Page 30 - Programmable Logic Controllers, Fifth Edition Mobile 2

controller. In this example, the Allen-Bradley Pico con-
Motor
troller equipped with 8 inputs and 4 outputs is used to Pressure Temperature starter
control and monitor the process. Installation can be sum- switch switch coil
I/1
O/1
I/2
marized as follows:
• Fused power lines, of the specified voltage type and
level, are connected to the controller’s L1 and L2 Manual
terminals. pushbutton
I/3
• The pressure switch, temperature switch, and push-
button field input devices are hardwired between
L1 and controller input terminals I1, I2, and I3,
respectively. Figure 1-23 PLC ladder logic program for the modified
• The motor starter coil connects directly to L2 and in process.
series with Q1 relay output contacts to L1.
• The ladder logic program is entered using the front 1.5 PLCs versus Computers
keypad and LCD display. The architecture of a PLC is basically the same as that of
• Pico programming software is also available that a personal computer. A personal computer (PC) can be
allows you to create as well as test your program made to operate as a programmable logic controller if you
using a personal computer. provide some way for the computer to receive informa-
tion from devices such as pushbuttons or switches. You
also need a program to process the inputs and some way
1.4 Modifying the Operation to turn devices on and off.

One of the important features of a PLC is the ease with However, some important characteristics distinguish
which the program can be changed. For example, assume PLCs from personal computers. First, unlike PCs, the
that the original process control circuit for the mixing op- PLC is designed to operate in the industrial environ-
eration must be modified as shown in the relay ladder dia- ment with wide ranges of ambient temperature and
gram of Figure 1-22. The change requires that the manual humidity. A well-designed industrial PLC installa-
pushbutton control be permitted to operate at any pres- tion, such as that shown in Figure 1-24, is not usually
sure, but not unless the specified temperature setting has affected by the electrical noise inherent in most indus-
been reached. trial locations.
If a relay system were used, it would require some re- Unlike the personal computer, the PLC is programmed
wiring of the circuit shown in Figure 1-22 to achieve the in relay ladder logic or other easily learned languages.
desired change. However, if a PLC system were used, no The PLC comes with its program language built into its
rewiring would be necessary. The inputs and outputs are memory and has no permanently attached keyboard, CD
still the same. All that is required is to change the PLC drive, or monitor. Instead, PLCs come equipped with
ladder logic program as shown in Figure 1-23.

L1 L2
120 VAC
Pressure Temperature
switch switch OL
M
Motor
starter
coil

Manual
pushbutton

Figure 1-22 Relay ladder diagram for the modified Figure 1-24 PLC installed in an industrial environment.
process. Source: Courtesy of Softac Systems, Ltd.

Programmable Logic Controllers (PLCs) Chapter 1 11

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