<8. Detailed Description>
Scan cycle 1
Task A
Input
processing
Control/calculation processing
Output
processing
Scan cycle
Task B
AI001 AO001
Input
proces-
sing
Control/
calculation
processing
Output
proces-
sing
Input
proces-
sing
Control/
calculation
processing
Output
proces-
sing
Input
proces-
sing
Control/
calculation
processing
Output
proces-
sing
Scan cycle 2
Scan cycle 3
X
X
As shown above, with multiple tasks, if the task specified for updating a device label
at device label definition is different from the task using the device label in actual
control calculation, this may affect control adversely.
In the above example, Task A, which uses AI001 and AO001 in its control
calculation, must be specified as the task for updating AI001 and AO001.
2. Sharing of Global Variables
When sharing data between tasks for a multi-task control application, use global
variables, but observe the precautions described below for the sender task and the
recipient task.
Recipient task
Do not read global variables directly. Instead, copy its data to another
variable at the beginning of the recipient task, and then read the copied
value.
In the example shown in the next figure, global variable P, which is turned on and off
by Task A, is read by Task B, which has a different control cycle. Variable P is read
in the first half (at Logic M) and latter half (at Logic N) of Task B’s control/calculation
processing, but changed from ON to OFF by Task A between Logic M and Logic N.
If Task B reads variable P directly, the value will be ON when Logic M is executed
but OFF when Logic N is executed.
Similarly, if an application uses a global variable directly, its value may change
within the same control cycle.
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