8 CPU Unit Functions
8-46
NJ-series CPU Unit Software User’s Manual (W501)
Observe the following precautions when you use differential monitoring.
Loss of Communications with Sysmac Studio While Differential Monitoring Is
in Progress
Let’s assume that communications with the Sysmac Studio were cut off during differential monitoring
because the communications cable was disconnected or because the Sysmac Studio ended due to
an error. In such cases, the CPU Unit will continue execution of differential monitoring. To restart
execution of differential monitoring, you must resume communications with the Sysmac Studio and
stop differential monitoring.
Simultaneous Execution of Differential Monitoring
You cannot run differential monitoring from more than one copy of the Sysmac Studio running on the
same computer or from the Sysmac Studio running on different computers.
Specifying Global Variables and External Variables
You can specify global variables or external variables (which specify global variables in POUs) for
differential monitoring. Keep in mind that the values of global variables and external variables are
updated at different times. A global variable is updated as soon as the value is written. An external
variable, however, is updated only when the CPU Unit executes the POU in which that external
variable is declared. The following figure shows this. In this example, the following two variables are
monitored.
The GVar1 global variable is read and written by the P1 program that is assigned to the primary peri-
odic task. Therefore, it will be updated in the primary period as long as the program writes to it every
period. The P2.GVar1 external variable, however, is updated only when the CPU Unit executes the
P2 program that is assigned to the periodic task. This means the external variable is updated only in
the task period of the periodic task. Because the task period of the periodic task is longer than the
primary period, the count for P2.GVar1 is updated fewer times than the count for GVar1.
Precautions for Differential Monitoring
Variable Type of variable POU that executes the read/write
GVar1 Global variable The P1 program that is assigned to the primary peri-
odic task
P2.GVar1 This is an external variable that is
declared in the P2 program and points
to GVar1.
The P2 program that is assigned to the periodic task
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