Copyright Goodheart-Willcox Co., Inc. 116 Programmable Logic Controllers: Hardware and Programming PLCs can use the associated contacts of outputs to interlock outputs, 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) associated 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. Opening the R-PLT contact on rung 000 turns 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 Goodheart-Willcox Publisher Figure 6-41. PLC ladder logic diagram for Example 6-4, showing how to interlock two outputs. Example 6-4