PROFINET is one protocol — not three. But it comes in four conformance classes that define what a device can do, how fast it communicates, and what network infrastructure it needs.
Engineers often confuse PROFINET RT, PROFINET IRT, and PROFINET IO as separate protocols. They are not. They are all part of PROFINET IO, defined in IEC 61158 / IEC 61784-2. The conformance class determines which real-time features a device supports.
Choosing the wrong conformance class means either overspending on infrastructure you do not need or failing to meet the timing requirements of your application.
This guide explains each conformance class, what it requires, and when to use it.
Table of Contents
1. What Happened to PROFINET CBA
PROFINET originally had two variants:
- PROFINET CBA (Component Based Automation) — published in 2001, based on DCOM and RPC. Designed for modular plant engineering where autonomous subsystems communicate over TCP/IP.
- PROFINET IO (Input/Output) — published in 2003. Designed for real-time I/O data exchange between controllers and field devices, replacing PROFIBUS DP.
PROFINET CBA never gained market acceptance. It was complex, tied to Microsoft DCOM technology, and solved a problem that most users did not have.
PROFINET CBA was removed from IEC 61784-1 in the 4th edition (2014). It is no longer part of the active PROFINET standard. No new devices support it. No current engineering tools support it.
Today, when someone says “PROFINET,” they mean PROFINET IO. It is the only active variant.
2. PROFINET IO: The Only Active PROFINET Variant
PROFINET IO is defined in:
- IEC 61158-5-10 — Application layer service definition
- IEC 61158-6-10 — Application layer protocol specification
- IEC 61784-2-3 — Communication profile
The current specification version is V2.4 (with maintenance updates V2.4MU3 through MU6). Version 2.5 is in draft.
PROFINET IO uses the IO-Controller / IO-Device model:
| Role | Description |
|---|---|
| IO-Controller | PLC or DCS that manages the PROFINET network (e.g., Siemens S7-1500) |
| IO-Device | Field device — remote I/O, drive, sensor, valve (e.g., ET 200SP, SINAMICS drive) |
| IO-Supervisor | Engineering tool (e.g., TIA Portal) — used for configuration and diagnostics |
Within PROFINET IO, the conformance class defines the device’s capabilities.
3. What Is a Conformance Class
A conformance class is a defined set of features that a PROFINET device must support. It determines:
- Which real-time communication class the device uses (RT, IRT, or TSN)
- Which diagnostic features are mandatory
- Which network infrastructure is required (standard switches, managed switches, or IRT/TSN switches)
- Which certification tests the device must pass
The conformance class is stated in the device’s GSDML file and on its certification certificate.
4. Quick Comparison Table
| Feature | CC-A | CC-B | CC-C (IRT) | CC-D (TSN) |
|---|---|---|---|---|
| Real-time class | RT | RT | IRT | TSN (IEEE 802.1) |
| Min cycle time | 1–10 ms typical | 250 µs – 1 ms | 31.25 µs | 31.25 µs |
| Determinism | Soft | Soft | Hard (isochronous) | Hard (time-aware shaper) |
| Switch requirement | Any (unmanaged OK) | Managed (SNMP, LLDP) | IRT-capable managed | TSN-capable (IEEE 802.1Q-2018) |
| Topology detection (LLDP) | Not required | Required | Required | Required |
| Media redundancy (MRP) | Not required | Required | Required | Required |
| Device replacement without tool | Not required | Required | Required | Required |
| Fiber optic support | Optional | Optional | Optional | Optional |
| PROFIsafe support | Optional | Optional | Optional | Optional |
| Typical application | Building automation, infrastructure | Standard factory automation | Motion control, robotics | Next-gen automation, converged networks |
| IEC 61784-2 profile | CP 3/4 | CP 3/5 | CP 3/6 | CP 3/6 (with TSN extensions) |
5. Conformance Class A (CC-A)
CC-A is the simplest conformance class. It requires the minimum set of PROFINET features.
What CC-A Supports
- RT communication — cyclic I/O data using EtherType 0x8892 with VLAN priority tagging
- DCP — Discovery and Configuration Protocol for device name and IP address assignment
- Basic diagnostics
- Standard Ethernet cabling
What CC-A Does NOT Require
- No LLDP topology detection
- No MRP ring redundancy
- No managed switches — standard unmanaged switches are fine
- No device replacement without engineering tool
When to Use CC-A
CC-A is suitable for simple networks where timing is not critical: building automation, tunnel ventilation, infrastructure monitoring, or any application with update times above 10 ms.
Network Infrastructure
Standard Ethernet switches, standard Cat5e cables, no special requirements. This is the lowest-cost PROFINET configuration.
6. Conformance Class B (CC-B)
CC-B adds network management, diagnostics, and redundancy features on top of CC-A.
What CC-B Adds Over CC-A
- LLDP (Link Layer Discovery Protocol) — automatic topology detection. The IO-Controller knows which device is connected to which switch port.
- MRP (Media Redundancy Protocol) — ring topology with automatic failover. Recovery time typically < 200 ms (configurable down to 10 ms for small rings).
- Device replacement without engineering tool — a replacement device with the same name automatically receives its IP address and configuration from the IO-Controller.
- SNMP — Simple Network Management Protocol for switch and device management.
- Neighborhood detection — each device reports its neighbor via LLDP, enabling the engineering tool to display the full network topology.
When to Use CC-B
CC-B is the standard choice for most factory automation applications: machine control, conveyor lines, packaging machines, process control. It covers over 90% of all PROFINET installations.
Network Infrastructure
Managed industrial Ethernet switches are required. The switches must support LLDP, SNMP, and 802.1Q VLAN tagging with priority queuing. Standard industrial switches from Siemens (SCALANCE), Phoenix Contact, Hirschmann, or Cisco IE meet these requirements.
7. Conformance Class C (CC-C) — IRT
CC-C adds isochronous real-time (IRT) communication for applications that need deterministic cycle times with minimal jitter.
What CC-C Adds Over CC-B
- IRT (Isochronous Real-Time) — the communication cycle is divided into two phases:
- IRT phase — reserved time slots for isochronous data. Only IRT frames are allowed. This guarantees zero jitter.
- Open phase — standard RT and TCP/IP traffic is allowed.
- PTCP (Precision Transparent Clock Protocol) — synchronizes all device clocks on the network. Every device knows exactly when to send its data.
- Cycle times down to 31.25 µs with jitter below 1 µs.
- Hardware-based frame forwarding — IRT switches use cut-through forwarding with dedicated time-slot scheduling.
How IRT Works
In a normal RT network, Ethernet switches forward frames based on priority queues. High-priority RT frames go first, but they can still be delayed by other frames in the queue. This causes jitter.
IRT eliminates jitter by scheduling every frame in advance. The IO-Controller calculates a precise time plan for the entire network. Every switch and every device knows exactly when to send and when to listen. The time plan is downloaded during device startup.
When to Use CC-C
CC-C is required for high-speed motion control: coordinated multi-axis drives, robotics, CNC machines, printing presses, and any application where microsecond-level synchronization between devices is essential.
Network Infrastructure
IRT-capable managed switches are required. These are specialized industrial switches with hardware support for time-slot scheduling and cut-through forwarding. Standard IT switches do not work for IRT.
IRT-capable devices also need special Ethernet controller ASICs (e.g., Siemens ERTEC 200P/400) that support the IRT scheduling hardware.
⚠️ Important: IRT requires the entire data path — every switch and every device between the IO-Controller and the IO-Device — to be IRT-capable. One non-IRT switch breaks the time-slot schedule.
8. Conformance Class D (CC-D) — TSN
CC-D is the newest conformance class, introduced in PROFINET V2.4. It replaces the proprietary IRT mechanism with the IEEE 802.1 TSN (Time-Sensitive Networking) standards.
What CC-D Adds Over CC-C
- TSN (Time-Sensitive Networking) — uses IEEE 802.1Q-2018 standards instead of the proprietary PTCP/IRT mechanism:
- IEEE 802.1AS — Precision Time Protocol for clock synchronization (replaces PTCP)
- IEEE 802.1Qbv — Time-Aware Shaper (TAS) for scheduled traffic (replaces IRT time slots)
- IEEE 802.1Qbu / IEEE 802.3br — Frame Preemption — a low-priority frame can be interrupted mid-transmission by a high-priority isochronous frame
- RSI (Remote Service Interface) — acyclic communication without IP. Enables PROFINET to operate entirely on Layer 2, freeing IP for other protocols like OPC UA.
- Same cycle times as CC-C: down to 31.25 µs
- Convergence — PROFINET CC-D and other TSN-based protocols (OPC UA TSN, EtherNet/IP with TSN) can share the same physical network
Why TSN Matters
IRT (CC-C) works well but uses a proprietary scheduling mechanism. Only PROFINET devices and PROFINET-specific switches understand it. TSN uses IEEE standards that are supported by switches from many vendors — not just industrial automation vendors.
TSN enables converged networks where PROFINET real-time traffic, OPC UA data, and standard IT traffic coexist on the same Ethernet infrastructure with guaranteed quality of service.
When to Use CC-D
CC-D is for new high-performance installations where convergence with IT networks and interoperability with other TSN-based protocols is important. It is still in early adoption — most installations today use CC-B or CC-C.
Network Infrastructure
TSN-capable switches conforming to IEC/IEEE 60802 (the joint profile for industrial automation TSN). These switches support IEEE 802.1AS, 802.1Qbv, and 802.1Qbu/802.3br.
9. Real-Time Classes: RT vs IRT vs TSN
RT, IRT, and TSN are not separate PROFINET protocols. They are communication classes within PROFINET IO.
| Class | Mechanism | OSI Layer | EtherType | Determinism | Jitter |
|---|---|---|---|---|---|
| RT (Real-Time) | Priority queuing (802.1Q priority 6) | Layer 2 | 0x8892 | Soft — frames prioritized but not scheduled | < 1 ms |
| IRT (Isochronous Real-Time) | Time-slot scheduling (PTCP) | Layer 2 | 0x8892 | Hard — every frame has a reserved time slot | < 1 µs |
| TSN (Time-Sensitive Networking) | IEEE 802.1Qbv Time-Aware Shaper | Layer 2 | 0x8892 | Hard — scheduled gates per IEEE standards | < 1 µs |
All three use the same EtherType (0x8892) and the same PROFINET IO application protocol. The difference is in how the Ethernet layer handles frame scheduling.
10. Cycle Times by Conformance Class
| Conformance Class | Min Cycle Time | Typical Range | Use Case |
|---|---|---|---|
| CC-A | ~4 ms | 4–512 ms | Building automation, monitoring |
| CC-B | 250 µs | 1–32 ms | Standard factory I/O, drives |
| CC-C (IRT) | 31.25 µs | 31.25 µs – 1 ms | Motion control, multi-axis synchronization |
| CC-D (TSN) | 31.25 µs | 31.25 µs – 1 ms | Next-gen motion control, converged networks |
The cycle time is configured in the engineering tool (e.g., TIA Portal) per IO-Device. Each device can have a different cycle time.
The MinDeviceInterval attribute in the GSDML file defines the shortest cycle time a device supports. It is specified in multiples of 31.25 µs (e.g., MinDeviceInterval = 32 means 1 ms minimum).
11. Switch and Hardware Requirements
| Requirement | CC-A | CC-B | CC-C (IRT) | CC-D (TSN) |
|---|---|---|---|---|
| Unmanaged switch | ✅ | ❌ | ❌ | ❌ |
| Managed switch (LLDP, SNMP) | Optional | ✅ | ✅ | ✅ |
| IRT-capable switch (PTCP, cut-through) | ❌ | ❌ | ✅ | ❌ |
| TSN-capable switch (802.1Qbv, 802.1AS) | ❌ | ❌ | ❌ | ✅ |
| Integrated 2-port switch in devices | Optional | Recommended | Required | Required |
| Special Ethernet ASIC in devices | ❌ | ❌ | ✅ (e.g., ERTEC) | ✅ (TSN PHY) |
12. Conformance Classes and PROFIsafe
PROFIsafe is independent of the conformance class. It works on all four classes (CC-A through CC-D).
PROFIsafe adds a safety layer on top of standard PROFINET frames. It uses the “black channel” principle — the safety protocol does not trust the transport network. It adds its own CRC, watchdog, and sequence counter inside the PROFINET user data.
This means you can run PROFIsafe over a simple CC-A network with unmanaged switches. You do not need IRT or TSN for safety.
13. How to Identify a Device’s Conformance Class
| Method | Where to Find It |
|---|---|
| GSDML file | Look for ConformanceClass attribute in the DeviceAccessPointItem element |
| Device datasheet | Usually listed under “Communication” or “PROFINET features” |
| Certification certificate | PI International issues certificates stating the tested conformance class |
| PI product database | Search on profibus.com — lists all certified devices with their conformance class |
Example from a GSDML File
xml
<DeviceAccessPointItem
ConformanceClass="CC-B"
SupportedRT_Classes="RT_CLASS_1"
...RT_CLASS_1 = RT (CC-A/CC-B). RT_CLASS_3 = IRT (CC-C).
14. Which Conformance Class Should You Use
| Application | Recommended CC | Why |
|---|---|---|
| Building automation, tunnels, HVAC | CC-A | Simple, cheap, timing not critical |
| Standard factory I/O, packaging machines | CC-B | Best balance of features and cost |
| Machine control with drives (no multi-axis sync) | CC-B | 1 ms cycle is fast enough for most drives |
| Multi-axis motion control, robotics | CC-C (IRT) | Microsecond synchronization required |
| CNC machines, high-speed printing | CC-C (IRT) | Jitter below 1 µs required |
| New plants with OPC UA + PROFINET convergence | CC-D (TSN) | Future-proof, converged IT/OT network |
💡 Tip: If you are not sure, start with CC-B. It handles over 90% of factory automation applications. Only upgrade to CC-C if your application requires cycle times below 1 ms or isochronous multi-axis synchronization.
15. Migration Path: CC-A → CC-B → CC-C → CC-D
Each conformance class is a superset of the previous one:
- CC-B includes everything in CC-A plus LLDP, MRP, and device replacement
- CC-C includes everything in CC-B plus IRT
- CC-D includes everything in CC-B plus TSN (CC-D replaces IRT with TSN, not extends it)
This means:
- A CC-B device works on a CC-A network (it just does not use the extra features)
- A CC-C device works on a CC-B network in RT mode (without IRT features)
- A CC-D device works on a CC-B network in RT mode (without TSN features)
You can mix conformance classes on the same network. The IO-Controller manages each device according to its capabilities.
Summary
PROFINET IO is the only active PROFINET variant. PROFINET CBA was removed from IEC 61784-1 in 2014. IRT is not a separate protocol — it is a real-time class within PROFINET IO.
The key points:
- CC-A — basic PROFINET. Standard switches. Timing above 4 ms. Cheapest option.
- CC-B — standard factory automation. Managed switches. LLDP topology. MRP redundancy. Covers 90%+ of applications.
- CC-C (IRT) — isochronous real-time. Cycle times down to 31.25 µs. Requires IRT-capable switches and device ASICs.
- CC-D (TSN) — replaces proprietary IRT with IEEE TSN standards. Same performance as CC-C but with IT convergence. Still in early adoption.
- PROFIsafe works on all conformance classes.
- When in doubt, choose CC-B.