If an input relay is “true” the path continues, and if it is “false” it does not.

A path is traced on the left side, across “rungs” consisting of various inputs. Ladder logic follows a relay function diagram, as shown in Figure 4.2. Ladder logic gets its name from the legacy method of implementing discrete logic via electromechanical relays and was initially referenced as “relay ladder logic.” Ladder logic can be thought of as a set of connections between inputs (relay contacts) and outputs (relay coils). Programmable logic controllers can use “ladder logic” or “ ladder diagrams (LD),” which is a simplistic programming language included within the IEC-61131-3 standard that is well suited for industrial applications. Ladder logic diagrams can easily become unwieldy and difficult to maintain unless a certain methodology is followed to give them structure. A short pulse on the input X000 can be extended into a longer pulse appearing at the contacts of the output Y400. The ladder logic above acts like a pulse extender. When the countdown period has expired, the normally closed contacts of T300 become open, thus interrupting the logic continuity to the output Y400 and so Y400 is de-energised. During the countdown period, the output device Y400 remains energised by the latched path through the contacts Y400. Now, when X000 goes false, a true signal is sent to the timer to begin the countdown period. Thus, when the input X000 goes false, the output Y400 remains on, it is latched by its own contacts. This true state is fed back into the input to the normally closed contacts of the timer.

In this example, when the input X000 is true, there is logic continuity through the normally closed timer contacts T300 to the output Y400.

Allen bradley ladder logic program