Chapter 5 Program Configuration and Operation Method
(c) Precautions in using internal device task program
1) Accordingly, even if the execution condition of internal device task program occurs in Scan Program
or Task Program (Cycle time, I/O), it shall not be executed immediately but executed at the time of
completion of Scan Program.
2) If the demand to execute Internal Device Task Program occurs, the execution condition shall be
examined at the time of completion of Scan Program. Accordingly, if the execution condition of Internal
Device Task occurs by Scan Program or Task Program (Cycle time) during ‘1 scan’ and disappears, the
task shall not be executed as it is not possible to detect the execution at the time of examination of
execution condition.
(8) Verification of task program
(a) Is the task setting proper?
If task occurs frequently more than needed or several tasks occur in one scan at the same time, scan time
may lengthen or be irregular. In case not possible to change the task setting, verify max. scan time.
(b) Is the priority of task arranged well?
The low priority task program shall be delayed by the high priority task program, which results in disabling
the processing within the correct time and even task collision may occur as next task occurs in the state
that the execution of previous task is delayed. Consider the emergency of task and execution time etc
when setting the priority.
(c) Is the Task Program written in shortest?
If the execution time of Task Program is longer, scan time may lengthen or be irregular. Even it may cause
the collision of task program. Write the execution time as short as possible. (Especially, when writing the
cyclic task program, write the execution time so that the task program can be executed within 10% cycle
of the shortest task among several tasks.)
(d) Is program protection for the high priority task needed during program execution?
If other task is inserted during task program execution, complete the task in execution and operate the
standby tasks in the order of high priority. In case that it is not allowed to insert other task in Scan
Program, prevent the insert partially by using ‘DI’ and ‘EI’ application instruction. The problem may occur
while processing the global variables used commonly with other program or special or communication
module.
(9) Program configuration and processing example
If task and program are registered as below.
Interrupt type Interrupt name Priority Task No. Program
Cycle time 10 ㎳_cycle time 3 0 Program 1
Internal device Internal device_%MX0 5 16 Program 2
I/O I/O_%IX0.0.0 2 8 Program 3
1) Scan program name: “ Scan Program”
2) Execution time respective program: Scan program = 17 ㎳, Program 1 = 2 ㎳, Program 2= 7 ㎳,
Program 3 = 2 ㎳
5- 17
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