PROFINET IRT: When Real-Time is not enough
Why do I need PROFINET IRT?
Industrial communication protocols such as PROFINET, EtherCAT and Ethernet/IP are classified as real-time communication protocols. These protocols can already achieve cycle times as low as 1 millisecond and jitter between 10 and 100 microseconds.
In some cases, however, even this level of performance isn’t enough. The fact of the matter is: it takes time to transmit and process data. In certain applications, it is critical that this time be precisely synchronized to ensure that production steps are performed correctly.
This is particularly true in motion control, where the demands for speed and performance in data communication have increased to the point where the cycle times of standard real-time PROFINET are no longer fast enough, and other key metrics such as jitter and determinism no longer meet the required standards.
For example, a robotic arm must be in a precisely defined position at a specific time to pick up or process a part. If the arm is even slightly early or late, the process can be severely disrupted. In such cases, the transmission and processing of data must be both fast and reliable.
PROFINET IRT is used wherever standard real-time communication reaches its limits and extremely accurate synchronization at exceptionally high speeds in data communication are required.
How Does PROFINET IRT Work?
PROFINET IRT requires strict determinism in the transmission of specific data packets within the network. This means that data must be received, processed, and forwarded in a precise sequence within a predetermined time frame, without being affected by delays such as network collisions or jitter.
To do this, a PROFINET IRT network allocates specific time slots for handling time-critical data. For instance, if 20% of the network traffic is reserved for IRT communication, i.e., PROFINET IRT, the network traffic is divided into five time segments:
- One segment is dedicated to IRT communication
- The remaining four segments are reserved for regular real-time communication
In this case, the network operates as a standard PROFINET network 80% of the time. During the remaining 20%, only PROFINET IRT-relevant data packets are processed, ensuring synchronized and deterministic transmission. While IRT communication is taking place, regular PROFINET data packets are buffered and processed when the IRT time segment is complete.
For this system to work effectively, a PROFINET IRT network must meet two essential requirements:
Synchronization across all IRT devices
A consistent synchronization cycle must be maintained across all PROFINET IRT devices. This is managed by a high-precision clock that is shared by all devices. This and additional mechanisms such as the Precision Time Protocol (PTP) are defined in the IEEE 1588 standard. PROFINET IRT extends this standard with the Precision Transparent Clock Protocol (PTCP). PTCP not only maintains a shared clock between PROFINET IRT devices, but also calculates delays between network switches and the associated wiring.
Buffering capabilities
Additional circuitry within the network switch is required to buffer any non-IRT Ethernet traffic received during the designated IRT communication time.
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