Execution Rules; Controller Load And Performance Considerations - Allen-Bradley Micro830 User Manual

Chapter 6 Program Execution in Micro800
Controller Load and
Performance
Considerations
62
• __SYSVA_TCYCURRENT – Current cycle time
• __SYSVA_TCYMAXIMUM – Maximum cycle time since last start.

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.
1. Read inputs
(1)
2. Execute POUs
3. Write outputs
4. Housekeeping (datalog,
recipe, communications)
(1) Program Organizational Unit.
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.
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
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Rockwell Automation Publication 2080-UM002G-EN-E - March 2015