Rockwell Automation Publication 2080-UM002L-EN-E - November 2021 73
Chapter 6 Program Execution in Micro800
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 that have higher priority than the main steps. Such activities include,
1. User Interrupt events, including STI, EII, and HSC interrupts (whe
n
appl
icable);
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.
Power Up and First Scan In Program mode, all analog and digital input variables hold their last state,
and the LEDs are always updated. Also all analog and digital output variables
hold their last state, but only the analog outputs hold their last state while the
digital outputs are off.
When transitioning from Program mode to Run mode, all analog output
variables hold their last state but all digital output variables are cleared.
Two system variables are also available from revision 2 and later.
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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