The user-specified Requested macrocycle option runs all blocks on the port at the same scan rate (options are: 150 ms, 250 ms, 500 ms, 1 second, 2 seconds, or 5 seconds). When this option is selected, the Block Scan Rate selection is disabled for fieldbus blocks. Furthermore with this option you must carefully coordinate module execution rates with the macrocycle or scheduling problems can occur. Also, control performance can be impacted when modules with different scan rates run on the same segment.
The DeltaV system attempts to run the blocks at the requested rate. However if the requested rate cannot be achieved, the DeltaV system uses the greater of the requested or calculated macrocycle. This is called the actual macrocycle. These guidelines explain how the actual macrocycle is determined:
The actual macrocycle is equal to the requested macrocycle if the calculated macrocycle is less than or equal to (<=) the requested macrocycle.
The actual macrocycle is equal to the calculated macrocycle if the calculated macrocycle is greater than (>) the requested macrocycle.
The following table provides some examples that show how the actual macrocycle is determined:
|
If the requested macrocycle is: |
and the calculated macrocycle is: |
then the actual macrocycle is: |
|---|---|---|
|
1 second |
0.6 seconds |
1 second |
|
1 second |
1.2 seconds |
1.2 seconds |
|
0.5 seconds |
1 second |
1 second |
|
0.5 seconds |
0.8 seconds |
0.8 seconds |
Each segment that is connected to a Series 2 H1 card is capable of supporting at least one, 200 ms module by setting the requested macrocycle to 150 ms. To avoid communications problems such as download timeouts and slow AMS Device Manager updates, keep the number of devices on the segment to a minimum if a 150 ms macrocycle is used.