A programmable logic controller, or PLC, is a device that monitors computer processes by viewing inputs to a system and turning on or off the outputs based on a set of instructions programmed using one of five PLC languages. PLCs are most often used in situations in which applications with electrical controls—such as packaging, material handling, machining or automatic assembly—require too much monitoring of inputs and outputs to control with timers alone.
Using a PLC eliminates the need for numerous timers to control many machines. The PLC makes monitoring processes easier because the operations are depicted visually on a screen, and the cost of a PLC is lower than the cost of the numerous timers and coils that would be required to perform the same functions. The PLC is space efficient, and it offers a way to test the programing in the lab, enabling corrections, if necessary, before implementing the device in an actual work scenario.
IEC 1131-3 Standard
The International Electrotechnical Commission, or IEC, has developed Standard 1131, which defines the specifications required for languages that operate programmable controllers such as the PLC. IEC 1131 dictates the semantics, syntax and display for the five PLC languages available, each of is best suited to certain types of applications.
Programmers may chose from among five PLC languages. Ladder diagram (LD) is best for programs controlled by multiple files, subroutines and code sectioning. Sequential function charts (SFCs) are used to program systems that are more advanced than those run by LD. Function block diagram (FBD) is a graphical language that drives data from inputs to outputs by sending through blocks of nested data. Structured text (ST) resembles Basic or Pascal programming languages, in that it uses statements such as "If-Then-Else," "While" and "Repeat." Instruction list (IL) uses mnemonic instructions from the ladder diagrams and sends the instructions to the PLC via a programming terminal.
Industry PLC Use
PLCs are industrial computers, rather than basic home- or office-type personal computers. Machine-Information-Systems.com refers to the PLC as an "unsung hero controlling a massive range of equipment." Based on their programming, which is controlled via one of the five languages, PLC units are actively working in the following industries: manufacturing, aerospace, travel, food, textile, film, hospital, leisure, foundry, agriculture, plastics and printing, among others.