124 Rockwell Automation Publication 2080-UM002N-EN-E - November 2022
Chapter 7 Program Execution in Micro800
Execution Rules
This section illustrates the execution of a program. The execution follows four main steps
within a loop. The loop duration is a cycle time for a program.
When a cycle time is specified, a resource waits until this time has elapsed before starting the
execution of a new cycle. The POUs execution time varies depending on the number of active
instructions. When a cycle exceeds the specified time, the loop continues to execute the cycle
but sets an overrun flag. In such a case, the application no longer runs in real time.
When a cycle time is not specified, a resource performs all steps in the loop then restarts a
new cycle without waiting.
Optional Module Normally before the read inputs step, the controller will verify the presence of any configured
plug-in and expansion I/O modules. If a plug-in or expansion I/O module is missing, the
controller will fault. In Connected Components Workbench software release 10 or later, an
Optional Module configuration option is added to prevent a missing plug-in I/O or expansion
I/O module from faulting the controller if enabled. This option can be enabled separately for
each plug-in I/O or expansion I/O module.
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 (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.
In firmware revision 21.011, Ethernet Implicit I/O messaging performance can be affected by
significant usage of interrupts in the project. As the interrupt has a higher priority than
Implicit messaging, use the Ethernet Diagnostics in Connected Components Workbench
software to check if there are dropped or missed packets with the desired setup.
1. Read inputs
2. Execute POUs
(1)
/programs
3. Write outputs
4. Housekeeping (data log, recipe, communications)
(1) Program Organizational Unit.
4
1
2
3
1
2
3
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.
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