Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 8)

Chapter
2
2Ć1
An Introduction to Programmable Controllers
This chapter reviews general fundamentals common to our programmable
controllers (PC’s). When you are finished, you will have read several important
concepts that will help you understand this manual. You’ll be able to:
Describe what a programmable controller does.
List and describe the functions of the four major sections of a programmable
controller.
Describe how the four major sections of a programmable controller interact.
Give an example of a simple program.
If you can do all this now, then turn directly to chapter three.
You are probably familiar with the traditional methods of machine control
(Figure 2.1). Sensing devices located on the machine detect changes in the
machine’s condition. For instance, a part arriving at a work station would
contact and close a limit switch, the sensing device. As a result, an electrical
circuit is completed and a signal is sent to the control panel.
Figure 2.1
Traditional Methods of Machine Control
Relays
Control Panel
Output Sensing
Devices
Machine
Devices
10152ĆI
At the control panel, the electrical signal enters a bank of relays or other
devices, such as solid state modules. Circuits within the control panel open or
close causing additional electrical signals to be sent to output devices at the
machine. For example, a relay energized by the limit switch may complete
another circuit energizing the output device, a clamp, which secures the part at
the work station.
Objectives
Traditional Controls