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3 CPU Unit Operation
CJ2 CPU Unit Software User’s Manual
3-1 CPU Unit Internal Operation
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3-1-1 Overview
The CPU Unit will repeatedly perform four processes (overseeing, program execution, I/O refreshing,
and peripheral servicing) after startup initialization. The time it takes to complete one cycle is called the
cycle time.
•Overseeing
This process prepares to execute the user program. It checks the hardware and software required for
processing. The time required for overseeing is called the overhead time.
• Program Execution
This process executes the instructions in the user program. The instructions in the user program are
executed when the operating mode is set to RUN or MONITOR mode. I/O processing for the instruc-
tions is performed on bits and words in the I/O memory areas in the CPU Unit.
• Peripheral Servicing
This process occurs when messages are required from Programming Devices or Units. Data is writ-
ten from the CX-Programmer or performed for instructions for Network Communications Units.
• I/O Refreshing
This process exchanges data with the Units by using the I/O memory areas. This process is always
performed after program execution. I/O refreshing is performed within the current cycle without inter-
ruption (i.e., without time slicing).
Processing Cycle
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