8 - 3 8 - 3
MELSEC-Q
8 MULTITASK PROCESSING
(2) Priority of the programs (Task priority)
An execution priority can be assigned to each program executed in multitask
processing. All programs are set to have the same priority when BASIC is started
up, which means that the main CPU executes all programs equally through the
scheduling by the OS.
[Priority]
B
A
C
1
1
1
When the priority of the program is changed by the ZURGENCY instruction, the
OS performs no scheduling and only the program with the highest priority is
executed, unless the program execution switching instruction shown in Appendix
3 is executed.
[Priority]
A
1
2
3
STOP
STOP
C
Only the program with
the highest priority is
executed.
Programs with low priorities are not executed until all programs with higher
priorities are put in the wait status or until the program execution switching
instruction shown in Appendix 3 is executed.
[Priority]
1
2
3
STOP
C
B
Executed when
programs with higher
priorities are put in the
wait status.
Cannot be executed
until programs with
priorities 1 and 2 are
put in the wait status.
It is recommended that the priority shall be raised only when a certain emergency
processing is required, and that the same priority is assigned to all programs
otherwise.
