Plain words · Linked to free courses
PLC glossary: terms explained simply
24 terms a PLC beginner meets, each in a sentence or two, with the free course that teaches it.
A PLC (programmable logic controller) is an industrial computer that reads inputs, runs a stored program in a repeating scan cycle and switches outputs. The terms below are the ones you meet first: ladder logic, rungs, contacts, coils, timers, counters, I/O, HMI and SCADA.
Updated . By EDWartens, the training division of Wartens Ltd (UK).
A to Z
Terms
Jump to a term: Analog signal · Coil · Contact · Counter · DCS · FAT · Fieldbus and industrial Ethernet · Function block · HMI · I/O · IEC 61131-3 · Interlock · Ladder logic · PLC · PLC simulation · Rung · SCADA · Scan cycle · Sequential function chart · Sinking and sourcing · Structured Text · Tag · Timer · VFD
- Analog signal (4-20 mA)
- A signal that varies continuously, such as a 4-20 mA loop from a pressure transmitter. The PLC scales it to engineering units such as bar. Learn it in the free Siemens TIA Portal course.
- Coil
- The output of a rung. When power reaches it, the coil turns its output or internal memory bit on. Learn it in the free CODESYS and IEC 61131-3 Programming course.
- Contact
- A condition on a rung. A normally open contact passes power when its input is on; a normally closed contact passes power when its input is off. Learn it in the free CODESYS and IEC 61131-3 Programming course.
- Counter
- An instruction that counts events, such as parcels passing a sensor, and turns an output on at a preset count. Learn it in the free Siemens TIA Portal course.
- DCS (distributed control system)
- A control system for continuous processes such as refineries and power plants, where control is spread across many controllers under one engineering environment.
- FAT (factory acceptance test)
- A test of a machine or panel at the supplier before delivery, against an agreed checklist.
- Fieldbus and industrial Ethernet
- Networks that connect PLCs, drives, I/O and HMIs, such as PROFINET, EtherNet/IP and Modbus.
- Function block (FB)
- A reusable piece of program with its own memory, used for things that repeat, such as controlling one motor. Learn it in the free Siemens TIA Portal course.
- HMI (human-machine interface)
- The screen where an operator sees the machine and gives commands, such as start, stop and set-points. Learn it in the free Siemens TIA Portal course.
- I/O (inputs and outputs)
- The connections between the PLC and the machine. Inputs bring in signals; outputs send commands. They can be digital (on or off) or analog (a range). Learn it in the free Electrical Fundamentals for Technicians: Volts to Multimeter to Drawings course.
- IEC 61131-3
- The international standard that defines PLC programming languages: ladder, function block diagram, structured text, instruction list and sequential function chart. Learn it in the free CODESYS and IEC 61131-3 Programming course.
- Interlock
- Logic or wiring that stops an unsafe or conflicting action, for example preventing forward and reverse contactors closing together. Learn it in the free Siemens TIA Portal course.
- Ladder logic (LAD)
- A graphical PLC language that looks like an electrical relay diagram: power flows left to right through contacts to a coil. It is the usual first language to learn. Learn it in the free CODESYS and IEC 61131-3 Programming course.
- PLC (programmable logic controller)
- A rugged industrial computer that reads inputs such as switches and sensors, runs a stored program and switches outputs such as motors and valves. It repeats this many times a second. Learn it in the free Siemens TIA Portal course.
- PLC simulation
- Software that behaves like a PLC so a program can be tested without hardware. Simulation teaches logic but does not control real machinery. Learn it in the free Siemens TIA Portal course.
- Rung
- One horizontal line of a ladder program. Contacts on the left are conditions; the coil on the right is the result. Learn it in the free CODESYS and IEC 61131-3 Programming course.
- SCADA
- Supervisory control and data acquisition: software that monitors and controls many machines or a whole plant from a central place, with alarms, trends and history.
- Scan cycle
- The loop a PLC runs forever: read the inputs, solve the program from top to bottom, write the outputs, then start again. How long one loop takes is the scan time. Learn it in the free Siemens TIA Portal course.
- Sequential function chart (SFC)
- A graphical language that describes a process as steps and the transitions between them, suited to sequences. Learn it in the free CODESYS and IEC 61131-3 Programming course.
- Sinking and sourcing
- Two ways of wiring DC inputs and outputs, depending on whether the PLC point provides or receives the current. Mixing them up is a common wiring mistake. Learn it in the free Siemens TIA Portal course.
- Structured Text (ST or SCL)
- A text PLC language like a simple programming language, good for maths and loops. Siemens calls its version SCL. Learn it in the free Siemens SCL Structured Text in TIA Portal course.
- Tag
- A named address for a value in the PLC or HMI, such as Motor1_Run, used instead of a raw number. Learn it in the free Allen-Bradley Studio 5000 course.
- Timer
- An instruction that waits for a set time. An on-delay timer (TON) turns its output on after its input has been on for the preset time. Learn it in the free Siemens TIA Portal course.
- VFD (variable frequency drive)
- A device that controls an AC motor's speed by changing the frequency and voltage it supplies.
Questions
Frequently asked questions
What is a PLC in simple words?
A rugged industrial computer that reads inputs such as switches and sensors, runs a stored program and switches outputs such as motors and valves. It repeats this many times a second.
What is ladder logic?
A graphical PLC language that looks like an electrical relay diagram: power flows left to right through contacts to a coil. It is the usual first language to learn.
What is the scan cycle of a PLC?
The loop a PLC runs forever: read the inputs, solve the program from top to bottom, write the outputs, then start again. How long one loop takes is the scan time.
What is the difference between HMI and SCADA?
An HMI is usually the operator screen for one machine or line. SCADA supervises many machines or a whole plant from a central place, with alarms, trends and history. The two overlap and are often built together.
Sources and how this page was checked
- IEC 61131-3: the PLC programming language standard, published by the International Electrotechnical Commission; definitions here are plain-language summaries, not quotations.
- EDWartens free courses: linked courses are EDWartens courses, not official vendor courses.
Keep reading
Definitions are simplified for beginners. For safety-critical work follow the machine maker's documentation and the standards that apply to your site.