How PROFINET Works ?

 

How PROFINET Works ?

The Engineering Guide to PROFINET: Architecture, Operation, and Industrial Ethernet Excellence

In the world of industrial automation, communication is the central nervous system of the factory. Among the various protocols that have emerged to bridge the gap between the office floor and the factory floor, PROFINET (Process Field Net) stands as the global leader. Developed by PROFIBUS & PROFINET International (PI), it is an open Industrial Ethernet standard that leverages the speed of IT networking with the rugged determinism required for high-speed manufacturing.

This article explores the mechanics of PROFINET, explaining how it achieves real-time performance, how it manages data, and why it has become the backbone of Industry 4.0.

 

What is PROFINET? The Convergence of IT and OT

PROFINET is not simply "PROFIBUS over Ethernet." While it shares the same user profiles and much of the philosophy of its predecessor, PROFINET is built natively on IEEE 802.3 (Ethernet).

The brilliance of PROFINET lies in its ability to handle "Office" traffic (HTTP, SNMP, DHCP) and "Industrial" traffic (I/O data, motion control, safety) on the same cable. It achieves this by bypassing certain layers of the standard TCP/IP stack for time-critical data—a process known as Channel Hopping or Direct L2 Communication.

The Functional Roles: Controllers, Devices, and Supervisors

To understand how PROFINET works, we must define the three primary "actors" in a PROFINET network:

A. The IO Controller (The "Brain")

Typically a Programmable Logic Controller (PLC) or a high-end Industrial PC. The Controller is the master of the configuration. It holds the hardware configuration for all connected devices and is responsible for establishing connections and exchanging cyclic I/O data.

B. The IO Device (The "Hands")

These are field devices such as Remote I/O modules, VFDs (Variable Frequency Drives), Servo Drives, and Sensors. A Device waits for a Controller to establish a connection. Once connected, it sends its inputs and receives its outputs at a predefined "Update Rate."

C. The IO Supervisor

Typically a laptop running engineering software (like TIA Portal) or an HMI. Supervisors are used for commissioning, diagnostics, and monitoring, but they do not control the cyclic I/O logic.

Communication Classes: Solving the Real-Time Challenge

Standard Ethernet is non-deterministic; collisions and "best-effort" delivery can cause unpredictable delays. PROFINET solves this by categorizing data into three communication classes:

Class 1: Non-Real-Time (NRT) / TCP/IP

For data that is not time-sensitive (e.g., configuring a device, downloading a program, or viewing a web page hosted on the PLC). This data uses the standard TCP/IP or UDP/IP stack (Layers 3 and 4). It is flexible but slow, with latencies in the 100ms range.

Class 2: PROFINET Real-Time (RT)

Used for the vast majority of industrial I/O. RT communication bypasses the TCP/IP layers and communicates directly at the Ethernet Layer (Layer 2). By removing the overhead of IP headers and routing, PROFINET RT can achieve update rates of 1ms to 10ms, which is sufficient for most factory automation tasks.

Class 3: PROFINET Isochronous Real-Time (IRT)

For high-speed motion control (e.g., 32 axes of servos synchronized to within 1 microsecond). IRT uses a hardware-based "Time Slot" method. The network cycle is split into a "Reserved" phase for IRT data and an "Open" phase for standard traffic. This ensures that the IRT data is never delayed by other network traffic.

 

The GSDML File: The Device's Passport

Every PROFINET device is accompanied by a GSDML (General Station Description Markup Language) file. This XML-based file tells the PLC:

• What sub-modules the device has.
• What parameters can be configured.
• The structure of the input and output data.
• The supported update rates.

Without a GSDML file, the Controller cannot communicate with the Device. It is the "driver" that allows the PLC to understand the device's "language."

Device Identification: Name vs. IP

In a standard home network, devices are found via IP addresses. In PROFINET, things are different.

1. MAC Address: Fixed by the manufacturer.
2. Device Name: Assigned by the engineer (e.g., Conveyor_Motor_1).
3. IP Address: Assigned by the PLC based on the Device Name.

How it works: When the PLC starts, it broadcasts a "DCP" (Discovery and Configuration Protocol) request: "Is there a device named Conveyor_Motor_1?" The device responds with its MAC address, and the PLC then sends it an IP address. This makes replacing a faulty device easier, as the PLC can often automatically assign the name to a new device based on its physical position in the network topology.

Cyclic vs. Acyclic Data Exchange

Cyclic Data (The Heartbeat)

This is the process data (Start/Stop bits, Speed setpoints, Sensor values). It is exchanged continuously. If a device fails to send its cyclic packet within a "Watchdog Time" (usually 3 cycles), the PLC will trigger a "Module Plug/Pull" fault and stop the process for safety.

Acyclic Data (On-Demand)

This is used for "Events."

• Alarms: A drive over-temperature alarm.
• Diagnostics: Reading the firmware version or serial number.
• Record Data: Changing a parameter value while the machine is running. This data is only sent when needed and does not impact the speed of the cyclic I/O.

Network Topologies: Flexibility by Design

Unlike old fieldbuses that required a "Daisy Chain," PROFINET supports:

• Line: Devices connected one after another (Integrated switches).
• Star: Devices connected to a central industrial switch.
• Tree: A combination of stars.
• Ring (MRP): Using Media Redundancy Protocol. If one cable breaks, the network detects the break and re-routes data in less than 200ms, preventing a machine shutdown.

Why PROFINET is the Standard for Industry 4.0

PROFINET is more than just I/O; it includes specialized "Profiles":

1. PROFIsafe: Allows safety data (E-Stops, Light Curtains) to travel over the same Ethernet cable as standard data, eliminating miles of expensive safety wiring.
2. PROFIenergy: Provides a standard way to put devices into "sleep mode" during breaks, significantly reducing factory energy consumption.
3. PROFIdrive: A standardized way for controllers to talk to drives, making it easier to swap drive brands without rewriting the entire PLC code.

 

PROFINET works by taking the standard Ethernet infrastructure we use in our offices and "hardening" it for the factory. By utilizing different communication classes (NRT, RT, IRT), it ensures that a simple web request won't delay a critical E-Stop signal or a high-speed servo move.

Its reliance on Device Names, GSDML files, and Layer 2 communication makes it a fast, robust, and user-friendly protocol. As we move deeper into the era of Big Data and the Industrial Internet of Things (IIoT), PROFINET’s ability to move massive amounts of data while maintaining microsecond precision ensures it will remain the dominant force in global automation.