Mitsubishi Electric MELSEC iQ-R Series Programming Manual page 28

Ex.
Fixed period execution of program block without phase
The execution time of A differs every time.
800ms
A
B
The program blocks A and B have the same execution cycle of 800ms, and the program block A is first executed.
In this case, the program blocks are executed simultaneously, and the program block A is executed at an interval of 800ms and the program block B is executed
after the execution of the program block A ends. Therefore, the execution time of program block A decides the execution cycle of the program block B, which will
not be exactly 800ms. The on-time performance thus degrades.
The undetermined factor () in the execution cycle of the program block B results from the fluctuation of execution time of the program block A.
■Program block with delay
If the program execution time exceeds the phase due to an interrupt execution, the next program block is not executed in the
execution cycle, and the on-time performance may degrade.
Ex.
Fixed period execution of program block with delay
200ms 200ms
#1 #2
A
B
C
D
The program block A is executed in the phase #1, the program block B in the phase #2, the program block C in the phase #3, and the program block D in the
phase #4.
(1) The execution time of the program block D is 200ms or longer due to an interrupt start program or other causes.
(2) The program block A cannot be executed until the execution of the program block D ends. Therefore, the execution cycle of the program block A is not
800ms, and the on-time performance will degrade.
(3) However, when the total sum of the exceeding time of the program block D and the execution time of the program block A is 200ms or shorter, the program
block B is executed in the normal execution cycle.
1 PROCESS CONTROL FUNCTION BLOCK
26
1.5 Program Execution Control
(800±α)ms
800ms
200ms 200ms 200ms
#3 #4
(1)
800ms
800ms
200ms 200ms
#1 #2 #3
(2)
(3)
200ms
#4