Page 122 - Programmable Logic Controllers, Fifth Edition

IEC NEMA IEC NEMA NEMA symbol
symbol symbol symbol symbol
Normally open Normally closed Break-make
Figure 6-10 Commonly used types of pushbutton switches.

Symbol 12 34 56 7
Contacts Hand O Auto
Position 1 2 3
A B
1 X
2 A ON
3 X
B
OFF
Figure 6-11 Three-position selector switch.
Source: Image Courtesy of Rockwell Automation, Inc. Figure 6-12 DIP switch.

The selector switch is another common manually op- range on a particular input module may be selected by
erated switch. The main difference between a pushbut- means of DIP switches located on the back of the module.
ton and selector switch is the operator mechanism. A
selector switch operator is rotated (instead of pushed) 6.5 Mechanically Operated Switches
to open and close contacts of the attached contact block.
Figure 6-11 shows a three-position selector switch. A mechanically operated switch is controlled automati-
Switch positions are established by turning the opera- cally by factors such as pressure, position, or tempera-
tor knob right or left. Selector switches may have two ture. The limit switch, shown in Figure 6-13, is a very
or more selector positions, with either maintained con- common industrial control device. Limit switches are
tact position or spring return to give momentary contact designed to operate only when a predetermined limit is
operation. reached, and they are usually actuated by contact with
Dual in-line package (DIP) switches are small an object such as a cam. These devices take the place
switch assemblies designed for mounting on printed cir- of a human operator. They are often used in the control
cuit board modules (Figure 6-12). The pins or terminals circuits of machine processes to govern the starting, stop-
on the bottom of the DIP switch are the same size and ping, or reversal of motors.
spacing as an integrated circuit (IC) chip. The individual The temperature switch, or thermostat, shown in
switches may be of the toggle, rocker, or slide kind. DIP Figure 6-14 is used to sense temperature changes. Al-
switches use binary (on/off) settings to set the parameters though there are many types available, they are all actu-
for a particular module. For example, the input voltage ated by some specific environmental temperature change.

Cam (on machine)
Operating IEC symbols
force Operator NEMA symbols

Normally open Normally closed
limit switch limit switch
Enclosure
containing Normally Normally
contact open closed
mechanism

Figure 6-13 Mechanically operated limit switch.
Source: Photo courtesy Eaton Corporation.

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 103

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