transitioning from the Reserved state to the Committed state and then to the Running state before performing the read operation. When the read operation completes, the task does not remain in the running state. If this were the case, the result is unexpected behavior, because you need to stop the task and unreserve its resources despite the fact you never explicitly reserved the resources or started the task. Instead, after the finite read operation completes, the task is implicitly transitioned from the Running state to the Committed state to the Reserved state. This results in the task returning to the state before you performed the read operation.

Keep in mind that setting the value of a channel, timing, or triggering attribute/property does not implicitly transition the task back to the Unverified state. Instead, the task remains in its current state and is implicitly verified when the next state transition occurs. For example, if the task is in the Reserved state and you set the value of timing attribute/property, the task remains in the Reserved state. The next time the task, either implicitly or explicitly, is committed, the task is verified. Because the task is implicitly verified when the next state transition occurs, NI-DAQmx can return an error specifying that the value of attribute/property is invalid.


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