What Is a PLC? Types and Selection Guide
A PLC is an industrial control computer that processes sensor signals according to a program and drives outputs; a guide to selecting CPUs, mini PLCs and modules.
What is a PLC and where is it used
A PLC (programmable logic controller) is an industrial control computer that processes signals from sensors and push buttons according to the user program and drives outputs such as relays, contactors, valves and drives. In packaging machines, conveyor lines, and process and building automation, it runs the machine's logic inside the panel. It is also called a "programmable logic controller"; DIN-rail smart relays do a similar job on a smaller scale.
In the simplest terms, a PLC program runs cyclically: it reads the inputs, executes the program, writes the outputs and repeats this continuously. The programming languages under IEC 61131-3 include ladder diagram, function block diagram and structured text; which of them are supported varies by manufacturer.
Types of PLC systems
This category is divided into four subgroups:
- PLC CPUs: central processor modules that execute the program.
- Mini PLCs with Display: compact devices with a display for small applications.
- PLC Accessories: expansion cables, connection elements and auxiliary parts.
- Positioning Modules: modules for axis positioning tasks.
Input and output cards are in a separate category: PLC Input and Output Modules.
What to look at when choosing a PLC
| PLC Output / Module Type | Switching and Signal Structure | Advantage and Typical Use | Catalog Link |
|---|---|---|---|
| Relay Output CPU / Module | Dry contact (switches both AC and DC loads, ~2 A) | Isolates different voltages; ideal for contactor coils and solenoid valves | PLC Systems · Digital Output Modules |
| Transistor Output (PNP/NPN) | Solid-state 24 V DC switching (~0.5 A) | Wear-free high-speed pulse output (PTO/PWM) for servo/stepper drives and SSRs | PLC Systems · Solid State Relays |
| Digital Input Module | 24 V DC (sink/source) or 230 V AC discrete signals | Reads pushbuttons, limit switches, inductive and photoelectric sensors | Digital Input Modules |
- Number of inputs and outputs: Work out the number of digital inputs and outputs from the list of sensors, push buttons and actuators. In the list, the number of digital inputs and the number of digital outputs appear as filter properties.
- Output type: The examples include devices with relay outputs. The load type and switching frequency determine the output type.
- Supply voltage: Compact devices may be supplied with 24 V DC or 230 V AC; this information often appears in the product code.
- Expansion: In modular systems, the CPU, input-output modules and backplane cables are selected separately. Some compact devices accept an expansion unit and some do not; look at the terms "expandable" and "non-expandable" in the product name.
- Communication: Ports such as built-in Ethernet determine the connection to HMIs, drives and other devices.
- Program capacity and cycle time: Compare according to the size of the program and the response speed requirement.
- Security: If a safety function is required, safety PLCs and the SIL level are evaluated separately; the level is determined according to the project and the manufacturer data.
The series name (such as Modicon M221, M262, Zelio, Delta SE and SS2) also determines the manufacturer's software and accessory compatibility.
Difference between PLC, HMI and SCADA
The PLC is the controller that runs the machine. HMI and operator panels let the operator see values and give commands; they do not replace the PLC. SCADA is a central software layer that collects data from many PLCs and performs monitoring and logging.
Brands in this category
- ABB: safety PLC modules, PLC processors and accessories.
- Schneider Electric: Modicon series logic PLCs, input modules and backplane expansion cables.
- GEMO: DIN-rail smart relay / PLC devices.
- Delta Electronics: DVP series compact PLCs.
- GMT: GLC series PLC CPU modules.
Order notes
Products are sold by the piece. Some products are listed as "lead-time" items; for these, the delivery time is confirmed. To reach the right product, search by stock code or manufacturer model number.
Frequently asked questions
What is the difference between a PLC and a smart relay?
A smart relay is a compact control device, usually DIN-rail mounted, with a small number of inputs and outputs, and it is used for simple logic tasks; models that cannot be expanded by adding modules are common. A PLC, on the other hand, can offer modular expansion, more communication options and larger programs. The choice depends on the number of inputs and outputs and the expansion need.
What is SCADA and how does it relate to a PLC?
SCADA is a central software system that collects data from PLCs and other devices in the field and provides monitoring, alarms and logging. It is the PLC that runs the machine directly; SCADA brings the data of these controllers together on a computer screen. For this reason, SCADA is added on top of a PLC, not in place of it.
What is an HMI, and is it mandatory for a PLC?
An HMI is a touch panel or display that lets the operator see the machine status and enter parameters. A PLC works without an HMI; control with push buttons and lamps is possible. If the operator needs to adjust values, read alarms or select recipes, an HMI is added and connected to the PLC through a communication port.
In which programming languages is a PLC programmed?
The IEC 61131-3 standard defines graphical languages such as ladder diagram (LD) and function block diagram (FBD), and text languages such as structured text (ST) and instruction list (IL); sequential function chart (SFC) is used to structure the program. Which languages are supported varies by manufacturer and software; see the device's software documentation.
Are the CPU and the input-output modules bought separately?
In modular systems, yes: the CPU executes the program, and the input and output modules connect the signals in the field. In compact PLCs, on the other hand, the inputs and outputs are on the device itself. In a modular design, the number of modules the CPU can carry and the compatible series must be checked according to the manufacturer data.