Intermittent function block errors that show up as a red X on an output in Control Studio in online mode and/or block errors that come and go in Event Viewer could be caused by an incorrect module execution time or an incorrect requested macrocycle. The module execution time and the macrocycles are independent. Module execution time determines how often a module executes while the macrocycle is the execution time for a single iteration of the fieldbus function blocks running in all the devices on the port. The requested macrocycle is user-specified and the calculated macrocycle is system-calculated. The calculated macrocycle time includes an adjustment that allocates time for unscheduled data transfers. As a rule of thumb, the module execution (scan) rate should be >= 2 times the actual macrocycle. For example, if the macrocycle is 1, set the module scan rate to 2 or greater. Modules with all blocks running in field devices or the H1 card can run with a module scan rate equal to or greater than the macrocycle.