Planning guidance
Fundamental questions must be resolved before implementing the design of a PROFINET network. In this chapter, you will find broad guidelines to support you in defining requirements
Content of the planning documentation
Once the requirements have been specified and the planning process has come to an end, the following information should be available to you:
● System configuration
● Topology
● Selection of components
● Selection of transmission medium
● Connector selection
● Communication relationships
● Estimate of the data volumes to be transferred
Preliminary considerations and analysis during the planning stage 1. Selecting the devices
Create a list of devices. The selection of devices is based, among other things, on the application class (conformance class), the time and communication requirements, the function, environmental influences, and the degree of protection.
2. Position of the devices in the machine
The position of the devices in the machine has implications for the degree of protection, EMC, device dimensions and the cables used, e.g. whether fiber-optic cable should be used rather than copper cable. This in turn influences device selection.
3. Defining the communication properties
The time requirements concerning the application (isochronous/cyclic) and communication via PROFINET must be defined, i.e. does communication take place in real-time (IRT/RT) or is it acyclic via TCP/IP or UDP/IP.
4. Network planning (topology)
Specify the network topology (ring, star, line). Depending on the type of topology selected, the following must be taken into account: switches (with IRT capability), EMC, extent of network, WLAN (IRT not possible) and, if applicable, media redundancy MRP (possible with SIMOTION V4.3 and higher, and SINAMICS V4.5).
– Set up your PROFINET in a point-to-point architecture where this is useful (for example, use a switch to branch off into a point-to-point topology downstream of a CPU).
– In the case of PROFINET with IRT, a line structure with 64 IRT devices is permissible.
The amount of data transmitted has certain implications. If longer message frame lengths are configured for each device, the possible number of devices per line may be reduced. However, this is detected early on during configuration with HW Config and signaled by means of an error message. The 64 IRT devices in the line are only applicable to PROFINET after V2.2.
– Maintain a low interconnection depth for the switches. This will reduce the effect of a worst-case jitter scenario with RT communication.
Topology
Star If you connect communication nodes to a switch,
you automatically create a star-shaped network topology.
With this structure (unlike with other structures), if an individual PROFINET device fails, this does not automatically lead to the failure of the entire network. Only the failure of a switch causes the failure of devices downstream of the switch.
Tree If you interconnect several star-shaped struc‐
tures, you obtain a tree network topology.
Line All the communication nodes are connected in
series as a bus.
If a switch fails, communication downstream of the failed switch is no longer possible.
Devices with a 2-port switch must be used in or‐
der to set up a linear structure.
Linear network structures require the least amount of cabling.
Ring With V4.3 or higher, you can establish a ring top‐
ology for MRP or MRPD. For the ring topology you must define a redundancy manager and re‐
dundancy clients.
5. Defining the communication relationships (logical assignment of partners)
Specify which communications partners are connected as well as the spatial and functional assignment of these partners.
6. Defining the data volumes
Define the possible data volumes of the network nodes and those at the communication junctions.
Note
Communication within a PROFINET network should only be as fast as the technology of the system requires it to be and not as fast as technically possible.
7. Defining the sending cycle and update time
The sending cycle is determined by the PROFINET device which has to be updated the most frequently. The update time is a multiple of the sending cycle and determines the PROFINET network load. The network load generated by PROFINET should always remain below 50% to allow reserve capacity for peak loads. Please note that the PROFINET network load increases in linear relation to the number of PROFINET devices and the sending cycle.
Note
Always configure the update times as required by the process, even if the bus system allows for a very large number of shorter update times. This reduces the PROFINET network load and the load on the PROFINET controller to what is strictly necessary.
8. Checking the network load
In order to determine the network load, you must take into account the PROFINET network load as well as the network load generated by standard Ethernet devices. These can take the form of video cameras for monitoring the system or data servers for production data, for example. Ethernet devices with a low data volume such as engineering workstations or HMIs, for example, are normally non-critical. To prevent the PROFINET RT data stream from being compromised, you should, if necessary, adapt the network topology in cases where high network utilization by Ethernet nodes is anticipated. However, you also need to make sure you have sufficient bandwidth reserves for future expansion.
Note
Devices which exert a high IP load on the network should be located, where possible, in a separate area of the network. The diagnostics server and HMI server are examples of such devices.
9. Connection to the company network
Various points must be considered if the automation system is to be connected to the company network. Normally, you should establish the connection via a router or a SCALANCE S with firewall to prevent unauthorized access. PROFINET IRT or RT communication via routers is not possible. You should only establish further connections (such as remote access or VPN, for example) in individual cases following consultation with the IT department.
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Figure 5-5 Optimized topology of a company network connection
Configuring the topology
Communication scheduling requires knowledge of the network topology. This includes information about interconnecting the individual devices to create a communication network.
Topology scheduling is only relevant for IRT High Performance. The topology editor that has been integrated in the hardware configuration enables user-friendly configuration of the network topology.