Plc or programmable controller

Redundancy[edit]
Some special processes need to work permanently with minimum unwanted downtime. Therefore, it is necessary to design a system that is fault-tolerant and capable of handling the process with faulty modules. In such cases to increase the system availability in the event of hardware component failure, redundant CPU or I/O modules with the same functionality can be added to hardware configuration for preventing total or partial process shutdown due to hardware failure. Other redundancy scenarios could be related to safety-critical processes, for example, large hydraulic presses could require that both PLCs turn on output before the press can come down in case one output does not turn off properly.
Programming[edit]

Example of a ladder diagram logic
Programmable logic controllers are intended to be used by engineers without a programming background. For this reason, a graphical programming language called Ladder Diagram (LD, LAD) was first developed. It resembles the schematic diagram of a system built with electromechanical relays and was adopted by many manufacturers and later standardized in the IEC 61131-3 control systems programming standard. As of 2015, it is still widely used, thanks to its simplicity.^[24]
As of 2015, the majority of PLC systems adhere to the IEC 61131-3 standard that defines 2 textual programming languages: Structured Text (ST; similar to Pascal) and Instruction List (IL); as well as 3 graphical languages: Ladder Diagram, Function Block Diagram (FBD) and Sequential Function Chart (SFC).^[24][25] Instruction List (IL) was deprecated in the third edition of the standard.^[26]
Modern PLCs can be programmed in a variety of ways, from the relay-derived ladder logic to programming languages such as specially adapted dialects of BASIC and C.^{[citation needed]}
While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization, and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible.^{[citation needed]}