5
The CPU evaluates the status of inputs, outputs, and other
variables as it executes a stored program. The CPU then sends
signals to update the status of outputs.
Output modules convert control signals from the CPU into
digital or analog values that can be used to control various
output devices.
The programming device is used to enter or change the PLC’s
program or to monitor or change stored values. Once entered,
the program and associated variables are stored in the CPU.
In addition to these basic elements, a PLC system may also
incorporate an operator interface device to simplify monitoring
of the machine or process.
Programming
Device
Operator
Interface
Central Processing Unit
(CPU)
Input
Module
Output
Module
In the simple example shown below, pushbuttons (sensors)
connected to PLC inputs are used to start and stop a motor
connected to a PLC output through a motor starter (actuator).
No programming device or operator interface are shown in this
simple example.
Motor Starter
Start
Pushbutton
Stop
Pushbutton
Inputs
Output
PLC
Motor
SF/DIAG
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