PROFINET Network Planning

Parameters for network planning

PROFINET network structure

The key properties of PROFINET are a variable network structure and the unlimited combinations provided by exploiting all topological shapes of the standard Ethernet. The topology results from the following criteria:

• Spatial arrangement of the components
• Distances to be bridged
• Requirements for the use of primary infrastructure / increased availability
• Consideration of netload (netload planning) and TCP/IP traffic
• Update rate in consideration of line depth
• Communication quality and telegram traffic
• Requirements for potential isolation/EMC

Selecting the right topology is important for the further planning of a PROFINET automation system. If required, the topology has to be adapted in a later inspection step.

Update time

The update rate is the interval at witch the data between the controller and IO device is updated. It can be set individually for each device in the controller (standard setting: 2 ms). In addition to considering the requirements of the process, also consider the PLC cycle time. Indu-Sol recommends setting the update time for devices to at least half the PLC cycle time. The guiding principle is “Update only as necessary – not as much as possible.“

The switching behavior of each device and the installed line depth, i.e. the number of passing devices (switches and IO devices with integrated switches) on the line, are critical for compliance with the update rate. An increasing number of passing devices prolongs the duration of the telegram. The variation of the real from the set update rate keeps increasing. It is called “jitter” and indicated in percent. Measurements to show compliance with the set update rates and their variation may serve as a basis for an assessment of system stability and provide an early warning of potential weak spots.

Netload

The cyclical netload generated by each PROFINET device has a maximum in the connection between the controller and the first device. In order to permanently ensure the flawless functioning of the PROFINET network, the following tolerances for planning and realization have to be ensured:

It can be seen from the practical applications that the existing netload consists of both PROFINET and TCP/IP communication. Although the PROFINET communication is generally prioritized at switches (network nodes), TCP/IP communication may sometimes jump the queue. Whether and to what extent this happens – or can happen – can be seen from the load ratio (PROFINET to TCP/IP communication).

Since different netloads (peak loads) affect the compliance with update rates, and devices of different netload classes are especially sensitive to peak loads, demonstrating the network quality in consideration of utilization is especially important during the acceptance test or troubleshooting of a system.

Line depth

Because in PROFINET (RT), it cannot be determined whether a device (switch) is operating in store-and-forward or in cut-through mode, a delay has to be expected for every passing device.

This delay has to be considered when designing a network in the planning phase. Fundamentally, the maximum line depth depends on the update time and the switch mode (see the “line depth” tables). Indu-Sol recommends observing the values from the PI guideline “Line depth for store-and-forward switches”.

Line depth for store-and-forward switches

Line depth for cut-through switches

Access points

When planning PROFINET networks, the guideline recommends passive access points for network diagnostics.

Why?

• During commissioning or maintenance, to analyze the network traffic or read out devices
• To connect diagnostic devices during running operation without interruption
• For troubleshooting or long-term diagnosis/preventative maintenance of the network condition

How?

• Using passive, feedback-free TAPs (e.g. PROFINET measuring points PNMA II / PNMX IP67)
• Using a smart TAP (e.g. PROFINET-INspektor® 1G / iPNMA)

Please note:
In principle, an initial rough analysis of the network traffic (telegram analysis) is also possible via a mirror port on the switch using analysis software (e.g. Wireshark). However, this port only delivers 100 Mbps.

But the cable carries incoming data to the controller at 100 Mbps on one wire pair and outgoing data from the controller at 100 Mbps on the other wire pair. Thus, the port of the switch cannot diagnose more than one half of the data flow.

PROFINET network planning software

PROnetplan V2

The PROnetplan V2 software supports the user with the planning of Ethernet based networks, like PROFINET. PROnetplan not only allows a rough planning of the network, but also a detailed planning including port assignment, support for correct switch selection and transmission speeds.

Due to the integrated diagnostic mode, which evaluates the network based on the PROFINET guidelines, incorrect planning of the network is hardly possible and subsequent changes are a thing of the past.

With PROnetplan V2, the user has the possibility to integrate his planned applications like PROFINET, but also camera monitoring and SCADA systems, into the network to estimate which infrastructure components are necessary to integrate all applications in the best possible way. In addition to the bandwidth of the network, the port utilization and the respective transmission media are evaluated.

In the diagnostic mode, the IP addresses of the network devices are checked for correctness and address duplication. The topology created with PROnetplan V2 can be used for network optimization, as a discussion platform with the client, as well as a document for a network installation.

PROnetplan in conjunction with PROscan® Active V2

The topologies scanned with the PROscan® Active V2 analysis software can easily be fed into PROnetplan. This enables a comparison between the plan and reality. This procedure is very useful during planning an expansion or optimization of the network.

Keep learning:

PROFINET

ServiceTool

PROFINET-INspektor®

PROFIBUS