Basic PLC Operation

PLCs consist of input modules or points, a Central Processing
Unit (CPU), and output modules or points. An input accepts a
variety of digital or analog signals from various field devices
(sensors) and converts them into a logic signal that can be used
by the CPU. The CPU makes decisions and executes control
instructions based on program instructions in memory. Output
modules convert control instructions from the CPU into a digital
or analog signal that can be used to control various field devices
(actuators). A programming device is used to input the desired
instructions. These instructions determine what the PLC will do
for a specific input. An operator interface device allows process
information to be displayed and new control parameters to be
entered.


Pushbuttons (sensors), in this simple example, connected to
PLC inputs, can be used to start and stop a motor connected to
a PLC through a motor starter (actuator).