When an action is triggered (such as a Control, or One Shot), the relevant driver creates a request for the action.
- When a request can be performed immediately, it will be completed straight away. For example, requests that do not require any communication with an outstation or similar device are usually completed immediately.
- If the request cannot be performed immediately, it is placed in a queue of pending requests. When a channel is available, the pending request with the highest priority becomes active and is passed to the channel's scanner to be performed.
By defining the request timeout, you can determine the maximum amount of time a request can be placed in the pending requests queue. If the request has not started within the defined timeout time, the request is aborted and removed from the queue. However, if the request is in progress, it is given an additional amount of time in which to complete. If the request has not completed at the end of the additional amount of time, ClearSCADA detects that the request is timed out and then fails the request.