Hannover Messe Hall 9, Stand A33
Profinet is increasingly used for communication standard for industrial Ethernet applications. It allows users from the experience gained over the years in the use of Profibus also benefit the Profinet. This example is the realization that not an externally visible network function along the lines of "It really works ..." should be used as a measure for assessing the network quality. The contribution of Indu-Sol shows which Qualtitätskriterien be met and how this can be implemented.
The advantages of Profinet are obvious: It can realize all network structures such as ring, star, line or network to the decreasing susceptibility by point to point wiring realize. Moreover Profinet convinced as an open, vendor-neutral, internationally standardized bus standard and can point to an ever-increasing user acceptance. Finally Profinet complements very well the PC-based automation technology and allows easy vertical integration from the field level to the enterprise level. But experts warn for network communications from being too innocent to approach the topic. Karl-Heinz Richter, Managing Director of Marketing & Sales Indu-Sol GmbH urges, rather, to learn from the experience with Profibus.
While (produced by cables, connectors) for Profibus 90% of the problems encountered in the bus communication to physical causes are, depends on Profinet by the possibility of transmission of multiple protocols within a network of perspective more on the evaluation of logical communication links. Anyone know the actual quality of its communications and not only want to judge by plant function or number of failures need concrete values.
The network previously interpreted correctly
Comparing Profibus Profinet with the amount of data to be transferred and the transfer speed seems almost unlimited. In contrast to the Profibus but the communication load is not constant, which makes the prediction of reliable communications. It must therefore advance the necessary load reserves are scheduled and the network structure is to think well. Like ones on the road heavy transport possible unleashing not on country roads or at peak times, it is in the Profinet important to design the communication paths according to the load. For example, a security camera connected as directly as possible with the visualization system or to stay in the picture, are not guided by passage through various localities. Another keyword in the planning line depth (Cascading of I / O devices) is. It is necessary first to determine whether the line depth for each application fits, if a particular update rate is observed. Line depth is transferred to the image of the road, how many traffic lights and traffic light junctions (switches) must be negotiated on the way to the goal.
Three measurable network state variables
In measurement practice of networking experts from Indu-Sol have for Profinet three measurable network state variables emerged that could have quite succeed as general quality criteria in the Profinetkommunikation: the telegram jitter, the number of failed messages and the load spectrum as the ratio of Profinet to TCP / IP. With respect to such quality criteria as well as their limits and relationships to each other, networks are a priori plan reliable and optimal design.
On the difference between theory and practice, says Richter, "Two general causes usually lead to that theory and practice do not match. First, they talk over the implementation often not exactly to the plan. Here, measurements can help to control planning data. Secondly, it is for complex systems in the planning phase is often not possible to consider all eventualities and the performers are aware of certain freedoms granted in the network design (installation). Here, the measurement in real operation help optimize settings and also the means of communication. "Will you bring together theory and practice, this is only possible with reliable measurement results, such as using a network monitoring under production conditions over a longer period of such 14 working days. Who does not want to judge the quality of the communication on the "Status LED" on the control or the number of failures, needs one thing above all, clear and transparent quality criteria.
Quality criteria for the Profinet communication
In practice, the network professionals have for Profinet three network state variables emerged that could have quite succeed as general quality criteria in the Profinet communication: the message jitter, the number of failed messages and the load spectrum (eg, the ratio of Profinet to TCP / IP) (Fig. 2).
The jitter is a measure of the deviation from the update rate. Judges said: "We can" compare jitter "for example, with the arrival times of buses. If we expect a bus every hour and this time comes too early and sometimes too late, we can speak of a temporal deviation from the originally expected time clock. Profinet when we set the 'arrivals' on the update intervals. Should any time be more than 50 percent, so the question 'why' is a must. From the outside, but you would not notice this fact early indication of a problem. "
Detecting unusual frames a second criterion is the Profibus see similar to the 'Retry limit'. Important difference is that, at the Profibus repeated the messages and the Profinet up to three missing telegram packets can be tolerated without being visible to the operator externally.
The third criterion Profinet, the overall bus load or traffic comprises the sum of all cyclic and acyclic data traffic. Richter argues: "Here too, a comparison of the road to understanding 'bus load' or 'Traffic' help. When Profibus only one car type "car" moves from A to B or vice versa. Profinet when we have to let go beside the car traffic and the possibility of trucks or motorcycles on the same street. For the purposes of the review, it is now important to know to what percentage of the respective participants each received in the traffic load. A guide for the load range is a ratio of 100: 1, speak to 100 Profinet telegrams should for example a maximum of one Ethernet frame is transmitted. "
It is also important in a thoughtful network planning to consider in advance, which loads can occur when and how they can be useful to distribute a good overall utilization of the network. Judge calls also an empirical value: "With a maximum bus load of 20 percent, a network communications without surprising communication failures definitely be ensured. Figuratively speaking, if here in Schmölln the roads are busy to 20 percent, one can be sure as motorists that there are always free ride and green wave, no matter where you come from his car or breaks forth. "
Calibrate lines yesterday
To ensure the Profibus communication reliability from the outset, they usually check out the physical line quality. It is also important to hang on to a Profibus but often lead to 30 participants. In Profinet, where participants come to 30 30 lines, this procedure is not only complex and expensive but also poses the risk that is the buying and parting actively intervened in the installation and thus can create new sources of error. Can "brought also a discussion with experts no clear majority for 'For or recovery' in terms of a line test in Profinet." From previous experience of Indu-Sol is in a properly conducted online test on the line test as an acceptance criterion: Judge this Profinet refrain. Are in fact satisfied in the online test all three of these quality criteria, one can also assume that the line quality is right. Conversely, errors based on deviations of the quality parameters in lines are already visible. Through the use of diagnostic tools such as Profinet inspector, these three sets of criteria for assessing the quality of communication to reason, to elaborate and expensive offline cable tests in terms of the decrease can be omitted.
Long term on the safe side
In addition, the 'online check' with the Profinet inspector has the advantage that all abnormalities are recorded by means of web-based visualization and time-stamped and real values indicating clearly displayed. This facilitates the analysis of what the problem was caused thereby. This allows networks to plan not only makes sense from the outset, but to ensure reliable communication in the long term. For this purpose it is useful to use the inspector throughout the entire plant life than 'Holter' of the system. All changes, especially unexpected, caused by acyclic occurring amounts of data, can detect at all times safe and avoid communication problems.