July 2009

Covering all aspects of industrial automation

Industrial Ethernet refers to networks that use Ethernet supplemented by mechanisms for assuring quality of service, for the purpose of networking nodes in automation systems.

Industrial Ethernet describes variants that use Ethernet with modifications or that use radio technologies.

Various manufacturers and associations have expanded their fieldbus variants to include an Industrial Ethernet variant, in which automation applications known up to now in fieldbus networks can continue to work.

One of these is Profinet, which covers all aspects of industrial communication.

There are three approaches for Industrial Ethernet. We can distinguish them by their different adaptations to Ethernet and the TCP/IP protocol.

1. Use of standard Ethernet as defined in IEEE 802.3 and TCP/IP

This approach precisely corresponds to implementations in the office world. The office-specific applications exchange to automation-specific applications on the application layer only. This type of solution can work in environments whose performance requirements are equivalent to those in the office world.

2. Use of standard Ethernet as defined in IEEE 802.3 and prioritization according to IEEE 802.1Q, as well as TCP/IP and specific automation protocols, on OSI layers 3 and 4

Dependent on the performance requirements for automation communication, this  approach provides different "communication channels"-one TCP/IP channel for applications whose performance requirements are equivalent to those of the office world (Best Effort), and one real-time channel that bypasses or replaces the TCP/IP protocol to enable use of applications with more stringent real-time performance requirements.

3. Use of optimized Ethernet stack, as well as TCP/IP and specific automation protocols, on OSI layers 3 and 4

This approach is required for automation systems if applications with deterministic data transmission and maximum real-time performance requirements are necessary, like axis control. However, this approach also covers all communication types by offering various communication channels that can work in parallel: Based on adaptation of the Ethernet stack and scheduling, one TCP/IP channel and one or more real-time channels that bypass the TCP/IP protocol are available. Determinism is by the use of a time slot procedure (scheduling) that provides fixed transmission cycles of the highest performance class and through cyclic synchronization of all nodes. This approach normally requires use of special switches.

Profinet conforms to variants 2 and 3 and works for many different automation scenarios that require real-time performance. Profinet includes all aspects of industrial communication and is an open system per IEC 61158.

Communication in parallel

The modular Profinet concept distinguishes among different types of communication for satisfying various response time requirements:

  • No specific performance requirements
  • Increased performance requirements-Real time (RT)
  • Maximum performance requirements-Isochronous real time (IRT)

In addition, Profinet distinguishes between different types of communication according to the nodes to be connected:

  • Profinet IO implements communication between controllers and IO devices with RT and IRT requirements.
  • Profinet Component Based Automation is suitable for communication among controllers via TCP/IP, as well as additional RT requirements.

It is possible to use all types of communication in parallel on the same network, and proxy components can work for transition to nodes in traditional fieldbus topologies, like Profibus and others. Likewise, Profinet supports communication via radio technologies, like WLAN, for connecting IO devices.


Most Profinet communication occurs via Ethernet and TCP/IP packets without any modification. This enables integration into a common network with office applications without limitations.

RT communication not only uses prioritization of Ethernet packets but also an optimized protocol stack on OSI layers 3 and 4. This protocol stack does not have any routing capability. This means RT communication is restricted to one logical subnet. However, a layer 3 topology enables parallel TCP/IP communication with centralized computers on the overall network.

IRT communication for maximum real-time requirements, especially for isochronous applications, is based on an extension of the Ethernet stack for the purpose of synchronizing all communication partners and on the use of scheduling, i.e., a time-slot technology.

IRT communication requires the relevant network area to be equipped with special IRT switches that reduce the integration into the overall network to the connection of closed communication areas based on layer 2.

The topology can have a transition to the overall network and can enable parallel TCP/IP communication.

Nicholas Sheble (nsheble@isa.org) writes and edits Automation Basics. The source for this piece is PTO North America (http://us.profibus.com).