Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 22)

An Introduction to
Programmable Controllers
Chapter 2
2Ć8
Isolation
The output isolates the more sensitive electronic circuitry of the programmable
controller from unwanted and dangerous voltages that occasionally occur at the
machine or the plant’s wiring system. Some situations require additional
external protection.
Power Supply
The fourth section is the power supply. It provides a low level dc voltage
source for the electronic circuitry of the processor. It converts the higher level
line voltages to low level logic voltages required by the processor’s
electronic circuitry.
Let’s look at a simple example to see the sequence of events that take place in
controlling a machine with a programmable controller (Figure 2.1). Suppose
you are making a part. The motor driven conveyor carries a unit to the work
area. The limit switch detects wen the part arrives at the work area. When that
happens, we want the conveyor to stop so you can work on the part.
Figure 2.1
A Simplified Example of a Machine with a Programmable Controller
Conveyor
Motor
Limit
Switch
Conveyor Unit
Controller
Input Output
11594
Notice how the limit switch and motor are wired to the programmable
controller. The limit switch, wired to terminal 02, is normally-closed. The
arriving part will open the switch. Therefore, the program statement controlling
the conveyor motor must read: “If there is voltage at input terminal 02 (limit
switch), then energize output terminal 02 (conveyer motor).” The conveyor
motor is wired to output terminal 02.
Control Sequence