PLC programming is the method for creating the instructions, sequence, and calculations used by the PLC to automate a machine or process. The PLC is the hardware and the program is the intelligence.
In a sense, the programmed PLC gives a machine a ‘brain’. PLC is an acronym for programmable logic controller.
A PLC is device who has its origins in the 1960’s by a company named Modicon. Its first use was in the automotive manufacturing plants to replace the old relays arranged to create essentially relay logic. It was a simple decision and control system effective for the era, but as usual, a better solution is always on someone’s creative mind.
Briefly, a relay is a mechanical switch controlled by an electrical source to turn it on and off.
Being mechanical in nature, it is large compared to electronics and moving parts tend to wear, degrade, and ultimately fail. Creating the desired relay logic requires many relays and many more wires to connect them. If the logic is wrong or needs to be updated then rewiring is needed. The PLC eliminates all of these physical relays to create logic circuits and moves them virtually into the program.
PLC programming has largely been and still is in Ladder Diagram or Ladder Logic. It is still the most commonly used language for programming a PLC, even though there are alternative languages. The reason is simple, it is very much like the electrical schematics used for relay logic. Ladder logic is programmed using common electrical symbols like those used for contacts and coils. A ladder diagram program looks like an electrical drawing that electricians are familiar and comfortable with, so easy adoption from the hard-wired relays to the PLC was accomplished.