PLC programming – ladder diagrams

Let’s build on our understanding from Chapter 5, OT and Industrial Control Systems. We have seen that ladder diagrams are the most popular frame of PLC programming adopted in the industry and they have evolved from electrical relay diagrams. The two vertical lines represent the power and ground, and each rung (horizontal line) represents the logic to complete the circuit conditionally. Contacts and coils form the two major components in each rung. A contact usually represents input while output is represented by a coil. If a path can be established from the left side of the rung through the asserted contacts to the right side, then the rung is true, and the output is asserted. If the path cannot be followed, then the actuator remains un-energized, thus deeming the rung false.

Important note

Contacts can also have outputs, which we will discover via an example in the last section of this chapter.

Let’s take the following...