PLC / API — Programmable Logic Controller
Definition
A PLC (Programmable Logic Controller)—known in French as an API, short for Automate Programmable Industriel—is a rugged industrial computer designed to automate production and control processes in real time. Unlike a desktop computer, the PLC is optimized to operate continuously in harsh environments: extreme temperatures, vibrations, humidity, dust, and electromagnetic interference.
The PLC executes a control program stored in memory and interacts with the physical world through inputs (sensors, buttons, flow meters, thermocouples) and outputs (actuators, valves, motors, indicator lights). It serves as the brain of production lines, water treatment plants, electrical substations, and a wide variety of other industrial equipment.
Acronyms
| Acronym | Language | Meaning |
|---|---|---|
| PLC | English | Programmable Logic Controller |
| API | French | Industrial Programmable Controller |
| PAC | French | Programmable Automation Controller (commonly used for modern variants) |
How It Works — The Scan Cycle
A PLC operates based on a scan cycle (or controller cycle) that repeats indefinitely as long as the controller is running:
- Reading Inputs: The PLC samples the status of all its physical inputs (digital and analog) and copies them into an internal memory area called the input image table.
- Program Execution: The processor executes the user program (ladder, FBD, ST, IL, or SFC according to the IEC 61131-3 standard) using the input image table.
- Updating the outputs: The program’s results are written to the output image table and then transferred to the physical outputs to control the equipment.
- System tasks: Communications, internal diagnostics, and updating the timestamp.
The duration of a scan cycle typically ranges from 1 ms to 100 ms, depending on the complexity of the program and the PLC model. Safety applications (SIL) require cycle times of less than 10 ms with watchdog mechanisms.
Major Manufacturers and Product Lines
The programmable logic controller (PLC) market is dominated by several major industrial manufacturers:
Siemens
- S7-1200: Compact series for small to medium-sized applications, TIA Portal-compatible, native Ethernet communication, supports Modbus TCP, Profinet, and OPC-UA.
- S7-1500: high-performance series with integrated safety (F-CPU), ultra-fast cycle times, and advanced trace and diagnostic capabilities.
- S7-300 / S7-400: legacy series still widely used at existing sites.
Schneider Electric
- Modicon M340: mid-range PLC, modular rack, with native Ethernet and Modbus TCP communication.
- Modicon M580: Ethernet-based PLC with CPU redundancy and OPC-UA communication, serving as the core of many new installations.
- Modicon Quantum: high-availability series for process industries (petrochemicals, energy).
Rockwell Automation (Allen-Bradley)
- MicroLogix: Compact, entry-level series.
- CompactLogix: versatile modular series with native Ethernet/IP protocol, widely deployed in North American manufacturing industries.
- ControlLogix: high-performance series with redundant chassis.
Mitsubishi Electric
- MELSEC iQ-R and MELSEC-F (FX) series: widely used in Asia and in the automotive and pharmaceutical industries.
Beckhoff
- CX / BX Series: PC-based PLC (PC-based automation), with native support for EtherCAT, OPC-UA, and TwinCAT. Particularly well-suited for motion control applications.
Supported Communication Protocols
Modern controllers support one or more of the following protocols, depending on the manufacturer and product line:
| Protocol | Physical Layer | Primary Use |
|---|---|---|
| Modbus RTU | RS-232 / RS-485 | Serial communication with sensors, drives, and meters |
| Modbus TCP | Ethernet | Network communication with SCADA, gateways, and HMIs |
| OPC-UA | Ethernet | Standardized data exchange with heterogeneous systems and the cloud |
| Profinet | Ethernet | Siemens deterministic communication between PLCs and decentralized devices |
| Ethernet/IP | Ethernet | Rockwell/ODVA standard for industrial communication |
| S7Comm / S7Comm+ | Ethernet | Proprietary Siemens protocol (requires a library or OPC-UA for third-party access) |
| EtherCAT | Ethernet | Ultra-fast communication for motion control (Beckhoff) |
Connecting to the Cloud via Eziwan
PLCs generally do not have native cloud connectivity. The connection is established through an industrial gateway such as the Eziwan Gateway, which acts as a bridge between the OT (PLC) network and the cloud.
The three main methods are:
- Modbus RTU via RS-485: A+/B- wiring between the gateway and the PLC's serial port. Ideal for older PLCs or devices without Ethernet.
- Modbus TCP via Ethernet: Direct network connection to the PLC’s Ethernet port. Standard port 502. Requires that the PLC and the gateway be on the same subnet.
- OPC-UA: Connection to the OPC-UA server built into modern PLCs (Siemens S7-1500, Schneider M580, Beckhoff). Offers the highest level of interoperability and security (authentication, certificates).
Once the connection is established, the Eziwan Gateway uploads data to the cloud in real time and opens an encrypted VPN tunnel for secure remote access.
Differences Between PLCs and RTUs
Although the two terms are sometimes used interchangeably, there are important distinctions:
- A PLC is designed for the local control of a machine or production line, with fast scan cycles and complex sequencing logic.
- An RTU (Remote Terminal Unit) is optimized for remote site management, featuring low power consumption, long-distance communication (4G, radio), and monitoring protocols such as DNP3 or IEC 60870.
For more information, see the complete definition of RTU.
IEC 61131-3 Programming Languages
The IEC 61131-3 standard specifies five programming languages for PLCs, which are supported by all modern PLCs (Siemens TIA Portal, Schneider Unity Pro, Rockwell Studio 5000, Codesys).
| Language | Abbrev. | Type | Suitable for | Example of use |
|---|---|---|---|---|
| Ladder Diagram | LD | Graphical | Digital logic for contacts/coils, electricians | Safety sequences, interlocks |
| Function Block Diagram | FBD | Graphical | Control, functional blocks | PID, counters, timers |
| Structured Text | ST | Text | Complex calculations, algorithms | Recipe processing, communications |
| Instruction List | IL | Text | Low-level (deprecated in IEC 61131-3 Ed. 3) | Legacy systems, older equipment |
| Sequential Function Chart | SFC | Diagram | Step-by-step process sequences | Assembly lines, machine tools |
A PLC project often combines several languages: SFC for sequencing steps, FBD for controlling continuous processes, and ST for calculations or communication with external devices.
Safety PLCs — Safety Controllers (SIL)
A Safety PLC is a programmable logic controller certified for safety instrumented systems (SIS) in accordance with IEC 61508 and its industrial application, IEC 61511. The safety level is expressed in SIL (Safety Integrity Level), ranging from SIL 1 to SIL 4.
| SIL | Probability of Failure (PFD) | Application Examples |
|---|---|---|
| SIL 1 | 10⁻² to 10⁻¹ | Simple process shutdown, presence detection |
| SIL 2 | 10⁻³ to 10⁻² | Emergency shutdown (ESD), burner protection |
| SIL 3 | 10⁻⁴ to 10⁻³ | Chemical reactor protection, pipeline |
Safety PLCs use redundant architectures (1oo2, 2oo3) with online diagnostics. Major manufacturers: Siemens S7-1500F (TÜV SIL 3), Schneider Modicon M580 Safety, Rockwell GuardLogix, Pilz PSS4000.
A safety PLC often operates in parallel with a standard PLC: the standard PLC manages the normal process, while the safety PLC monitors safety conditions and triggers the ESD if a limit is exceeded. Both can be connected to the same Eziwan Gateway, with separate access rights (read-only for the safety PLC, read/write for the standard PLC).
Soft-PLC — Software PLC on an Industrial PC
A Soft-PLC (or PC-based automation) implements the functionality of a PLC on a standard industrial PC, without proprietary hardware. Advantages: the PC's computing power, Linux/Windows flexibility, and native network connectivity.
The Codesys runtime (3S-Smart Software Solutions) is the European standard for Soft-PLCs. Many OEMs offer it under their own brand names: Beckhoff TwinCAT, Wago, Bosch Rexroth ctrlX, and Phoenix Contact PLCnext.
The Eziwan Gateway can read data from a Codesys Runtime via OPC-UA (native Codesys OPC-UA server) or via the Codesys Gateway Protocol—see the SCADA over 4G integration guide.
Related Resources
Frequently Asked Questions
What is the difference between a PLC and an RTU?
PLCs excel at local real-time control (logic, machine safety); RTUs are optimized for remote site management (long-distance protocols, autonomy, data logging). Modern product lines are converging, with a networked PLC often fulfilling both roles.
What are the leading brands of PLCs?
Siemens (S7), Schneider Electric (Modicon M221/M340/M580), Rockwell Allen-Bradley (CompactLogix, ControlLogix), Beckhoff (TwinCAT), WAGO, ABB, Omron, and Mitsubishi account for the vast majority of the installed base in Europe.
How do you connect a PLC to the cloud?
Through an industrial gateway that reads variables in Modbus TCP, S7, or OPC-UA on the machine side, then publishes them via MQTT to the cloud platform through a 4G VPN or Ethernet tunnel—without modifying the PLC program.
Is it possible to securely access a PLC remotely?
Yes, through industrial remote access: encrypted outbound tunnel, MFA authentication, access limited to the target device, and logged sessions. The manufacturer's consoles (TIA Portal, EcoStruxure) then function just as they would locally.
Related Resources
- Remote Access to Industrial PLCs — the secure access solution
- Connecting a PLC to the Cloud — all brands
- PLC vs. RTU — a detailed comparison
- Guide: Connecting a PLC — step-by-step Modbus / OPC-UA
- PLC vs. RTU for Remote Monitoring — the in-depth article