2 Motion Control Configuration and Principles
2-18
NJ/NX-series CPU Unit Motion Control User’s Manual (W507)
Axis Variable Update Timing
Axis Variables are system-defined variables for some of the axis parameters and for the monitor
information, such as the actual position and error information for the axes controlled by the MC
Function Module.
If you access an Axis Variable of the primary periodic task during the priority-16 periodic task, the
values of the variable that were read at the start of the priority-16 periodic task are used.
Also, the values of an Axis Variable are not written when a motion control instruction (FB) is exe-
cuted. They are written in motion control processing (MC) at the start of the next priority-16 periodic
task.
*1 The values of an Axis Variable of the primary periodic task are read at the start of user program execution for
the priority-16 periodic task.
*2 The values of an Axis Variable are not written when a motion control instruction (FB) is executed in the priority-
16 periodic task.
*3 The values are written during this motion control processing (MC).
Precautions for Correct UsePrecautions for Correct Use
• When motion control instructions are placed in a priority-16 periodic task, the response time of
the Servo Drive or other device will increase if the task period of the priority-16 periodic task is
lengthened.
• Make sure that all axes can be stopped safely for emergency stops, including emergency
stops commanded from external devices.
• The execution timing of motion control instructions in a priority-16 periodic task is not the same
as the execution timing for I/O control. Design the user program to allow for this.
• You cannot use the priority-16 periodic task on NX102 CPU Units and NX1P2 CPU Units.
For information on Axis Variables, refer to 3-1-3 Introduction to Axis Variables.
UPG
FB
UPG
IO MC
UPG
IO MC
UPG
IO MC
UPG
IO MC
*3
UPG
IO MC
UPG
I/O
OUT
Primary period
Execution of motion control instructions
*2
Servo
Execution
command
Priority-16
periodic task
Task period (primary period × 3)
Primary
periodic task
Values of
Axis
Variable
are read
*1
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