Holistic thinking now defines industrial automation

When it first considered an ethernet extension to the Profibus protocol in late 1999, Profibus International initially considered embedding the Fieldbus protocol inside an ethernet frame. It did not take long to realise that this was a dead end street for both openness and efficiency. A fundamental reappraisal resulted in Profinet: it places an underlying emphasis on 'automation solution' rather than just 'communications' as the core value. The resulting flexibility promises a fundamental shift in automation strategies at a speed and scope that each user can determine individually.

Recognising that a sudden, quantum leap is unrealistic, Profibus International has adapted Profinet to meet the present needs of distributed I/O just as easily and completely as the likely future trends such as distributed automation systems which will demand an object-oriented approach. International standards are embraced throughout to make Profinet truly open. In particular, Profinet is fully compatible with existing ethernet TCP/IP standards, which means it can easily integrate with higher level IT systems and tools.

Also taken into account are extensions covering the real-time needs of automation, for which a number of important innovations have been necessary. This is especially relevant to the hard real-time needs of motion control, which have been addressed by a new software/hardware approach that offers unprecedented real-time performance and is unique to Profinet. Later on, Profinet will also seek to meet the needs of machine safety and process automation, though at present, both these are open topics.

Finally, Profinet recognises that no ethernet-based solution for automation will succeed unless it provides backwards-compatible connectivity for legacy fieldbus systems, thereby protecting customers' investments. Profinet offers an easy way to integrate existing networks. It can embrace any fieldbus system, not just Profibus.

Profinet is best understood as a set of automation functions and services, all based on an ethernet communications structure designed to suit many different application environments. Not all pieces have to be used all the time or for all applications, and mix and match is possible. The full value of this flexibility will become apparent in coming years as the demands of automation users escalate - particularly in areas such as motion control - as competition encourages product differentiation and as markets move towards distributed automation structures.

At the heart of Profinet are three communication options covering the automation spectrum from boardroom to factory floor: standard TCP/IP and UDP offer typical cycle times of 100ms and provide all important vertical integration. The first stage of real time enhancement delivers communications synchronism with 5-10 ms cycle times. The second stage, which uses isochronous techniques, delivers communications with a response time of 1 ms and an associated jitter of just 1µs for hard real-time applications.

These three options are implemented in standard ethernet frames using two channels which share bandwidth and can run independently or simultaneously.

Hard real time

For more critical automation environments, particularly the 1ms cycle times and 1µs jitter demanded by high end motion control applications, additional communications support is required. Profinet introduces a concept called IRT, or isochronous real time, which exists as an optional second communications channel alongside the open channel.

Isochronous means that the functionality delivered by each ethernet frame occurs at precisely the same time in each cycle. This requires accurate synchronisation and, if the system is to handle multiple participants, very precise control of multiple, synchronised, data streams.

The IRT channel exists side-by-side with the open channel by partitioning the ethernet frame into two segments and sharing the bandwidth.

A time-slice system is used to break up the IRT channel into fixed time slots, each representing an end device.

IRT's time slicing technique cannot be supported by current ethernet switching technology so it requires specialist hardware support.

Embedded IRT switches

High-end motion control can be divided into two market areas - powerful motion controllers and sophisticated drives or field devices. Two IRT switches are therefore envisaged: a 4-port switch for motion controllers and a 2-port switch for field devices.

The diagram indicates how these will be used. Each type will be based on an asic. The 4-port switch is required where controllers and other devices such as HMIs communicate horizontally or upwards, and here, since cost is usually not so much of an issue, the 4-port chip will be board-mounted as a sub-module of the controller. For lower priced field devices, the 2-port chip will be embedded in the product itself. The chips are called ERTEC 400 and ERTEC 200. They work by responding to the time-slice technology of the IRT channel, ensuring that system connections are made at precisely the right moment to pass messages between designated devices. Data intended for another recipient in the system is simply switched onwards. Both types support normal ethernet switching functions and they replace external ethernet switches, allowing complex automation structures to be created easily. ERTEC 400 will not include the ethernet physical layer and will probably be connected as a PCI sub-system in the motion controller. Dual sourcing has been announced by Siemens and NEC Electronics.

Market objectives

Two major market segments are envisaged for Profinet - decentralised control and distributed automation. The former is driven by the currently conventional view of automation and may be seen as a traditional approach comparable with Profibus DP.

The latter will grow in importance as the separate elements of automation systems are concatenated into single production units. At present only a few industries see automation in this way - machine tools are one example - but more end users are moving in the same direction, particularly in the automotive sector.

Profinet supports both scenarios, with the decentralised control solution expected to take the lead in the next few years.

The decentralised control approach is called Profinet I/O and it offers a distributed I/O solution similar to Profibus DP, but with the benefits of industrial ethernet. In essence, it will provide the same kind of I/O functionality as conventional remote I/O blocks and thus it goes a long way to protect the investments of vendor companies. However, it will involve a different communications structure, new configuration/maintenance tools and it therefore has many implications for end user training, inventory management, etc.

Each Profinet I/O device is allocated an Ident number and, like Profibus, has a configuration file (similar to a GSD). This file, called GSDML, is written in XML. The GSDML file defines an application relationship between controller and field devices and allows communications relationships to be established based on Profinet's standard and real-time options. UDP/IP is used during set up and once this is accomplished, data is transferred cyclically between controller and field devices using the first stage synchronism. Distributed automation systems have been one of the key drivers for Profinet.

Based on object-oriented techniques used in the computing industries for many years, Profinet considers an automation module as an 'object' or a 'component' of a larger system. However, Profinet simplifies the computing world's view of objects by describing each component as a block of functionality having a set of inputs and outputs. This suits the functional nature of real-world automation system applications.

Use of XML imposes a standard programming approach allowing blocks to be copied from a library, placed into a PC-based Connection Editor and configured like the interconnections of a PCB. This CAD approach cuts design, configuration and commissioning dramatically, especially when the re-useability factor is taken into account. Furthermore, no communications knowledge is needed because the complexity of the underlying network is hidden and communications relationships are executed automatically. To support the concept, each block of automation is shipped with an XML-based PCD (Profinet component description) file incorporating descriptions of the hardware and software used. Once configured, a Profinet program is downloaded over the network into the hardware and run, with screen-based diagnostics enabling errors to be pin pointed and fixed easily. Profinet's open channel communications are used in a typical component-based system, with additional help from DCOM and RPC.

Integration with the enterprise

Being based on ethernet TCP/IP means Profinet systems are fully open to the IT world and, perhaps more importantly, the internet and web-based services and tools.

For example, it can use DHCP to assign addresses to slaves in the network automatically. SNMP can be used for network management, allowing each connected device to be configured remotely and examined for status and diagnostic information. Web integration is defined in Profinet to allow use of common browsers for commissioning, diagnostics and maintenance.

A further aid to IT integration relates to the use of OPC as a common interface for extracting data. Profinet defines how data is presented in different types of enabled devices. OPC DX, created by OPC to bridge the cross fieldbus divide, is also used by Profinet.

Impact on legacy systems

Despite some areas of overlap, it is extremely unlikely that Profinet will displace Profibus for a considerable time, if ever.

Market analyses suggest that a crossover point may not even be reached until 2010.

The reasons are all practical. Few users will jettison a serviceable investment in Profibus systems even if the attractions of ethernet are strong. Furthermore, the fieldbus will continue to meet the market's needs for many years, particularly in specialist areas such as machine safety and process control. Market surveys suggest that the number of Profibus installations will double from 10 to 20 million nodes over the next four years. Finally, Profinet costs will take time to fall to those of Profibus.

However, a gradual change to ethernet-based automation is inevitable and it is therefore vitally important that existing systems and technology can be easily integrated with Profinet architecture. Two solutions have been recognised - proxy- and component-based. The component-based approach was described earlier. The proxy solution however is unique to Profinet and deserves attention: it can embrace any fieldbus protocol.

Essentially, a proxy is a gateway between a fieldbus system and Profinet. One half of the device faces the Profibus world - it appears as a Profibus master - while the other half faces the Profinet world. Transparent communications between the two 'worlds' allow the proxy to represent the fieldbus slave devices on ethernet. No tunnelling of protocols is required so communications efficiency is as good as a pure Profinet system. Proxy technology can be used with directly connected systems, and component-based sub-networks. The proxy approach is flexible enough to accommodate any fieldbus system, which means that an end user migrating plant to an ethernet environment could choose to adopt the Profinet approach to automation regardless of the industrial network in use. With major European automotive manufacturers calling for a common approach to ethernet in plants this aspect recommends itself. However, Profinet holds some powerful aces. The three-part communications solution offers powerful opportunities for innovative system design and implementation. The IRT real-time capability may be a world-beater, especially taking into account its innovative switch technology. XML-based engineering guarantees universal support. The distributed automation component solution promises a secure growth path. TCP/IP compatibility means open IT integration and simpler enterprise-wide plant management, reducing costs through a mixture of standardised tools and common platforms. Furthermore, the proxy concept has already demonstrated its relevance. Indeed, this ability of Profinet to 'embrace not displace' other fieldbus protocols may prove to be the critical factor.

* Geoff has marketing affiliations with Profibus International.