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1-2 The Role of the PC
The Programmable Controller, or PC, is the part of the Control System that di-
rectly controls the manufacturing process. According to the program stored in its
memory, the PC accepts data from the input devices connected to it, and uses
this data to monitor the controlled system. When the program calls for some ac-
tion to take place, the PC sends data to the output devices connected to it, to
cause that action to take place. The PC may be used to control a simple, repeti-
tive task, or it may be connected to other PCs, or to a host computer in order to
integrate the control of a complex process.
1-2-1 Input Devices
PCs can receive input from either automated or manual devices. The PC could
receive data from the user via a pushbutton switch, keyboard, or similar device.
Automated input could come from a variety of devices: microswitches, timers,
encoders, photosensors, and so on. Some devices, like the Limit Switch shown
below, turn ON or OFF when the equipment actually makes contact with it. Other
devices, like the Photoelectric Switch and Proximity Switch shown below, use
other means, such as light or inductance, in order to get information about the
equipment being monitored.
Photoelectric switch Limit switch
Proximity switch
1-2-2 Output Devices
A PC can output to a myriad of devices for use in automated control. Almost any-
thing that you can think of could be controlled (perhaps indirectly) by a PC. Some
of the most common devices are motors, Solenoids, Servomotors, Stepping
Motors, valves, switches, indicator lights, buzzers, and alarms. Some of these
output devices; such as the motors, Solenoids, Servomotors, Stepping Motors,
The Role of the PC Section 1-2
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