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16
Structure and Operation
Section 1-3
I/O Memory
The program reads and writes data in this memory area during execution.
Part of the I/O memory contains the bits that reflect the status of the PLC’s
inputs and outputs. Parts of the I/O memory are cleared when the power is
turned ON and other parts are retained.
Note Refer to SECTION 4 Memory Areas for more details on I/O memory.
Program
This is the program written by the user. The CPM2B executes the program
cyclically. (Refer to 1-3-5 Cyclic Operation and Interrupts for details.)
The program can be divided broadly into two parts: the “main program” that is
executed cyclically and the “interrupt programs” that are executed only when
the corresponding interrupt is generated.
PLC Setup
The PLC Setup contains various startup and operating parameters. The PLC
Setup parameters can be changed from a Programming Device only; they
cannot be changed from the program.
Some parameters are accessed only when PLC’s power supply is turned on
and others are accessed regularly while the power is on. It will be necessary
to turn the power off and then on again to enable a new setting if the parame-
ter is accessed only when the power is turned ON.
Note Refer to 4-5 PLC Setup for more details.
Communications
Switches
The Communications Switches determine whether the peripheral port and
RS-232C port operate with the standard communications settings or the com-
munications settings in the PLC Setup. Refer to 2-2 Board Components and
their Functions for more details.
1-3-2
Operating Modes
CPM2B CPU Boards have 3 operating modes: PROGRAM, MONITOR, and
RUN.
PROGRAM Mode
The program cannot be executed in PROGRAM mode. This mode is used to
perform the following operations in preparation for program execution
• Changing initial/operating parameters such as those in the PLC Setup
• Writing, transferring, or checking the program
• Checking wiring by force-setting and force-resetting I/O bits
!Caution
The PLC continues to refresh I/O bits even if the PLC is in PROGRAM mode,
so devices connected to output points on the CPU Board or Expansion I/O
Boards may operate unexpectedly if the corresponding output bit is turned ON
by changing the contents of I/O memory from a Programming Device.
MONITOR Mode
The program is executed in MONITOR mode and the following operations can
be performed from a Programming Device. In general, MONITOR mode is
used to debug the program, test operation, and make adjustments.
• Online editing
• Monitoring I/O memory during operation
• Force-setting/force-resetting I/O bits, changing set values, and changing
present values during operation
RUN Mode
The program is executed at normal speed in RUN mode. Operations such as
online editing, force-setting/force-resetting I/O bits, and changing set values/
present values cannot be performed in RUN mode, but the status of I/O bits
can be monitored.
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