Copyright Goodheart-Willcox Co., Inc. Chapter 6 PLC Programming 115 PLCs have the capability to lock their outputs in position. In Figure 6-39, pressing pushbutton #1 (PB-1) turns on the pilot light at address O:2/0. Once turned on, the pilot light locks itself in position by closing its associated contact (O:2/0) in parallel with PB-1. Pressing pushbutton #2 (PB-2) breaks the “lock-in-position” pass and turns off the pilot light. Illustrating the concept of output sealing or locking itself in position. 000 001 O:2/0 Pilot Light End I:1/1 PB-2 I:1/0 PB-1 O:2/0 Pilot Light Goodheart-Willcox Publisher Figure 6-39. PLC ladder logic diagram for Example 6-2, showing how an output can lock itself in position. Example 6-2 Latch and unlatch are functions that use two inputs to control the state of a bit that affects a single output. Refer to the diagram in Figure 6-40. Pressing the pushbutton PB-1 at address I:1/0 latches the B3:0/0 bit and turns on the green pilot light at address O:2/0. To turn off the pilot light, the B3:0/0 bit must be unlatched. Pressing the pushbutton PB-2 at address I:1/1 unlatches the B3:0/0 bit and turns off the green pilot light. Lach/Unlatch Functions 000 001 002 003 B3:0/0 L B3:0/0 U O:2/0 G-PLT End I:1/0 PB-1 I:1/1 PB-2 B3:0/0 Goodheart-Willcox Publisher Figure 6-40. PLC ladder logic diagram for Example 6-3, showing how to use the latch/unlatch functions. Example 6-3