110 Programmable Logic Controllers: Hardware and Programming
0000
0001
Pushbutton Switch (SW1) Limit switch (LS1) Motor
Red_PLT
O:0 I:0 I:0 I:0
O:0
2
1 0
0
1
End
Stop Start
M1 1, 2
M1
M1
L1 L2
G
1
2
Figure 6-12. PLC ladder logic diagram for the PLC logic diagram displayed in Figure 6-9.
Figure 6-13. Relay logic diagram.
6.4 Programming Ladder Logic Diagrams
with Normally Open and Normally Closed
Contacts
Figure 6-13 displays a relay logic diagram in which an electric
motor remains energized to operate in a continuous mode. When
the normally closed stop pushbutton is closed, the green light is on.
Pressing the start pushbutton turns the motor on. When the motor is
energized, its associated normally open contact closes. Therefore, the
motor runs until the stop pushbutton is closed. Also, when the motor
is energized its associated normally closed contact opens to turn the
green light off.