Industrial Networking Basics: How PLCs Communicate

Why PLCs Need Networks

A programmable logic controller (PLC) rarely works alone. In a modern plant or large commercial building, dozens of PLCs, sensors, motor drives, and operator panels must share data constantly. Industrial networks are the highways that carry that data. As an electrician, you will pull cable, terminate connectors, troubleshoot communication faults, and verify that devices appear on a network—so understanding what each network does and where it fits is essential job knowledge.

The Industrial Network Hierarchy

Industrial networks are organized into three levels, sometimes called the automation pyramid.

Field Level (Device Level) — This is the lowest level, closest to the physical process. Sensors, push buttons, limit switches, solenoids, and variable-frequency drives (VFDs) connect here. Networks at this level move small amounts of data very fast. DeviceNet and Modbus RTU are common choices at this level.

Control Level — PLCs, remote I/O racks, and motion controllers communicate here. Speed is still important, but messages can be slightly larger. EtherNet/IP and Profibus DP are widely used at this level.

Information Level (Enterprise Level) — At the top, the plant's business computers, supervisory control and data acquisition (SCADA) systems, and manufacturing execution systems (MES) collect production data. Standard office Ethernet and TCP/IP protocols run here, connecting the factory floor to the front office.

Think of it as a chain: a temperature sensor reports to a field-level network, the PLC reads that data and acts on it at the control level, and a SCADA server logs the result at the information level.

Common Industrial Network Protocols

EtherNet/IP — EtherNet/IP (the "IP" stands for Industrial Protocol, not Internet Protocol, though it runs on standard Ethernet hardware) was developed by Rockwell Automation and is now managed by ODVA. It uses standard Ethernet cable, switches, and RJ-45 or M12 connectors that electricians already know. Data transfer rates are typically 100 Mb/s or 1 Gb/s. EtherNet/IP is popular at both the control level and increasingly at the field level for smart devices.

DeviceNet — Also managed by ODVA, DeviceNet uses a four-conductor trunk-and-drop cable (two for power, two for signal, plus a shield) that carries both data and 24 V DC power to connected devices. Data rates top out at 500 kb/s. Its rugged connectors and built-in power delivery make it practical for shop-floor devices like photoelectric sensors and pushbutton stations. The maximum trunk length depends on the selected baud rate—always check the manufacturer specification before pulling cable.

Modbus — One of the oldest industrial protocols, Modbus is simple, open, and still widely used for communicating with instruments such as meters, drives, and HVAC equipment. Modbus RTU runs over RS-485 wiring; Modbus TCP runs over Ethernet. Its simplicity makes it a first choice when connecting third-party devices to a PLC.

Profibus — Profibus DP is a European standard common in process industries and on equipment manufactured overseas. You will encounter it when working on imported machinery. It uses a shielded twisted-pair cable with a specific violet jacket and DB-9 connectors.

How an HMI Exchanges Data with a PLC

An HMI (Human-Machine Interface) is the operator panel—a touchscreen or keypad display—that lets a person monitor and control the PLC's program. The HMI does not run the control program; the PLC does. The HMI only reads and writes values in the PLC's memory.

Here is the data path when an operator presses a START button on an HMI touchscreen:

1. The HMI sends a write request over the network (often EtherNet/IP) to the PLC, asking it to set a specific memory tag to the value 1.

2. The PLC receives the request, writes the value to the tag, and its ladder logic detects the tag change.

3. The PLC outputs a signal to the motor starter coil.

4. The PLC sends the updated status tag back to the HMI.

5. The HMI display updates—perhaps changing a button color from red to green.

All of this happens in milliseconds. Tags are named memory locations inside the PLC. The HMI project is configured to read and write specific tags by name. If a tag name is mistyped or the network address is wrong, the HMI shows a communication error.

Electrical Safety on Industrial Networks

• Always apply lockout/tagout (LOTO) before touching wiring on a live control panel, even low-voltage network cables. Other circuits inside the enclosure may be at line voltage.
• DeviceNet carries 24 V DC on the trunk cable. While 24 V DC is generally not a shock hazard at typical current levels, a short circuit can cause a fire or damage devices.
• Wear appropriate PPE—at minimum safety glasses and insulated gloves rated for the voltage present—whenever working inside an energized enclosure.
• Never defeat network termination resistors or disconnect trunk cables on a live DeviceNet segment without first understanding the impact on connected devices.
• Industrial Ethernet runs at extra-low voltage (ELV) and presents minimal shock risk, but cable routing near high-voltage conductors can cause interference; follow separation requirements in the applicable installation standard.

Tying It to Your Career

NCCER Electrical Level 3 and Level 4 coursework introduces industrial controls and motor control circuits. Understanding network communication is a direct extension of that knowledge. State journeyman exams may not test specific brand protocols, but they do test your ability to work safely in control environments. OSHA 10 training reinforces the lockout/tagout and PPE practices that protect you every time you open a control cabinet.