PROFINET defines three Conformance Classes — CC-A, CC-B, and CC-C — that determine what a device can do. CC-A handles basic real-time communication. CC-B adds network diagnostics and topology discovery. CC-C adds Isochronous Real-Time (IRT) — the hardware-scheduled, sub-microsecond-jitter communication required for motion control, synchronized drives, and time-critical process control.
CC-C cannot be implemented with standard Ethernet hardware. It requires specialized ASICs or FPGAs that perform hardware-level frame scheduling, sub-microsecond synchronization, and cut-through forwarding with bridge delays under 3 µs. A regular managed switch or PC NIC will not work — even if it’s gigabit-grade.
This guide covers exactly what hardware is required to implement PROFINET CC-C, verified against IEC 61784-2:2014 (Communication Profile Family 3, CP 3/6) and PI Conformance Test Specification.
Table of Contents
1. CC-C vs CC-A vs CC-B: What Changes
The three Conformance Classes are defined in IEC 61784-2:2014, Section 8.1.7, Table 55:
| Feature | CC-A (CP 3/4) | CC-B (CP 3/5) | CC-C (CP 3/6) |
|---|---|---|---|
| Communication Class RT_CLASS_1 | Mandatory | Mandatory | Mandatory (or optional if IsochroneModeRequired) |
| Communication Class RT_CLASS_3 (IRT) | – | – | Mandatory |
| Cut-through forwarding | – | Recommended | Mandatory |
| Isochronous application (motion) | – | – | Optional |
| High performance application | – | – | Optional |
| IEEE 802.1Q VLAN tagging | – | Yes | Yes |
| LLDP topology discovery | Optional | Mandatory | Mandatory |
| Time sync (PTCP) | – | – | Mandatory |
| MRP (Media Redundancy) | Optional | Optional | Optional |
| MRPD (Seamless redundancy) | – | – | Optional |
The fundamental shift at CC-C is that the network becomes hardware-deterministic. Frame transmission, forwarding, and reception are scheduled by hardware in nanosecond-precise time slots, not by software.
For a broader conformance class overview, see: PROFINET Conformance Classes Explained
2. The Communication Classes Inside CC-C
CC-C uses three communication classes simultaneously, each with its own traffic priority:
| Communication Class | Traffic Class | Purpose |
|---|---|---|
| RT_CLASS_3 | RED | Isochronous Real-Time — guaranteed delivery in reserved time slots |
| RT_CLASS_1 | GREEN | Real-Time (non-isochronous) — cyclic I/O for non-critical signals |
| RT_CLASS_UDP | (Optional) | RT over UDP/IP for routable RT — optional |
The orange traffic class is reserved for legacy RT_CLASS_2 (no longer used per the standard — Table 28 explicitly states “the use of RT_CLASS_2 is not intended”).
3. RT_CLASS_3 — The IRT Communication Class
RT_CLASS_3 is what makes a device CC-C. It carries the cyclic IRT data with:
- Bandwidth reservation in the RED phase of the cycle
- Hardware-scheduled transmission at precise time offsets
- No collisions — every IRT frame has its own time slot
- Jitter < 1 µs end-to-end across the IRT domain
This requires the device’s MAC, switching fabric (if any), and physical interface to be scheduled by hardware, not software.
4. Timing Requirements for IRT Hardware
The most critical CC-C parameters from IEC 61784-2:2014 Table 29 (Communication Performance Parameters):
RT_CLASS_3 Specific Parameters
| Parameter | Minimum | Maximum | Meaning |
|---|---|---|---|
| REDBeginSafetyMargin | 0 ns | 1 640 ns | Time buffer before RED phase begins |
| MinSupportedFSO | 1 120 ns | 5 000 ns | Minimum supported Frame Send Offset |
| MinRTC3Gap | 1 120 ns | 2 000 ns | Minimum gap between RT_CLASS_3 frames |
| REDEndSafetyMargin | 0 ns | 0 ns | Time buffer at end of RED phase (fixed) |
RT_CLASS_1 / RT_CLASS_UDP Parameters (also required)
| Parameter | Minimum | Maximum | Meaning |
|---|---|---|---|
| MaxFrameStartTime | 960 ns | 5 000 ns | Maximum frame start time |
| MinNRTGap | 960 ns | 2 000 ns | Minimum gap for non-real-time frames |
These are nanosecond-level requirements. Standard Ethernet MACs cannot meet them. The hardware must include a hardware-scheduled MAC that transmits frames at exact, pre-calculated offsets.
5. Bridge (Switch) Requirements for CC-C
For PROFINET CC-C, the switch is part of the timing system. IEC 61784-2:2014 Table 30 — Parameters for RT_CLASS_3 bridges:
| Parameter | Value | Notes |
|---|---|---|
| Max Retention Time | Mandatory ≥ 20 µs; Optional < 4 ms | How long the bridge can hold a frame |
| Frame send offset deviation | Mandatory ≤ 10 ns | Maximum deviation from scheduled send time |
| SafetyMargin | 0 ns ≤ Mandatory ≤ 100 ns | Buffer for FrameDataProperties.ForwardingMode = “Relative mode” |
A 10 ns frame send offset deviation is extraordinary — far stricter than any commercial Ethernet switch. This is achievable only with dedicated PROFINET IRT ASICs.
Cut-Through Forwarding (Mandatory for CC-C)
Standard Ethernet switches use store-and-forward — receive the entire frame, check CRC, then forward. This adds latency proportional to frame size.
CC-C mandates cut-through forwarding — start forwarding the frame as soon as the destination address is read. This typically gives bridge delays under 3 µs for full-size frames.
From Table 55 (Conformance class behaviors):
CC-C: “CutThroughMode is mandatory.” CC-A/CC-B: “CutThroughMode is recommended” (CC-B) or “optional” (CC-A).
High Performance Application Class Parameters
If the bridge supports the optional “High Performance” application class (motion control, drive coordination), additional limits apply per IEC 61784-2:2014 Table 44:
| Parameter | Maximum Value |
|---|---|
| BridgeDelay | 1 µs |
| MaxPortTxDelay | 90 ns |
| MaxPortRxDelay | 210 ns |
| MaxDFP_Feed | 500 ns (Dynamic Frame Packing) |
| Peer to peer jitter | 250 ns |
These are sub-microsecond requirements per port — only achievable with hardware-MAC implementations.
6. End Node (IO Device / IO Controller) Requirements
Frame Send Offset Deviation (Table 31)
For an end node transmitting on RT_CLASS_3:
| Node Type | Frame Send Offset Deviation |
|---|---|
| Node with RT_CLASS_3 bridge | Mandatory ≤ 10 ns |
| End node (no embedded bridge) | Recommended ≤ 10 ns |
Minimum FrameSendOffset (Table 33)
| SendClockFactor | Minimum FrameSendOffset |
|---|---|
| SCF ≥ 8 (cycle time ≥ 250 µs) | Mandatory ≤ 5 000 ns |
| SCF < 8 (cycle time < 250 µs) | Mandatory ≤ 2 000 ns |
The Send Clock Factor (SCF) determines the cycle time: cycle = SCF × 31.25 µs. SCF=1 means 31.25 µs cycle — the fastest configurable in PROFINET.
7. PTCP Time Synchronization Requirements
CC-C uses PTCP (Precision Time Control Protocol) — a PROFINET-specific protocol carried on Ethertype 0x88F7 at Layer 2. It synchronizes every CC-C device on the IRT domain to a master clock with sub-microsecond accuracy.
IEC 61784-2:2014 Table 34 — PTCP control loop:
| Parameter | Value | Meaning |
|---|---|---|
| Allowed frequency changing speed | Mandatory ≤ 5 µHz/s | Maximum rate at which the slave clock can adjust its frequency |
This is the hardware oscillator stability requirement. The clock cannot drift faster than 5 microhertz per second — equivalent to ~5 ns per second worst case. This needs a temperature-compensated or oven-controlled oscillator (TCXO/OCXO) in the PROFINET IO controller acting as time master.
PTCP is similar in concept to IEEE 1588 but optimized for PROFINET. It is NOT compatible with IEEE 1588/PTP — devices must support PTCP specifically for IRT.
8. The 3-Phase IRT Cycle: RED, ORANGE, GREEN
In CC-C, every communication cycle is divided into three phases:
| Phase | Traffic Class | Purpose | Hardware Behavior |
|---|---|---|---|
| 🔴 RED | RT_CLASS_3 (IRT) | Isochronous data | Hardware-scheduled. Each frame has a reserved time slot. |
| 🟠 ORANGE | (Reserved) | Bandwidth guard | Buffer between RED and GREEN. |
| 🟢 GREEN | RT_CLASS_1 + standard Ethernet | Cyclic I/O + IT traffic | Standard Ethernet forwarding. Subject to QoS prioritization. |
A typical cycle:
|<------- 1 ms total cycle ------->|
|<-- 250 µs -->|<200µs>|<-- 550 µs -->|
| RED |Orange | GREEN |
| IRT data | | RT + IT |The RED phase is short — typically 10–20% of the cycle — but its frames have guaranteed timing. The remaining bandwidth is available for standard PROFINET RT and best-effort traffic.
For the standard to work, every device in the IRT domain must know the cycle schedule — which frame is sent when, on which port. This is the role of the IRT engineering tool (e.g., Siemens TIA Portal with Synchronization settings).
9. Hardware Architecture: Why an ASIC/FPGA Is Needed
A standard NIC or managed switch cannot deliver CC-C performance. The hardware must include:
| Hardware Block | Function |
|---|---|
| Hardware-scheduled MAC | Transmits frames at exact pre-programmed time offsets (deviation ≤ 10 ns) |
| PTCP synchronization engine | Receives PTCP messages, adjusts local clock, maintains < 5 µHz/s drift |
| Cut-through switching fabric | Forwards frames with < 1 µs bridge delay |
| Cycle scheduler | Manages RED/ORANGE/GREEN phase transitions |
| High-precision oscillator | Provides time reference for hardware MAC scheduling |
| GSDML parser / config interface | Receives IRT configuration from engineering tool |
This is why CC-C devices use dedicated PROFINET ASICs or FPGA IP cores — implementing all this in software is impossible at gigabit speeds.
10. Certified CC-C ASICs and FPGA Solutions
Siemens ERTEC Family
The ERTEC (Enhanced Real-Time Ethernet Controller) is the most widely deployed CC-C ASIC:
| Device | Description |
|---|---|
| ERTEC 200P | 2-port PROFINET IRT ASIC with ARM Cortex-M3 core. Used in Siemens IO devices. |
| ERTEC 400 | 4-port PROFINET IRT ASIC. Used in switches and complex IO devices. |
Hilscher netX Family
Hilscher netX SoCs combine ARM core + PROFINET IRT hardware:
| Device | Description |
|---|---|
| netX 51 / netX 52 | Compact SoCs for PROFINET IRT slaves |
| netX 90 | Newer generation, lower power, integrated CC-C support |
| netX 100 / netX 500 | Multi-protocol (PROFINET, EtherCAT, EtherNet/IP) with CC-C support for PROFINET |
Renesas R-IN32M3 / TPS-1
| Device | Description |
|---|---|
| R-IN32M3 | ARM Cortex-M3 + PROFINET IRT slave hardware |
| TPS-1 | Single-chip PROFINET IRT slave (jointly developed with Phoenix Contact) |
FPGA IP Cores
For custom designs:
- Softing PROFINET IRT IP Core for Xilinx and Intel FPGAs
- port GmbH PROFINET IRT Stack
- Molex Profinet IRT IP
These IP cores implement the hardware MAC and PTCP engine in FPGA fabric — used in motion controllers, drives, and high-end IO modules where standard ASICs don’t fit.
11. PROFINET CC-C Switches
A switch in a CC-C network must be certified for IRT forwarding. Key requirements:
| Feature | Requirement |
|---|---|
| Bridge delay | < 1 µs (for High Performance application class) |
| Frame send offset deviation | ≤ 10 ns |
| Cut-through forwarding | Mandatory |
| PTCP support | Mandatory (acts as PTCP transparent clock) |
| Per-port hardware MAC | Required |
| Number of priorities | At least 2 (4 recommended) |
Certified Examples
| Manufacturer | Product Family |
|---|---|
| Siemens | SCALANCE X-300/X-400/X-500 (PROFINET IRT models) |
| Phoenix Contact | FL SWITCH 2300/2400/2700 PN |
| Hirschmann (Belden) | OCTOPUS, MACH 1040 PROFINET |
| Hilscher | NSCM (Network Component Master) |
| MOXA | TN-G6512 series (PROFINET IRT models) |
⚠️ Critical: A switch certified for PROFINET CC-B does NOT support CC-C. CC-C certification is separate and stricter. Always verify the specific certification level before specifying.
12. Cabling and Physical Layer Requirements
CC-C imposes strict physical layer requirements:
| Parameter | Requirement | Source |
|---|---|---|
| Link speed | 100 Mbps full-duplex (minimum) | IEC 61784-2 Table 55 footnote i |
| Auto-negotiation | Mandatory at 100 Mbps full-duplex | IEC 61784-2 Table 55 footnote i |
| Higher speeds | 1 Gbit/s / 10 Gbit/s / 100 Gbit/s full-duplex optional | IEC 61784-2 Table 55 footnote i |
| Cabling | CAT5 / CAT5e / CAT6 copper or fiber (multi-mode / single-mode) | PROFINET Cabling Guideline |
| Max segment length | 100 m (copper), 2 km (multi-mode fiber), 14 km (single-mode) | Standard Ethernet |
| Cabling type | PROFINET-certified Type A (fixed), Type B (flexible), or Type C (highly flexible) | PROFINET Cabling Guideline |
Media converters that are not PROFINET-aware add asymmetric delay and break IRT timing. Use only PROFINET-certified media converters or fiber-to-copper modules in IRT segments, or avoid them entirely in the IRT path.
13. Media Redundancy: RED_CLASS_3 (MRPD)
PROFINET defines two media redundancy classes:
| Class | Protocol | Use With |
|---|---|---|
| RED_CLASS_1 | MRP (Media Redundancy Protocol) | RT_CLASS_1 + RT_CLASS_3 |
| RED_CLASS_3 | MRPD (MRP with Planned Duplication) | RT_CLASS_3 — seamless redundancy |
For CC-C with the highest availability requirements, MRPD provides bumpless redundancy — IRT frames are duplicated on both ring paths simultaneously. No reconfiguration time on cable break. The receiver discards the duplicate.
MRPD is optional in CC-C, but mandatory for safety-critical motion applications where even the ~30–60 ms MRP recovery time is unacceptable.
14. Topology Constraints for IRT
PROFINET IRT (CC-C) is sensitive to topology. The engineering tool calculates frame schedules based on the exact topology — wrong cabling breaks IRT.
Allowed Topologies
- Line (daisy chain) — most common
- Star (through PROFINET switches) — common
- Tree (line + star combinations) — supported
- Ring (only with MRP or MRPD) — for redundancy
Disallowed Topologies / Configurations
- Mixed CC-C and non-CC-C devices on the same IRT segment — non-CC-C devices break the schedule
- Standard (non-PROFINET) Ethernet switches in the IRT path — bridge delay too high, no PTCP support
- Spanning Tree Protocol (RSTP) on CC-C ports — incompatible with IRT timing
- VLAN tag modification by intermediate devices — breaks priority handling
LLDP is mandatory in CC-C for topology discovery — the engineering tool must know exactly which port connects to which neighbor to compute the IRT schedule.
15. How to Specify CC-C Hardware in a Tender
Use this checklist when writing a procurement spec:
Required Certifications
- IEC 61784-2:2014 CP 3/6 (PROFINET Conformance Class C)
- PI Conformance Test passed at CC-C level (mandatory PI certification, not vendor self-declaration)
- GSDML file provided with CC-C-specific records (IsochroneModeRequired, MinFSO, etc.)
Required Hardware Specifications
- Frame send offset deviation: ≤ 10 ns
- Bridge delay (for switches): ≤ 1 µs (High Performance) or ≤ 3 µs (general)
- MaxPortTxDelay: ≤ 90 ns
- MaxPortRxDelay: ≤ 210 ns
- PTCP master/slave/transparent clock support as appropriate to role
- Cut-through forwarding (mandatory for CC-C)
- 100 Mbps full-duplex minimum with auto-negotiation
- LLDP topology discovery (mandatory at CC-C)
- At least 2 traffic priorities (4 recommended)
- Frequency drift: ≤ 5 µHz/s (Table 34)
Optional but Recommended
- MRP support (RED_CLASS_1) for ring redundancy
- MRPD support (RED_CLASS_3) for seamless redundancy in safety-critical applications
- High Performance application class (cycle times < 250 µs) — for motion control
- IEC 61850-3 / IEEE 1613 environmental certification (if used in substations or harsh environments)
- Dynamic Frame Packing support (for bandwidth optimization)
Documentation to Request
- PI Conformance Test report (CC-C level)
- Detailed PROFINET parameter sheet listing all Table 29 / Table 44 values
- GSDML file
- Maximum IRT cycle frequency supported
- Maximum number of IRT devices per switch / per controller
- Hardware block diagram showing the IRT MAC implementation
Summary
PROFINET Conformance Class C is a hardware-level certification — not just a software feature. The required nanosecond-precision timing, sub-microsecond bridge delays, and PTCP synchronization cannot be implemented in software or on standard Ethernet hardware.
The key things to remember:
- CC-C corresponds to CP 3/6 in IEC 61784-2:2014, Table 55
- The defining feature is RT_CLASS_3 (IRT) with Traffic Class RED
- Frame send offset deviation ≤ 10 ns — only achievable with hardware-scheduled MAC (ASIC/FPGA)
- Cut-through forwarding is mandatory for switches at CC-C
- PTCP (Ethertype 0x88F7) provides time sync with frequency drift ≤ 5 µHz/s
- Common CC-C ASICs: Siemens ERTEC 200P/400, Hilscher netX 51/52/90, Renesas TPS-1, R-IN32M3
- CC-C switches: Siemens SCALANCE X-300/X-400, Phoenix Contact FL SWITCH 2300/2400, Hirschmann MACH 1040
- A device certified at CC-B does NOT meet CC-C — separate certification required
- For tenders, demand a PI Conformance Test report at CC-C level — not vendor self-declaration