Types of PLC

How they are classified

There is no single official classification, but in practice PLCs are told apart by three things:

• Size: how many inputs and outputs they can handle.
• Architecture: whether everything comes in one box or is assembled from modules.
• Special function: for example, whether they are certified for safety.

Let’s look at the most common types, from smallest to largest.

Programmable relay

Also called a smart relay. It is the smallest step: a compact device with few inputs and outputs, often with a small display and buttons on the front for programming it directly.

• Ideal for: simple tasks such as lighting control, a pump, a gate or ventilation.
• Limitations: little memory, few I/O, basic functions and communications.

Compact PLC

The CPU, power supply and a fixed set of inputs and outputs come in a single housing. Many models allow a few expansion modules to be added on the side.

• Ideal for: small and medium machines, budget-limited projects and learning.
• Advantages: affordable, easy to install, takes little panel space.
• Limitations: expansion and performance have a ceiling.

Modular PLC

The CPU, power supply and each input, output or communication module are separate pieces mounted side by side, usually on a DIN rail.

• Ideal for: complex machines and production lines.
• Advantages: you choose exactly the modules you need and can grow later; if a module fails, only that one is replaced.
• Limitations: more expensive and requires more planning than a compact PLC.

Rack (chassis) PLC

Modules slide into a rack with a backplane that connects them. This is the architecture of large systems.

• Ideal for: entire plants and continuous processes, with hundreds or thousands of I/O points.
• Typical features: remote I/O racks spread across the plant, module replacement with the system powered (hot swap) and redundancy: two CPUs running in parallel, where one takes over if the other fails.

Safety PLC

A PLC certified for functional safety according to standards such as IEC 61508, IEC 62061 or ISO 13849. Internally it uses redundant processing, continuous diagnostics and special inputs and outputs able to detect a broken wire or a welded contact.

• Used for: emergency stops, light curtains, safety doors, two-hand controls, burner and boiler shutdown systems.
• Many current models run a standard program and a safety program on the same CPU, kept separate and protected from each other.

PAC (Programmable Automation Controller)

PAC is a term used for high-end controllers that combine the best of a PLC with capabilities close to a computer: lots of memory, several tasks in parallel, motion control, heavy data handling and advanced communications.

In practice, the line between a “high-end PLC” and a “PAC” is blurry and depends a lot on how each manufacturer names its products.

What about PC-based control?

There is also the software PLC (soft PLC): a program that turns an industrial computer into a PLC, running the same kind of IEC 61131-3 languages. It is used when a lot of computing power or integration with other systems on the same device is needed.

Quick comparison

Type	Typical size (I/O)	Scalability	Typical use
Programmable relay	Few (tens or fewer)	Very low	Simple automation
Compact PLC	Tens	Low to medium	Small and medium machines
Modular PLC	Tens to hundreds	High	Complex machines, production lines
Rack PLC	Hundreds to thousands	Very high	Plants and continuous processes
Safety PLC	Depends on design	Depends on design	Machine and process safety functions
PAC	Hundreds to thousands	Very high	Complex, data-heavy systems

Which one to choose?

Some questions integrators ask before choosing a PLC:

• How many inputs and outputs do I need today, and how many might I need in the future?
• Are there analog signals, fast pulses or motion control?
• Which other devices must it talk to, and over which protocol?
• Are there safety functions to protect people?
• Which brands have good technical support and spare parts in my country?
• What equipment is already in the plant? Sticking to one brand simplifies maintenance.

Summary

• From smallest to largest: programmable relay → compact → modular → rack.
• Safety PLCs are certified to protect people.
• PACs and soft PLCs are controllers with computer-like capabilities.
• The choice depends on I/O, communications, safety, local support and expected growth.