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3 CPU Unit Operation
3-1
CPU Unit Operation
This section gives an overview of the CPU Unit operation, describes the operating modes, and explains
how the Unit operates when there is a power interruption.
3-1-1
Overview of CPU Unit Operation
The CPU Unit reads and writes data to the internal I/O memory areas while executing user ladder pro-
grams by executing the instructions in order one at a time from the start to the end.
Overhead processing
(self-diagnosis)
Program execution
CPU Unit
processing
cycle
I/O refreshing
Peripheral servicing
Overhead Processing (Self-diagnosis)
Self-diagnosis, such as an I/O bus check, is performed.
Ladder Program Execution
Instructions are executed from the beginning of the program and I/O memory is refreshed.
I/O Refresh
Data to and from external devices, such as sensors and switches, directly connected to the built-in I/O
terminals and expansion I/O terminals, is exchanged with data in the I/O memory of the PLC. This pro-
cess of data exchange is called the I/O refresh.
Peripheral Servicing
Peripheral servicing is used to communicate with devices connected to the communications port or for
exchanging data with the CX-Programmer.
Cycle Time
The cycle time is the time between one I/O refresh and the next. The cycle time can be determined
beforehand for SYSMAC PLCs.
3-2
CPU Unit Internal Memory
I/O memory
0 0 0 0 0 0 0 0
Access
0 0 0 0 0 0 0 0
1 1 0 1 1 0 0 0
0 0 1 1 1 0 1 0
1 0 1 0 1 0 0 1
Refreshes external devices at this timing
Change in status
after all instructions
have been executed
0 1 0 1 0 1 0 0
Exchange
0 1 1 0 1 0 1 0
1 1 0 0 1 0 1 0
1 0 1 1 1 0 1 1
1 0 0 0 1 1 0 1
CP1E CPU Unit Software User's Manual(W480)
Inputs
Outputs
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