PROFINET RT is Layer 2 only
PROFINET RT uses EtherType 0x8892. It cannot be routed. Keep all PROFINET devices in the same VLAN.
10.1 PROFINET
The previous chapter covered OT network topologies. Ring topologies with MRP provide the redundancy that industrial protocols require. The most widely deployed of those protocols in Europe is PROFINET.
Why PROFINET Exists
Section titled “Why PROFINET Exists”PROFIBUS DP served factory automation well, but its RS-485 physical layer limits bandwidth to 12 Mbps and distance to 100 m at full speed. PROFINET (Process Field Network) moves industrial I/O to standard Ethernet, providing gigabit speeds, longer distances, and integration with IT infrastructure. Siemens developed it. PROFIBUS & PROFINET International (PI) manages it. The IEC standardizes it as IEC 61158 Type 10.
Communication Classes
Section titled “Communication Classes”PROFINET defines three communication channels, each serving a different latency requirement:
| Channel | Cycle Time | Transport | Use |
|---|---|---|---|
| NRT (Non-Real-Time) | > 100 ms | TCP/IP | Configuration, diagnostics, alarms |
| RT (Real-Time) | 1 to 10 ms | Ethernet Layer 2 (EtherType 0x8892) | Standard I/O, drives, sensors |
| IRT (Isochronous Real-Time) | 250 us to 1 ms | Scheduled Ethernet + PTP | Motion control, synchronized axes |
PROFINET RT bypasses IP and TCP entirely. It sends raw Ethernet frames with EtherType 0x8892. This eliminates the variable latency that TCP retransmission introduces.
Device Model
Section titled “Device Model”| Role | Description |
|---|---|
| IO-Controller | The PLC. Initiates communication, reads inputs, writes outputs. |
| IO-Device | Field device (I/O module, drive, sensor). Responds to the controller. |
| IO-Supervisor | Engineering tool. Configures and diagnoses devices. |
Communication is cyclic (periodic I/O data exchange) and acyclic (on-demand parameter reads/writes, alarms).
Addressing and Discovery
Section titled “Addressing and Discovery”PROFINET devices are identified by a Device Name (human-readable, assigned during commissioning), an IP Address (assigned by the IO-Controller at startup), and a MAC Address (used for discovery before IP assignment).
DCP (Discovery and Configuration Protocol) operates at Layer 2. The IO-Controller uses DCP to discover devices and assign IP addresses and device names. GSDML (General Station Description Markup Language) is an XML file that describes a device’s capabilities, imported by the engineering tool.
PROFINET and VLANs
Section titled “PROFINET and VLANs”Place PROFINET RT traffic in a dedicated VLAN. Mark PROFINET RT frames with PCP 6 (802.1p priority). This ensures PROFINET frames are processed before lower-priority traffic in switch queues.
PROFINET and MRP
Section titled “PROFINET and MRP”PROFINET supports MRP for ring redundancy. The IO-Controller configures MRP on all ring switches as part of the PROFINET project. The MRP domain uses the same VLAN as the PROFINET RT traffic. When an MRP ring failover occurs, PROFINET detects the topology change and re-establishes communication within the MRP convergence time (less than 200 ms).
Key Takeaways
Section titled “Key Takeaways”DCP assigns addresses at startup
The IO-Controller assigns IP addresses via DCP. Static IP is not used for PROFINET IO-Devices.
Mark RT frames with PCP 6
PCP 6 priority ensures PROFINET RT frames are processed before best-effort traffic during congestion.
What Comes Next
Section titled “What Comes Next”PROFINET dominates in Europe. In North America, the equivalent protocol is EtherNet/IP. The next chapter compares EtherNet/IP’s architecture, explaining how its use of UDP/IP makes I/O traffic routable across subnets.
References
Section titled “References”- IEC 61158-6-10:2019 — Industrial communication networks: Type 10 elements (PROFINET)
- PROFIBUS & PROFINET International. (2022). PROFINET System Description: Technology and Application.
- Hirschmann. (2023). Application Note: PROFINET with Hirschmann Switches. Belden/Hirschmann.