Page 135 - Programmable Logic Controllers, Fifth Edition

Inputs Ladder logic program Outputs
L1 L2
Motor 1 Motor 1
Motor 1 Stop PB Start PB M1 M1
Start PB OL

Motor 1
Stop PB M1 M2 OL

Motor 2 Motor 2
Motor 2 Stop PB Start PB M2
Start PB
M3
M1 OL

Motor 2
Stop PB M2

Motor 3 Motor 3
Motor 3 Stop PB Stop PB M3
Start PB
M2
Motor 3
Stop PB M3

Figure 6-49 PLC program equivalent of the hardwired sequential motor
control circuit.

Ladder logic program
Inputs Outputs
• When SOL-A pushbutton is pressed its NO contact SOL-A SOL-B
completes the circuit to SOL-A and its NC contacts L1 SOL-A PB PB SOL-A L2
PB
opens the current path to SOL-B. SOL-A
• When SOL-B pushbutton is pressed its NO contact
completes the circuit to SOL-B and its NC contacts SOL-B
opens the current path to SOL-A. PB SOL-B
SOL-B SOL-A
• When both buttons are pushed, neither solenoid will PB PB SOL-B
be energized.

Figure 6-51 shows a PLC program equivalent of
the hardwired circuit implemented using two NO Figure 6-51 PLC program equivalent of the hardwired
pushbuttons. pushbutton interlocking circuit.

L1 L2 6.10 Latching Relays
SOL-A SOL-B
PB PB SOL-A Electromagnetic latching relays are designed to hold the
relay closed after power has been removed from the coil.
Latching relays are used where it is necessary for contacts
SOL-B to stay open and/or closed even though the coil is ener-
gized only momentarily. Figure 6-52 shows a latching
relay that uses two coils. The latch coil is momentarily
Figure 6-50 Hardwired pushbutton interlocking circuit. energized to set the latch and hold the relay in the latched

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

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