IMPLEMENTING PROFINET IRT: Synchronous applications can be easily implemented with the PROFINET IRT protocol. With just a few additions to the system, an isochronous, low-jitter data exchange can be implemented via Industrial Ethernet interfaces even for simple real-time applications: with a PROFINET-capable device and an intermediate driver.

Because of its dominant position in the office world and in industrial applications, the Ethernet interface is especially important. Most intelligent devices used for automation therefore have an Industrial Ethernet connection for this reason. This standard is also suitable for small, individual applications, as Ethernet is very often the common denominator of a cost-effective implementation. An asynchronous data exchange is then possible. However, what is the situation with the cyclic data exchange when this task is not performed by an automation system such as SIMATIC or SIMOTION.

In this case, an alternative is provided by the use of the PROFINET IRT (Isochronous Real-time) protocol. This synchronizes the communication of all connected devices in a precisely maintained clock cycle, in which the read and write operations of all devices are performed. This reduces the maximum jitter during the acquisition of an asynchronously occurring event to the length of a basic cycle. In this way, the reproducibility of the processes is improved by up to five times for the same cycle times.

For simple applications with a manageable number of communication nodes and where the demands on the performance are not too extreme, the setting up of a PROFINET IRT communication does not require many technical resources: one or two standard Industrial Ethernet interfaces that have to be reserved exclusively for the PROFINET IRT application and a hardware device equipped with the IRT technology. This takes over the role of the master in a communication topology set up according to the master-slave principle.

Definition of the topology

Three basic steps are required to set up a real-time-capable PROFINET communication system. The device that is to be the master and which is to organize the cyclic communication in the application is defined in the first step. In principle, any PROFINET-capable device is suitable for this, for example, a drive of the Sinamics S120 series with an interface for PROFINET. All other devices are slaves. The master ensures that the sequence of the send and receive operations specified by the PROFINET protocol is maintained. It thus prevents collisions of datagrams – an essential precondition for the required high determinism of the communication.

Precondition: suitable driver

In the second step, the correct preconditions must be created for the real-time processing on the driver side. Under Windows and most Linux operating systems, the standard drivers of the network interface only support asynchronous data traffic. In order that the application software can access the transferred process image directly and without delay, the real-time data must be filtered out of the data traffic of the Industrial Ethernet interface, or channeled in. A simple way to perform this is provided by the Network Driver Interface Specification (NDIS) of Windows. It enables access between the protocol implementations on the operating system and the driver of the Industrial Ethernet block.

Only a hardware-independent NDIS intermediate driver is required for this. It extracts the real-time information of the PROFINET IRT protocol from the incoming data of the respective interface, clocks the user program and sends the data issued by the user program in the response datagram of the IRT cycle.

Parameterization of the synchronous communication

The parameterization is performed in the third step. For a simple application, the process image of an application can be considered as a unit in each cycle. The distribution of the components of the process image defined for the individual communication peers must be specified for this. The software required to perform this task must be installed on the device with the master function.

In the output direction, the device with the longest path is addressed first in every cycle. In the input direction, the communication peer with the shortest path transmits first, followed by the others. The configuration of the communication and the protocols from the controlling PC is performed via the asynchronous communication. Basic PROFINET protocols that can also be used independently of the IP-based operation are defined for this.

Engineering system for high-performance PROFINET communication

Uniform components that are equipped for PROFINET IRT operation are required to fully utilize the performance capability of the PROFINET IRT protocol. PROFINET with IRT is available in automation systems, in which isochronous real-time communication is provided as a system service of the engineering tools and the device firmware. In this way, the use of the PROFINET protocol in the machine and plant construction as well as in industrial applications becomes cost-effective and technically just as attractive as for simple applications on a small scale. SIEMENS

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