Finding Fieldbus Standards

Conflicting `standards’ emerge, but interoperability is still elusive, writes Jim Pinto, chairman and founder of US-based Action Instruments

Everyone in the industrial automation world seems to have been waiting for `Fieldbus’, the bi-directional communications protocol used to connect field-level instruments and controls.

The International Electrotechnical Commission IEC/ISA SP-50 international fieldbus specifications are still under development. However, even when the specification is complete and products are available, this will not eliminate the need for other industrial networks for a variety of reasons – cost, speed, complexity, compatibility with already-installed devices and future expansion requirements.

THE CHOICE BETWEEN THE BUSES

The industrial automation environment has a wide variety of networks available, and confusion arises because their capabilities overlap. Figure 1 shows a simple overview and comparison between the industrial networks available today.

At the lowest level are the sensor networks, which were originally designed primarily for digital (on/off) interface. These are fast and effective, but with only limited applications beyond relatively simple machine-control. ASi (actuator-sensor interface) is widely used in Europe.

At the next level in the hierarchy are the device buses which provide analogue and digital support for more complex instruments and products. DeviceNet interfaces very well with programmable controllers; however, it is still only a local, device-level network, operating typically with up to 64 points within about 1,000 feet.

LONWorks operates over greater distances and is currently the only practical peer-to-peer network, extendible to many thousands of points though it is still comparably slow and more complex. Profibus DP and Interbus are somewhat comparable to DeviceNet, as excellent, general-purpose device-level networks.

Profibus PA provides improved performance at the fieldbus level, for instruments and controls, replacing the features and functions which are provided by HART (originally developed for transmitter calibration and diagnostics). The primary differences are in speed, complexity and distance. LONWorks also extends into this realm. Fieldbus SP-50 goes beyond Profibus-PA and is still being extended to higher levels in the hierarchy of the automation environment. However, this is the primary area of contention in the ongoing IEC/ISA SP-50 committee discussions.

Then come the control networks which include ControlNet, clearly overlapping with some of the functionality and performance intended to be provided by Fieldbus SP-50.

At the highest, information and internet connectivity level, EtherNet TCP/IP is clearly the standard. EtherNet is gaining wide acceptance, even at the lower levels, because its performance is well established at the higher end, while commercial proliferation and plummeting prices have extended usage at all levels.

DeviceNet was introduced by Allen-Bradley (Rockwell Automation). To encourage industry-wide support for an open standard, responsibility for DeviceNet was transferred to ODVA – the Open DeviceNet Vendors Association – which is now responsible for coordinating specification changes and helping to assure interoperability between products supplied by multiple vendors. Many other major industrial automation vendors are now members of ODVA and are contributing strongly to its development and proliferation.

Profibus, on the other hand, was developed by Siemens in Europe and now has a significant group of allied vendors. PTO, the Profibus Trade Organisation, includes vendors and end users.

Interbus was developed by Phoenix Contact, ASi by Siemens and others, and Modbus by Modicon. Other open standards such as Echelon’s LONWorks are operated primarily by one company, with specifications published for any interested users and implementers.

The sheer volume of use of EtherNet in the commercial/office environment, however, has quickly made it a viable candidate for industrial environment networking, with comparable cost even at device-level.

A new Industrial EtherNet Association has been formed recently dedicated to the proliferation of EtherNet in industrial applications. This group of users and vendors will focus on EtherNet adapations and extensions which are suited to factory automation and process control applications, while still retaining interoperability and internet connectivity.

Since inter-operability between equipment from multiple vendors is a major factor for many users, all the major open-bus vendor organisations offer independently certified inter-operability – typically through independent third-party test establishments. Typically, the products are certified to work together. However, there may still be differences in speed of operation and other technical features, which still provide proprietary differentiation.

A `standard’ benefits end users, not vendors. Indeed, it is a reasonable and justified objective for any vendor to provide `differentiation’. Expecting vendors to support a standard that eliminates their advantage is futile. The vendors all support `interoperability’ within the standard, and the associations sponsor `interchangeability’ between products from different vendors through independent testing.

However, the incompatibility between vendor and user objectives will inevitably prolong the confusion until one – or more likely a `few’ – networks attain de-facto leadership. Until that leadership is clearly established in the marketplace, conflicting claims will continue to cause confusion.

It should be recognised that a committee-generated `standard’ is intrinsically a `bureaucratic’ thing that obsoletes itself in the present-day fast-moving technological environment. The realities of a practical competitive environment demonstrate that no one is really willing to pay the price for a standard, when performance is available at a fraction of the cost.

USAGE AND PROLIFERATION

Profibus has made progress to the extent that more than half a million nodes have been installed. Echelon claims to have installed several million LON nodes, although a large portion of these are in the building-automation markets and embedded in special non-inter-operable products. ODVA reports an installed base of approximately half a million nodes for DeviceNet. ASi and Interbus are estimated in the same region.

Ethernet usage is in the hundreds of millions, and it is anticipated that the sheer breadth of application will inevitably cause encroachment into the industrial device and fieldbus markets to compete with, if not replace, the primary industrial network contenders. Recognising this inevitability, most industrial networks are expanding their scope to include at least some level of EtherNet and/or TCP/IP compatibility.

This important fact must be noted: no single fieldbus product will emerge as the universal standard in the industrial automation environment. It is far more likely that several standards will emerge at the various levels of performance and complexity, in different applications and environments.

WHAT ARE THE REAL BENEFITS?

Industrial networking for all factory automation and process control devices has significant benefits – reduced wiring, interoperability among multiple-vendors, enhanced field-level controls, simpler systems integration, and easier troubleshooting and maintenance. A variety of bus networks will demonstrate these significant benefits over the next few years, paving the way for a complete re-organisation of the industrial environment based on networking of sensors, controllers, computers, I/O, actuators and more. Ultimately, industrial networks enable greater manufacturing flexibility, productivity, higher quality products and improved regulatory compliance.

PREDICTIONS

The great debate on the IEC/SP50 Fieldbus standard will continue, and even when it is finally complete, it will continue to attract lip-service and capitulation to demands by all major vendors.

But, in parallel with their interoperable Fieldbus products, vendors will continue to offer proprietary extensions and performance improvements which will continue to defeat the underlying users’ need for interchangeability.

The reality of interoperability is a marketing dichotomy that will never really be resolved. Major vendors will show interoperable products, when demanded by major users, but will always inevitably retain the best performance for their own products when utilised as a family.

And, of course, the reverse will be true. Some aspects of operation must, of necessity, remain proprietary – the fruits of intellectual property.

What will emerge is the use of gateways between buses and between proprietary products and open architectures. With the availability of cheap memory and processing power, it is far more practical to make a whole range of I/O products work with any bus through a gateway.

Gateways will theoretically appear to have intrinsic delays and performance limitations, but, practically, can be made to operate with very little disadvantage.

In the meantime, it is likely that at least a few de-facto standards will emerge from the current contenders. However, Fieldbus itself will remain a useful, more complex and costly, lesser used alternative, inhibited by its dependence on bureaucratic multi-vendor committee involvement.

Action Instruments Tel: +1 858 279-8836