These functions typically have an input terminal to enable the function execution (EN) and an output terminal when the function has finished successful execution (ENO). To handle more complex tasks, such as math, timer functionality, counters, or PID, all ladder representations today have the ability to call functions on the ladder rung. (Many ladder representations can perform immediate reads and writes, rather than wait until the end of the execution cycle.) At the end of the logic cycle, all the physical outputs are updated with the values from the memory table. The ladder logic is run by reading and writing from the memory table for inputs, outputs, and intermediate values. Physical inputs are read and stored in a memory table. Inherent to ladder logic is an unseen scanning engine and memory management stack. Series contacts act like an “and” and parallel contacts act like an “or.” Execution is left to right and top to bottom.Ī rung of ladder logic mirrors electrical circuits. In ladder logic, each contact and coil is linked to a Boolean memory location.
Open contacts block power and closed contacts pass power. Power flows through a series of normally open or normally closed contacts and powers a coil.
The illustration of “One rung of ladder logic” shows that the left rail is the power bus and the right rail is the ground bus. This software, called ladder logic or a ladder diagram, looks and executes similarly to an electrical diagram.
Later, a microprocessor-based system was introduced, and the software for the system was designed to mirror the electrical diagrams used to build the relay control panels. The ladder diagram evolved in the 1960s when the automotive industry needed a more flexible and self-documenting alternative to physical relay and timer cabinets. The five most popular programming methods for control and measurement software are: ladder diagram, textual, function block/data flow, state chart, and simulation and modeling.
These fundamentally different models of computation let programmers select the most appropriate tool for their tasks. Recognizing this need, software vendors often allow multiple programming methods to be used with their software. This holds true whether repairing a faucet or programming a measurement and control system.
Anyone who’s ever worked on a project knows that one of the differences between an amateur and a professional is using the right tool for the job.