How Machines Talk to Each Other: Industrial Networks
Why Do Machines Need to Communicate?
In a modern factory, dozens or hundreds of PLCs, sensors, drives, and robots each collect data. Industrial networks make this data flow between devices and toward monitoring and management systems.
Without a network: each machine is an isolated island. With a network: an integrated system that sees itself completely and adapts.
The OSI Model: Seven Layers
The OSI model divides communication into 7 layers, from physical cables (Layer 1) to application protocols (Layer 7). Key layers for industrial networking: Layer 1 (physical medium), Layer 2 (Ethernet frames), Layer 3 (IP addressing), Layer 4 (TCP reliability), Layer 7 (Modbus, OPC-UA protocols).
Serial Communication
RS-232: point-to-point, up to 15m, still found in legacy equipment.
RS-485: supports up to 32 devices on one cable, up to 1200m, resistant to electromagnetic interference — the physical layer for Modbus RTU.
Modbus: The Classic Protocol
Developed by Modicon in 1979 — the most widely deployed industrial protocol globally.
Works on Master/Slave model: the master sends requests, slaves respond. Each device has a unique address (1-247).
Data types:
- Coils: single bits, read/write
- Discrete Inputs: single bits, read only
- Holding Registers: 16-bit values, read/write
- Input Registers: 16-bit values, read only
Modbus TCP: same protocol over Ethernet/IP instead of RS-485.
Industrial Ethernet
- PROFINET (Siemens): real-time Ethernet, common in European industry
- EtherNet/IP (Rockwell): CIP protocol over TCP/UDP, common in North America
- EtherCAT (Beckhoff): sub-microsecond precision, ideal for robotics and servo drives
OPC-UA: The Future Standard
Open Platform Communications Unified Architecture solves vendor interoperability:
- Platform-independent: runs on Windows, Linux, embedded devices
- Secure: TLS encryption, authentication, authorization
- Rich information model: describes data meaning, not just values
- Works over TCP/IP: compatible with firewalls and routing
Example: A Siemens temperature sensor talks to an ABB SCADA system and a Mitsubishi PLC — all via OPC-UA without modification.
IIoT and MQTT
MQTT (Message Queuing Telemetry Transport) is the leading IIoT protocol:
- Publish/Subscribe model: devices publish data to "topics"
- Servers distribute to subscribers automatically
- Very lightweight: ideal for resource-constrained devices
- Persistent TCP connection
Sensor publishes: machines/line1/temperature → "35.2"
SCADA subscribed to: machines/+/temperature
→ receives all sensor data automatically
Protocol Comparison
| Protocol | Medium | Use Case |
|---|---|---|
| Modbus RTU | RS-485 | Legacy PLCs |
| Modbus TCP | Ethernet | Modern IP-connected devices |
| PROFINET | Ethernet | Siemens automation |
| EtherNet/IP | Ethernet | Rockwell/AB automation |
| EtherCAT | Ethernet | Robotics, servo drives |
| OPC-UA | TCP/IP | Cross-vendor system integration |
| MQTT | TCP/IP | IIoT, cloud connectivity |
Industrial Network Security
OT networks were historically air-gapped. IIoT connectivity creates risks: network segmentation, industrial firewalls between IT and OT, TLS encryption (OPC-UA, MQTTS), and regular firmware updates are essential.
Summary
Industrial networks are the nervous system of the modern factory. From simple Modbus for legacy devices to OPC-UA for cross-system integration to MQTT for cloud connectivity — each protocol has its place. Understanding these layers enables engineers to design systems that communicate effectively and securely.