ladder logic program examples lamp sequence

Correspondingly, each normally-open T2 contact appears colored, indicating its “closed” status, while each normally-closed T2 contact is uncolored. Another form of status indication seen in this PLC program is the values of certain variables in the PLC’s memory, shown in red text.įor example, you can see coil T2 energized at the upper-right corner of the screen (filled with light blue coloring), while coil T3 is not. Other instructions appear as rectangular boxes.Įach horizontal line is referred to as a rung, just as each horizontal step on a stepladder is called a “rung.” A common feature among Ladder Diagram program editors, as seen on this screenshot, is the ability to color-highlight those virtual “components” in the virtual “circuit” ready to “conduct” virtual “power.” In this particular editor, the color used to indicate “conduction” is light blue. Coils are somewhat different, appearing as either circles or pairs of parentheses. Normally-open contacts are empty within the space between the line segments, while normally-closed contacts have a diagonal line crossing through that space. The following computer screenshot shows a typical Ladder Diagram program being edited on a personal computer:Ĭontacts appear just as they would in an electrical relay logic diagram – as short vertical line segments separated by a horizontal space. None of the contacts or coils seen in a Ladder Diagram PLC program are real rather, they act on bits in the PLC’s memory, the logical inter-relationships between those bits expressed in the form of a diagram resembling a circuit. Ladder diagrams are to be thought of as virtual circuits, where virtual “power” flows through virtual “contacts” (when closed) to energize virtual “relay coils” to perform logical functions. While Ladder Diagram programming has many shortcomings, it remains extremely popular and so will be the primary focus of this chapter.Įvery Ladder Diagram program is arranged to resemble an electrical diagram, making this a graphical (rather than text-based) programming language. This language was invented for the express purpose of making PLC programming feel “natural” to electricians familiar with relay-based logic and control circuits. This is a graphical language showing the logical relationships between inputs and outputs as though they were contacts and coils in a hard-wired electromechanical relay circuit. In the United States, the most common language used to program PLCs is Ladder Diagram (LD), also known as Relay Ladder Logic (RLL).

Ladder logic program examples lamp sequence