01 November 2002
Ladder to success
Ladder logic is the most common language used in PLC applications. The original programmable controllers were designed to replace electrical relay-based control systems. Technicians and engineers designed those systems using a symbolic language: ladder diagrams. Ladder diagrams consist of two concepts: the power source, which forms the sides of the ladder (the rails), and the current flowing through the logic input devices that form the ladder rungs. If an electrical current flows through the relay contacts in a rung, the output relay coil turns on.
A push button (PB1) energizes a pump start control relay (CR1) if the level of the process fluid in the storage tank is not too high (LSH-1).