What is a PLC and what is it for?

What a PLC (Programmable Logic Controller) is

A PLC, or Programmable Logic Controller, is an industrial computer designed specifically to control physical processes: machines, production lines, conveyor systems, chemical plants. It reads signals from sensors, executes a logic program and sends commands to actuators like motors, cylinders or valves.

Where a home PC is optimised for multitasking, a PLC is optimised for one thing: running its program over and over, deterministically, 24/7, in environments where failure costs money. Dust, vibration, extreme temperatures, electrical interference. A PLC is built to survive.

If your plant has a big electrical cabinet and a machine that moves on its own, there's almost certainly a PLC inside. Or there should be.

A brief history: from wiring to software

Before PLCs, the logic of a machine was implemented with electromechanical relays wired inside huge cabinets. Changing a sequence meant redesigning the wiring — expensive, slow and error-prone work.

In 1968 General Motors publishes its famous specifications for an electronic standard machine controller, asking for a device that would do the same job as relays but was reprogrammable. Dick Morley and his team at Modicon respond with the Modicon 084, considered the first PLC in history.

Evolution has been steady since:

• 70s: first digital PLCs; ladder language mimicking relay diagrams.
• 80s-90s: serial communications, fieldbuses (Profibus, DeviceNet), integrated HMI.
• 2000s: industrial Ethernet (Profinet, EtherNet/IP), safety PLCs.
• Today: cloud integration, OPC-UA, digital twins, simulation before commissioning.

Internal architecture: what's inside a PLC

Every modern PLC shares five functional blocks:

• CPU (Central Processing Unit): the "brain". Runs the program in deterministic cycles.
• Memory: RAM for the running program, flash for code and persistent config.
• Input modules (I): receive signals from sensors (limit switches, thermocouples, pressure switches, encoders).
• Output modules (O): send signals to actuators (contactors, solenoid valves, drives).
• Communications: Profinet, EtherNet/IP, Modbus, OPC-UA. The PLC is the central node talking to HMIs, robots, SCADA, ERP.

The scan cycle: why a PLC is deterministic

The feature that defines a PLC and sets it apart from an industrial PC is the scan cycle: a sequence that repeats indefinitely with a guaranteed maximum time (typically 1-50 ms).

1. Read inputs: the PLC photographs the state of all sensors.
2. Execute program: resolves the logic with that snapshot.
3. Write outputs: updates all actuators simultaneously.
4. System tasks: diagnostics, communications, watchdog.

This cycle guarantees the system's response to a physical event is predictable. It's what lets a robotic cell operate without risk to an operator's life.

The five IEC 61131-3 languages

The international standard IEC 61131-3 standardises five PLC programming languages. A good automation engineer knows all of them and picks the right one:

• LD (Ladder Diagram): the classic. Graphical, mimics relay diagrams. Ideal for simple combinational logic and for the maintainer to understand it.
• FBD (Function Block Diagram): function blocks connected with lines. Very common in process control and analogue.
• ST (Structured Text): Pascal-like structured text. Unbeatable for calculations, loops and complex algorithms.
• IL (Instruction List): low-level, assembly-like. In decline today.
• SFC (Sequential Function Chart): graphical state machine. Perfect for batch processes and long sequences.

How to choose a PLC for your project

The question "which PLC brand do I use?" is only the last step. Before getting there, a serious integrator answers six others:

1. Number and type of I/O: digital, analogue, fast (encoders). Always oversize by 20%.
2. Required cycle time: servos? coordinated motion control?
3. Communications: robots? MES? corporate SCADA using a specific protocol?
4. Safety: is PL d / SIL 2 or above required? That forces F-CPU or equivalent.
5. End maintainer: if the neighbouring plant works with Siemens, fighting with Rockwell is a bad deal for the customer.
6. TCO: total cost over 10 years. Software licenses, training, spares, support.

The direct comparison between the two dominant platforms in Europe we develop in detail in Siemens vs Rockwell: when to choose each in a European project. Here we summarise the technical criteria before choosing the brand.

Safety PLC vs standard PLC

A Safety PLC (or F-CPU in Siemens vocabulary) is a controller certified to run safety functions with a measurable integrity level (PL under EN ISO 13849, SIL under IEC 62061).

It's not "a more expensive PLC": it's a CPU with dual processor, ECC memory, cross-diagnostics and an execution cycle validated by a certifying body. Its program — the emergency stop logic, guards, doors — runs in parallel to the normal program and is digitally signed.

The PLC in the Industry 4.0 era

The recurring question is: "does the PLC still make sense with all these edge gateways and AI in the cloud?". Short answer: yes, more than ever.

The PLC is the only device in the whole OT/IT pyramid that guarantees determinism, functional safety and 24/7 robustness. What has changed is its role: it's no longer an isolated controller — it's the node that produces reliable data upwards (OPC-UA, MQTT) and receives recipes and setpoints from MES/ERP. How to structure that level connection well we explain in ISA-95: how to connect PLC to ERP without breaking anything.

AI, cloud and pretty dashboards live above. But without the PLC doing its job well down there, everything else is just Power BI lying with charts.

Frequently asked questions

How much does a PLC cost?

Depends on the range (basic S7-1200, mid-range S7-1500, ControlLogix for large applications) and vendor. As a reference, the PLC usually represents 5-15% of the total cost of an automated system — the rest is engineering, periphery, electrical cabinet and commissioning. If you want a concrete estimate for your case, tell us about it.

Can a PLC be hacked?

Yes, and it has happened. Stuxnet attacked Siemens PLCs in 2010. That's why IEC 62443 is mandatory today in any serious project: network segmentation, industrial firewalls, user management and monitoring.

Can an Arduino or Raspberry Pi replace a PLC?

For prototypes or non-critical projects, yes. For a production line or a safety-rated machine, no. They aren't certified, don't have industrial warranty, and there aren't shop-floor technicians who know how to maintain them.

How long does it take a programmer to learn a new PLC?

Concepts (ladder, scan cycle, IEC 61131-3) are universal and transferable. A specific brand's environment (TIA Portal, Studio 5000, Unity Pro) takes 3 to 6 months of real work. Becoming an expert: 2-3 years.

Should the PLC documentation appear on the acceptance certificate?

It should be non-negotiable. Electrical schematics, variable list, commented code, backups and passwords, alarm list. Without those, the day your integrator disappears you're stuck.