Rockwell Automation Publication 2080-UM002K-EN-E - March 2019 77
Program Execution in Micro800 Chapter 6
Controller Load and
Performance
Considerations
Within one program scan cycle, the execution of the main steps (as indicated in
the Execution Rules diagram) could be interrupted by other controller activities
which have higher priority than the main steps. Such activities include,
1. User Interrupt events, including STI, EII, and HSC interrupts (when
applicable);
2. Communication data packet receiving and transmitting;
3. PTO Motion engine periodical execution (if supported by the controller).
When one or several of these activities occupy a significant percentage of the
Micro800 controller execution time, the program scan cycle time will be
prolonged. The Watchdog timeout fault (0xD011) could be reported if the
impact of these activities is underestimated, and the Watchdog timeout is set
marginally. The Watchdog setting defaults to 2 s and generally never needs to be
changed.
Periodic Execution of Programs
For applications where periodic execution of programs with precise timing is
required, such as for PID, it is recommended that STI (Selectable Timed
Interrupt) be used to execute the program. STI provides precise time intervals.
It is not recommended that the system variable __SYSVA_TCYCYCTIME be
used to periodically execute all programs as this also causes all communication to
execute at this rate.
ATTENTION: If the optional module feature is enabled, use the
MODULE_INFO instruction to verify that the module is present because
the controller will not fault if the module is missing.
WARNING: Communication timeouts may occur if programmed cycle
time is set too slow (for example, 200 ms) to maintain communications.
System Variable for Programmed Cycle Time
Variable Type Description
__SYSVA_TCYCYCTIME TIME Programmed cycle time.
Note: Programmed cycle time only accepts values in
multiples of 10 ms. If the entered value is not a
multiple of 10, it will be rounded up to the next
multiple of 10.
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