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Standard Program Sweep Variations
In addition to the normal execution of the standard program sweep, certain variations can be encountered or forced. These variations, described in the following paragraphs, can be displayed and/or changed from the programming software.
Constant Sweep Time Mode
In the standard program sweep, each sweep executes as quickly as possible with a varying amount of time consumed each sweep. An alternative to this is CONSTANT SWEEP TIME mode, where each sweep consumes the same amount of time. You can achieve this by setting the Configured Constant Sweep, which will then become the default sweep mode, thereby taking effect each time the PLC goes from STOP to RUN mode. A value from 5 to 200 milliseconds (or up to 500 milliseconds for the 350 and 360 series PLC CPUs) for the constant sweep timer (default is 100 milliseconds) is supported.
Due to variations in the time required for various parts of the PLC sweep, the constant sweep time should be set at least 10 milliseconds higher than the sweep time that is displayed on the status line when the PLC is in NORMAL SWEEP mode. This prevents the occurrence of extraneous oversweep faults.
Use a constant sweep when I/O points or register values must be polled at a constant frequency, such as in control algorithms. One reason for using CONSTANT SWEEP TIME mode might be to ensure that I/O are updated at constant intervals. Another reason might be to ensure that a certain amount of time elapses between the output scan and the next sweep’s input scan, permitting inputs to settle after receiving output data from the program.
If the constant sweep timer expires before the sweep completes, the entire sweep, including the windows, is completed. However, an oversweep fault is logged at the beginning of the next sweep.
Note
Unlike the Active Constant Sweep which can be edited only in RUN mode, the Configured Constant Sweep Mode can be edited only during STOP mode and you must “Store the configuration from the Programmer to the PLC” before the change will take effect. Once stored, this becomes the default sweep mode.
PLC Sweep When in STOP Mode
When the PLC is in STOP mode, the application program is not executed. Communications with the programmer and intelligent option modules continue. In addition, faulted board polling and board reconfiguration execution continue while in STOP mode. For efficiency, the operating system uses larger
Chapter 2 System Operation |
