PLC and automation glossary

The terms used in the course, explained in a few words. Each one includes its Spanish name, so you can follow material in both languages.

4–20 mA

Standard analog current signal in instrumentation. The 4 mA "live zero" makes a broken wire detectable.Read more

Analog inputIn Spanish: Entrada analógica

Input that measures a continuous value, such as 0–10 V or 4–20 mA, and converts it into a number.Read more

Bit

Smallest unit of memory: it is 0 or 1.Read more

Byte

Group of 8 bits.Read more

CODESYS

IEC 61131-3 programming environment from an independent company, used as a base by many PLC manufacturers.

CoilIn Spanish: Bobina

Ladder output instruction that writes the rung result to a bit.Read more

CommonIn Spanish: Común

Terminal shared by a group of inputs or outputs that closes the circuit of the whole group.Read more

Compact PLCIn Spanish: PLC compacto

PLC with CPU, power supply and inputs and outputs in a single housing. It usually accepts a few expansion modules.Read more

Contactor

Electromechanical power switch, driven by a coil, that connects large loads such as motors.Read more

CPU

The PLC central unit: it runs the program and manages memory and communications.Read more

CTU (up counter)In Spanish: CTU (contador ascendente)

Counter that adds 1 on each rising edge and turns its output on when it reaches PV.Read more

Digital inputIn Spanish: Entrada digital

Input with only two states (0 or 1), such as a push button or a proximity sensor.Read more

DIN railIn Spanish: Riel DIN

Standard 35 mm metal rail on which PLCs, terminal blocks and other panel devices are mounted.Read more

EdgeIn Spanish: Flanco

Detection of a signal change: 0 to 1 (rising) or 1 to 0 (falling). It lasts a single scan cycle.Read more

Emergency stopIn Spanish: Paro de emergencia

Red mushroom push button on a yellow background that stops the machine for safety. It uses NC contacts and acts through a safety circuit.Read more

EtherNet/IP

Industrial Ethernet backed by Rockwell Automation and the ODVA organization.Read more

Fail-safeIn Spanish: A prueba de fallas

Design in which a fault (broken wire, power loss) brings the machine to a safe state.Read more

FBD (function block diagram)In Spanish: FBD (diagrama de bloques de funciones)

Graphical language of blocks joined by signal lines.Read more

ForcingIn Spanish: Forzado

Manually fixing the value of an input or output, overriding the program or the wiring. Useful for testing, dangerous if forgotten.Read more

HMI

Human-machine interface: screen next to the machine to see states and alarms and give commands.Read more

IEC 61131-3

International standard defining PLC programming languages: LD, FBD, SFC, ST and IL (the latter removed in 2025).Read more

InterlockIn Spanish: Enclavamiento entre salidas

Condition that prevents two incompatible actions from happening at the same time, such as running forward and reverse.Read more

Internal markerIn Spanish: Marca interna

Bit or number in PLC memory that the program uses as its own memory, not connected to the outside.Read more

Ladder diagram (LD)In Spanish: Ladder (LD, diagrama de escalera)

Graphical language that imitates a relay wiring diagram with contacts and coils.Read more

Lockout/tagout (LOTO)In Spanish: Bloqueo y etiquetado (LOTO)

Procedure that isolates, locks and tags an energy source before working on equipment.Read more

Modbus

Simple open communication protocol, in RTU (serial) and TCP (Ethernet) versions. Almost every industrial device speaks it.Read more

Modular PLCIn Spanish: PLC modular

PLC made of separate modules (CPU, power supply, I/O, communications) combined to suit the application.Read more

NC contact (normally closed)In Spanish: Contacto NC (normalmente cerrado)

On a device, a contact that is closed at rest. In Ladder, a contact that passes power when its bit is 0.Read more

NO contact (normally open)In Spanish: Contacto NA (normalmente abierto)

On a device, a contact that is open at rest. In Ladder, a contact that passes power when its bit is 1.Read more

NPN sensorIn Spanish: Sensor NPN

Sensor whose output connects to 0 V when it detects (sinking). It connects to source inputs, with the common at +24 V.Read more

Online changeIn Spanish: Cambio en línea

Editing the program while the PLC is in RUN, without stopping the machine.Read more

OPC UA

Vendor-independent standard to exchange industrial data securely with SCADA, MES and IT systems.Read more

Overload relayIn Spanish: Relé térmico

Motor protection against overload. Its NC auxiliary contact tells the control circuit when it trips.Read more

PAC

Programmable automation controller: high-end controller combining PLC functions with computer-like capabilities.Read more

PLC (programmable logic controller)In Spanish: PLC (controlador lógico programable)

Industrial computer that reads inputs, runs a program in cycles and drives outputs to automate machines and processes.Read more

PNP sensorIn Spanish: Sensor PNP

Sensor whose output delivers +24 V when it detects (sourcing). It connects to sink inputs, with the common at 0 V.Read more

Process imageIn Spanish: Imagen de proceso

In-memory copy of the input and output states. The program works on this copy, which is refreshed at the start and end of each cycle.Read more

PROFINET

Industrial Ethernet backed by Siemens and the PI organization for I/O, drives and real-time control.Read more

Programmable relayIn Spanish: Relé programable

Small, low-cost controller for simple automation, with few inputs and outputs.Read more

Relay outputIn Spanish: Salida a relé

Output with a mechanical contact: it switches AC or DC and isolates, but it is slow and wears out.Read more

Retentive memoryIn Spanish: Memoria retentiva

Memory area that keeps its value when the PLC is powered off.Read more

RS-485

Differential, noise-resistant serial bus medium for several devices over long distances.Read more

RungIn Spanish: Peldaño

Each line of a Ladder program: conditions on the left, outputs on the right.Read more

Safety PLCIn Spanish: PLC de seguridad

PLC certified for safety functions, with internal diagnostics and redundancy. Used for emergency stops, light curtains and guard doors.Read more

SCADA

Supervisory control and data acquisition system for a whole plant: history, alarms and control rooms.Read more

Scan cycleIn Spanish: Ciclo de scan

Loop the PLC repeats endlessly: read inputs, execute the program, write outputs and housekeeping.Read more

Scan timeIn Spanish: Tiempo de ciclo

Duration of one complete scan cycle, usually a few milliseconds. It depends on the program size and the CPU.Read more

Seal-in (latch)In Spanish: Autorretención

Contact of an output in parallel with its start button to keep it on after release.Read more

Set / Reset

Coils that set a bit to 1 (set) or 0 (reset) and leave it there: they form a memory.Read more

SFC (sequential function chart)In Spanish: SFC (diagrama funcional secuencial)

Graphical language that organises a process into steps and transitions.Read more

ST (structured text)In Spanish: ST (texto estructurado)

High-level textual language with IF, CASE, loops and maths expressions.Read more

Tag (variable)

Symbolic name linked to a PLC data item, such as START for I0.0.Read more

Terminal blockIn Spanish: Bornera

DIN-rail connection block where field wiring lands before going to the PLC.Read more

TOF (off-delay timer)In Spanish: TOF (retardo a la desconexión)

Timer whose output stays on for PT after its input drops.Read more

TON (on-delay timer)In Spanish: TON (retardo a la conexión)

Timer whose output turns on after its input has been on for time PT.Read more

TP (pulse timer)In Spanish: TP (pulso)

Timer that gives a pulse of duration PT when its input turns on.Read more

Transistor outputIn Spanish: Salida a transistor

Electronic DC output: fast and long-lived, ideal for frequent switching and pulses.Read more

Variable frequency drive (VFD)In Spanish: Variador de frecuencia

Device that controls an induction motor speed by varying the frequency and voltage it supplies.Read more

WatchdogIn Spanish: Watchdog (perro guardián)

Supervision that stops the PLC if a scan cycle takes too long, for example because of an endless loop.Read more

WordIn Spanish: Word (palabra)

Group of 16 bits; it stores, for example, an INT or the value of an analog input.Read more

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