In brief
A programmable logic controller (PLC) is a digital control device that controls machines and plants cyclically on the basis of a stored program.
Contents
What is Programmable logic controller?
A programmable logic controller reads in signals from sensors and switches, processes them according to a user program and controls actuators such as motors, valves or displays. In industry it has largely replaced the hard-wired relay and contactor controls that used to be common, because the logic can be changed via software.
A modular design is typical, consisting of a power supply, central processing unit (CPU), digital and analogue input and output modules, communication modules and, where needed, special modules, for example for counting or positioning tasks. The program is executed cyclically: read inputs, execute program, write outputs. The duration of one pass is called the cycle time. The programming languages are standardised in IEC 61131-3, including ladder diagram (LD), function block diagram (FBD), structured text (ST) and sequential function chart (SFC).
Controllers often remain in service far longer than the respective device generation is available. Older series are discontinued, and replacement CPUs and modules are then only available to a limited extent. At the latest by then, the question arises of stockpiling or migrating to a current generation, which usually requires adjustments to the program, hardware and networking.
In practice: maintenance and procurement
The current user program should be backed up and version-controlled, together with the hardware configuration and the licences for the programming software. Spare parts enquiries require the complete item number, the hardware revision of the module and, where applicable, the firmware version, as these determine compatibility.
Synonyms and abbreviations: PLC · programmable controller · controller · PLC system · industrial controller
Frequently asked questions
What programming languages are there for a PLC?
The standard IEC 61131-3 describes several languages: graphically, ladder diagram (LD), function block diagram (FBD) and sequential function chart (SFC); textually, structured text (ST). Instruction list (IL) is considered deprecated in newer editions of the standard. Which languages a controller actually supports depends on the programming environment.
What is the cycle time of a PLC?
The cycle time is the time a controller needs for one complete pass: reading inputs, executing the program and setting outputs. It depends on program size, processing power and communication load. For time-critical tasks, a short and consistent cycle time is important; many controllers monitor it and go into a fault state if it is exceeded.
What should you do if a PLC has been discontinued?
First, the installed modules, their condition and existing reserves should be recorded. In the short term, stockpiling or a last time buy secures operation; in the medium term, migration to a current controller generation should be planned. This includes program conversion, checking the peripherals and planned commissioning. A backed-up, commented program version makes each of these steps easier.
What information is needed to replace a PLC module?
Required are the complete item number, the hardware revision and, for CPUs, the firmware version. Information on slot assignment, the bus system and the programming software used is also helpful. A different hardware revision can mean that a module is not recognised without adjusting the hardware configuration.