136 Programmable Logic Controllers: Hardware and Programming
Example 6-4
PLCs can use the associated contacts of outputs to interlock out-
puts, allowing only one output to be on at a time. In Figure 6-41,
actuating the GPB input at address I:1/0 turns on the green pilot light
(G-PLT) at address O:2/0 and opens the NC contact (G-PLT) associ-
ated with the green pilot light on rung 001. Actuating the RPB input at
address I:1/1 turns on the red pilot light (R-PLT) at address O:2/1 and
opens the NC contact (R-PLT) associated with the red pilot light on
rung 000. The opening of the R-PLT contact on rung 000 will turn off
the G-PLT output. Actuating the GPB input turns on the G-PLT output
and turns off the R-PLT output by opening the NC G-PLT contact on
rung 001. In this way, the outputs are interlocked so that only one light
can be turned on at a time.
Green Pushbutton (GPB) turns on Green Pilot Light (G-PLT). Red Pushbutton (RPB)
turns on Red Pilot Light (R-PLT). Note that only one light is on at a time.
000
001
002
O:2/0
G-PLT
O:2/1
R-PLT
End
I:1/0
GPB
O:2/0
G-PLT
I:1/1
RPB
O:2/1
R-PLT
O:2/1
R-PLT
O:2/0
G-PLT
Figure 6-41. PLC ladder logic diagram for Example 6-4. How to interlock two outputs.