What is IRT?
IRT (Isochronous Real-Time) is the highest real-time class of PROFINET, in which cyclic process data is sent in a pre-planned, reserved time phase of every bus cycle, while all devices and switches keep their clocks aligned to within one microsecond. This makes cycle times of 250 µs common and cycles down to 31.25 µs possible, with jitter below 1 µs. IRT belongs to conformance class C (CC-C) and is intended for motion control, servo axes and other applications in which several devices must act at exactly the same moment.
In cybersecurity, the abbreviation IRT is also used for the Incident Response Team. That meaning is covered further down this article.
🧠 What is PROFINET IRT used for?
Ordinary PROFINET RT (conformance classes A and B) achieves cycle times of around 1 ms over standard Ethernet, with jitter of roughly 10 to 100 µs. For most I/O that is more than enough. For synchronised axes it is too coarse: if two servos receive their setpoints a few tens of microseconds apart, the result is a visible error in cutting, printing or packaging work.
IRT solves this with two mechanisms:
- Bandwidth reservation - part of every cycle is reserved exclusively for IRT frames, so other traffic can never delay them
- Clock synchronisation - controller, devices and switches share one time base, so every frame is sent and forwarded at a pre-calculated moment
Isochronous transmission was added to PROFINET IO in 2004, specifically to make the protocol suitable for motion control. The PROFIdrive drive profile requires synchronised drives for application classes 4 to 6 (positioning and coordinated axes), and IRT is the standard route for this in PROFINET. Typical applications include presses, packaging machines, robotics, printing machines and measurement set-ups in which values from several stations must be captured at the same instant.
🔧 How does the IRT cycle work?
Every IRT cycle has a fixed length, the send clock, between 31.25 µs and 4 ms. Most applications run at 1 ms; motion control typically runs at 250 µs to 1 ms. The cycle is divided into phases:
| Phase | Content | Switch behaviour |
|---|---|---|
| Red | IRT frames only (real-time class 3, RTC3) | Time-triggered: each frame has a pre-calculated send offset and is forwarded cut-through |
| Orange (optional) | Other time-critical frames | Reserved time window |
| Green | RT frames (RTC1), alarms, synchronisation frames, TCP/IP and other traffic | Priority-based per IEEE 802.1Q, as in ordinary PROFINET |
| Yellow (end of green) | Only frames that still fit before the next red phase | Store-and-forward; the switch checks the frame length |
The yellow transition lasts 125 µs, the time a maximum 1,500-byte Ethernet frame needs at 100 Mbit/s. On top of that the switch keeps a safety margin of 640 ns, so a late frame never ends up in the red phase of the next cycle. Because the red phase only contains scheduled frames, the delay of every IRT frame can be predicted to the nanosecond.
For cycles below 250 µs, PROFINET V2.3 adds two extensions. Fragmentation splits large frames in the green phase so that phase can be shorter. Dynamic Frame Packing (DFP) bundles the output data for all devices in a line topology into a single frame; each device takes out its own part, so the frame shrinks along the way. On the return path, inputs are combined in the same way.
⏱️ How does IRT synchronise the clocks?
IRT uses PTCP (Precision Transparent Clock Protocol), a protocol derived from PTP (IEEE 1588). PTCP has two kinds of message:
- Line delay measurements - each pair of neighbours periodically measures the propagation time over the cable, independently of the connection state
- Sync frames - the sync master, usually the controller, distributes the system time; each device adds the measured cable delay and its own forwarding time (bridge delay)
All participants that take part in IRT together form a sync domain: one sync master with assigned sync slaves. Because cable length and switch delay count, the engineering tool must know the topology. In TIA Portal you therefore define for each port which device is connected to it. Based on that, the tool calculates the transmission schedule for every frame and every switch offline.
On top of IRT, the application itself can also run in step with the bus cycle: isochronous mode. Inputs are then read at a fixed time Ti before the cycle and outputs are applied to the process at a fixed time To after the start of the bus cycle. In a Siemens S7-1500, the program runs in a Synchronous Cycle OB, whose cycle may be an integer multiple of the send clock. This way all stations measure at the same moment, even when they are far apart.
⚙️ What is the difference between RT, IRT and TSN?
| Feature | PROFINET RT (CC-A/B) | PROFINET IRT (CC-C) | PROFINET over TSN (CC-D) |
|---|---|---|---|
| Scheduling | Priority via VLAN tag | Fixed transmission schedule per frame | Priority, with time windows per IEEE 802.1Qbv for time-critical streams |
| Clock synchronisation | Not required | PTCP, below 1 µs | IEEE 802.1AS (gPTP) |
| Cycle time | Around 1–10 ms, minimum 250 µs | 250 µs, down to 31.25 µs with V2.3 extensions | Short enough for motion control |
| Jitter | Around 10–100 µs | Below 1 µs | Low, provided traffic is time-scheduled |
| Hardware | Standard Ethernet, 802.1Q switches | IRT ASICs and CC-C certified switches | TSN-capable devices and switches |
| Topology planning | Not required | Mandatory | Required for time-critical streams |
IRT relies on dedicated hardware. Well-known chips include the ERTEC family from Siemens, the TPS-1 with two integrated ports and an IRT switch, and Hilscher’s netX controllers, where the IRT behaviour is implemented in microcode. Every component in the IRT path, including every industrial switch, must support CC-C. PROFINET over TSN replaces this proprietary mechanism with open IEEE standards, making it easier for time-critical and ordinary IT traffic to share the same network.
Outside the PROFINET world, EtherCAT and SERCOS III are the main alternatives for motion control. They take a different approach: a single frame processed on the fly by every device, instead of switched traffic following a planned schedule.
🧭 How do you set up an IRT network?
- Check the conformance class - ask for the PROFINET certificate with CC-C for the controller, drives, O and switches. A single switch without IRT support in the path breaks the schedule.
- Define the sync domain - assign the sync master and add all IRT devices as sync slaves. Select IRT as the RT class for those devices.
- Record the topology - link every port to its neighbour in the engineering tool and enter the cable lengths. The schedule only applies to this cabling; a swapped cable produces a topology error.
- Choose the send clock - start at 1 ms or 500 µs and only go lower if the control loop really needs it. Shorter cycles enlarge the red share and leave less room for other traffic.
- Configure isochronous mode - preferably set Ti and To to the same values for all stations when measured values from several stations must coincide.
- Arrange redundancy - use MRP with MRPD if the machine cannot tolerate any interruption (see below).
- Test and monitor - after commissioning, check the diagnostics for loss of synchronisation and late frames, and include those counters in your network monitoring.
🛡️ How do redundancy and security work with IRT?
Ordinary MRP switches over within 200 ms after a cable break. For a servo axis with a 250 µs cycle, that is an eternity. PROFINET therefore offers MRPD (Media Redundancy for Planned Duplication): the sender transmits every IRT frame around both sides of the ring at the same time and the receiver discards the duplicate. A cable break then causes no interruption at all.
For cybersecurity, the same limitations apply as for classic PROFINET: there is no authentication or encryption, and IRT frames use the same Ethertype 0x8892 at layer 2. Anyone with access to the segment can disrupt the schedule with extra traffic or with forged synchronisation frames. PROFINET Security Classes 2 and 3 add certificates and encryption, but in 2026 they are not yet widely available. An IRT cell therefore belongs in its own zone under the zones and conduits model of IEC 62443, with segmentation via VLANs and an industrial firewall towards the higher levels of the Purdue Model. Do not let layer 2 traffic leave the cell, and monitor the segment passively with an OT IDS or Wireshark, which decodes PROFINET and PTCP.
🚨 What does IRT mean in incident response?
In incident response, IRT stands for Incident Response Team: the team that detects, analyses, coordinates and resolves cyber incidents within an organisation. It is the operational executor of the Incident Response Plan, from detection through to recovery. An IRT is also called a CSIRT (Computer Security Incident Response Team) or a CERT.
| Role | Responsibility |
|---|---|
| IRT coordinator | Leads the team, manages communication and decision-making |
| Security analyst / SOC | Detection, triage and technical analysis using tools such as SIEM and EDR |
| Network and system administration | Containment and recovery |
| OT specialist | Assesses the impact on PLC, SCADA and production |
| Communications | Internal updates and press relations |
| Legal function | Notification obligations, liability and GDPR |
| CISO / IT manager | Ultimately responsible for escalation and decisions |
The team works through detection, classification, escalation, containment and recovery, root cause analysis, reporting and improvement measures. Its tools include playbooks, SOAR, an asset inventory or CMDB, a jump server for secure access and notification templates for the NCSC or the regulator, which also supports compliance. In complex IT/OT environments the team is multidisciplinary: recovery is aligned with engineering, planned downtime and backup strategies, and with the safety department, especially where a SIS or interlocks are involved. Read more about tasks, types and legislation under CSIRT.
❓ Frequently asked questions
What does IRT stand for in PROFINET?
In PROFINET, IRT stands for Isochronous Real-Time. It is the communication mode of conformance class C, in which frames are sent according to a fixed schedule in a reserved phase of every cycle. All participants are synchronised via PTCP to within one microsecond.
What cycle time can PROFINET IRT achieve?
PROFINET IRT supports send clocks from 31.25 µs to 4 ms. In practice, motion control usually runs at 250 µs to 1 ms. Cycles below 250 µs require the PROFINET V2.3 extensions fragmentation and Dynamic Frame Packing.
Do I need special switches for IRT?
Yes, PROFINET IRT requires switches that support conformance class C. Such switches synchronise their clock via PTCP, reserve the red phase and forward IRT frames cut-through. An ordinary managed switch, even with VLAN priority, cannot guarantee the IRT schedule.
Is IRT required for PROFIsafe?
No, PROFIsafe does not need IRT. PROFIsafe protects the safety data itself and uses PROFINET as a black channel, usually over ordinary RT. IRT is only needed when the application itself demands a tightly synchronised cycle, as with coordinated servo axes.
What is the difference between IRT and isochronous mode?
IRT handles network transport: frames arrive at a fixed moment in every cycle. Isochronous mode additionally ties the application to that cycle, so inputs are read at time Ti and outputs are set at time To. IRT is a prerequisite for isochronous I/O, but IRT can also run without an isochronous application.
Does TSN replace PROFINET IRT?
PROFINET over TSN (conformance class D) offers the same functions as IRT using open IEEE 802.1 standards instead of dedicated hardware. That makes TSN the future route for new, converged networks. Existing IRT installations keep working; which variant you choose depends mainly on what your PLC platform and drives support.
📌 In summary
PROFINET IRT reserves a fixed time phase in every cycle and synchronises all clocks via PTCP, enabling cycles down to 31.25 µs with jitter below 1 µs. This makes IRT the PROFINET choice for motion control and synchronised measurement, provided that every component supports conformance class C and the topology is planned in advance. With MRPD, IRT is also seamlessly redundant, but segmentation and monitoring remain necessary because the protocol has no built-in security. In cybersecurity, IRT means something else: the Incident Response Team.