Chapter 5. Power Supply / CPU
5-20
7) Examination on task program
After writing down a task program, be sure to examine the following items.
(1) Task setting has been correctly done?
y If tasks are invoked more frequently than necessary or several tasks are invoked simultaneously within one scan,
the scan time become longer and irregular. If the task setting cannot be changed, check the maximum scans time.
(2) Task priorities are properly arranged?
y The lower priority tasks still may not be processed after its time due to delay by higher priority tasks. In some cases,
if the prior tasks have been delayed and next task occurs task collision can occur. Set the priority with due
consideration of items such as urgency and execution time of a task.
(3) Task programs are written as shortly as possible?
y If execution time of a task program is long, the scan time may become longer and irregular and also collision of
task programs may occur. Therefore, write task programs as shortly as possible.
(4) Protection of lower priority programs against higher priority program isn’t needed during execution of those programs.
y If the priority of a task program (or a scan program) has been set to lower priority and other tasks must not interrupt
during its execution, use the function Dl and ‘El’ to protect the program partly. When processing global variables
used commonly in other programs, special modules or communications modules, problems can occur.
8) Example of program configuration and processing
When the task and program have been registered as below,
• Task registration : T_SLOW (interval T#10ms, priority = 0)
PROC_1 (internal contact point: %MX0, priority = 3)
E_INT1 (external contact point: %IX0.0.1, priority = 2)
• Program registration : program → P0
program → P1 with the task T_SLOW
program → P2 with the task PROC_1
program → P3 with the task E_INT1
If program execution time is equal to external interrupt occurrence time:
• Execution time for each program: P0= 17ms, P1= 2ms, P2= 7ms, P3= 2ms
• Interrupt E_INT1 occurrence time: Occurred at the 6, 7 and 20ms after the operation started.
• PROC_1 occurrence: Invoked during the execution of scan program
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