The module scan rate as defined in Control Studio and the macrocycle are independent. The module scan rate determines how often the module executes while the macrocycle determines how often the fieldbus function blocks on the port execute. The macrocycle is selected through the Requested macrocycle option on the fieldbus Port Properties dialog.
The DeltaV system supports two macrocycle options:
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A user-specified scan rate (150, 250, 500 ms, 1, 2, or 5 seconds) for all blocks running on the port
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Module-driven that runs blocks at different scan rates and allows fast and slow loops to run concurrently
The Calculated macrocycle value also appears on the General tab of the Port Properties dialog. The Calculated macrocycle time, which is produced by the DeltaV system, includes an adjustment that allocates time for unscheduled data transfers and minimum schedule spacing. The topic FOUNDATION Fieldbus Technology Overview has a discussion of unscheduled data transfers. The minimum schedule spacing is the spacing between consecutive compel data messages. Click the Advanced tab to read the minimum schedule spacing. The minimum schedule spacing value is writeable to maintain compatibility with older fieldbus devices; however, it should be changed only if recommended by Emerson. Reducing the value can cause communication problems and increasing the value can reduce control loop performance. The default value has been tested to work properly with all approved fieldbus devices. To access a visual representation of the macrocycle, click the View Schedule button on the Advanced tab to launch the Macrocycle Viewer application.