Skip to content
PLCampus
Course contents
  1. 1. What is a PLC?
  2. 2. How a PLC works inside
  3. 3. Types of PLC
  4. 4. Inputs and outputs
  5. 5. Wiring and connections
  6. 6. Ports and communications
  7. 7. Memory and addressing
  8. 8. IEC 61131-3 languages
  9. 9. Basic instructions
  10. 10. Real-world good practices
On this page

Hardware & wiring

Ports and communications

USB, RS-232, RS-485 and Ethernet; industrial protocols such as Modbus, PROFIBUS, PROFINET, EtherNet/IP, CC-Link, EtherCAT and OPC UA; and what an HMI and a SCADA are.

4 min readReviewed: October 5, 2026

Why a PLC communicates

A PLC rarely works alone. It communicates to:

  • Be programmed and checked from a computer.
  • Show data on and take commands from an HMI screen at the machine.
  • Send information to a SCADA system that supervises the whole plant.
  • Talk to other PLCs, variable frequency drives, energy meters or robots.
  • Read remote I/O: input and output modules installed far from the PLC, linked by a single communication cable instead of hundreds of signal wires.

It helps to separate two ideas: the physical medium (the cable and its electrical signals) and the protocol (the language devices use to understand each other). The same RS-485 cable can carry Modbus RTU or PROFIBUS, and the same Ethernet cable can carry Modbus TCP, PROFINET, EtherNet/IP or OPC UA.

Physical media

USB

Used to program and diagnose from a computer. It is convenient, but not meant for permanent connections or long distances.

RS-232

Point-to-point serial communication (one device with another). It reaches short distances, around 15 m. Today it appears mostly on older equipment, printers, barcode readers and scales.

RS-485

Serial bus communication: several devices share the same twisted pair. It is very resistant to noise because it uses differential signals (the voltage difference between wires A and B). It allows distances of up to about 1200 m at low speeds and has termination resistors at both ends of the bus. It is the medium of Modbus RTU and PROFIBUS DP.

Ethernet

The dominant medium today. It uses network cables with RJ45 connectors (or M12 connectors in the field), switches and copper segments of up to 100 m. For longer distances or very noisy environments, fibre optics are used. Many PLCs have two ports with a built-in switch to daisy-chain devices.

Industrial protocols

Protocol Associated with Physical medium Typical use
Modbus RTU Open (created by Modicon, now Schneider) RS-485 / RS-232 Drives, meters, simple devices
Modbus TCP Open Ethernet The same as Modbus RTU, over Ethernet
PROFIBUS DP Siemens and the PI organization RS-485 Remote I/O and drives in existing plants
PROFINET Siemens and the PI organization Ethernet I/O, drives and real-time control
EtherNet/IP Rockwell Automation and the ODVA organization Ethernet I/O and drives, very common with Allen-Bradley
CC-Link / CC-Link IE Mitsubishi Electric and the CLPA organization Dedicated cable / Ethernet Machines and lines of Asian origin
EtherCAT Beckhoff and the EtherCAT Technology Group Ethernet Very fast motion control
OPC UA OPC Foundation (vendor independent) Ethernet (TCP/IP) Data exchange with SCADA, MES and IT systems

A few ideas to read the table:

  • Modbus is simple, open and old (late 1970s). That is why almost every device speaks it, even as a second option. It works with a master (or client) that asks and slaves (or servers) that answer.
  • PROFINET, EtherNet/IP, CC-Link IE and EtherCAT are industrial Ethernet: they add mechanisms to exchange data cyclically and predictably, which is key to controlling machines.
  • OPC UA does not compete with them in fast control: it is used to carry data securely and in an organised way to higher-level systems, independently of the PLC brand.

HMI and SCADA

  • An HMI (Human-Machine Interface) is the screen, usually a touch screen, that the operator has next to the machine. It shows states, alarms and values, and accepts commands such as starting a cycle or changing a recipe.
  • A SCADA (Supervisory Control And Data Acquisition) system supervises a whole plant or installations spread over a region, such as a drinking-water network. It gathers data from many PLCs, keeps history, manages alarms and shows everything in control rooms.

In both cases, the control logic stays in the PLC. The HMI and the SCADA read and write PLC variables, but if communication is lost, the PLC must keep controlling the process safely.

Industrial network security

Plant networks (OT networks, for Operational Technology) control physical equipment, so an attack or a mistake can stop production or damage machines. Some basic rules:

  • Never expose a PLC directly to the internet.
  • Separate the control network from the office network with security devices such as firewalls.
  • For remote access, use solutions designed for it (for example, a VPN) and controlled by the plant.
  • Change default passwords and disable services that are not used.

The IEC 62443 series is the international reference for cybersecurity in industrial automation and control systems.

Summary

  • Tell the physical medium (USB, RS-232, RS-485, Ethernet) from the protocol (Modbus, PROFINET…).
  • RS-485 is a robust, long-range serial bus; Ethernet is the dominant medium today.
  • Each major brand pushes an industrial Ethernet; Modbus and OPC UA are open.
  • The HMI is the machine’s screen; SCADA supervises the plant. The logic stays in the PLC.
  • Control networks are protected: no PLC exposed to the internet.