Introduction

End users expect interoperability as the primary benefit of standardization. But, that remains elusive as vendors push to promote their proprietary differentiation. Even if or when a fieldbus standard 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.

Which Bus ?

At the next level in the hierarchy are the “device” busses - which provide analog 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. Profibus DP and Interbus are somewhat comparable to DeviceNet and are excellent, general-purpose device-level networks. 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 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 overlaps somewhat with Profibus-PA and Profibus-FM, and this is the primary area of contention in the ongoing IEC/ISA SP-50 committee discussions.

At the next level we have the “control” networks - which include ControlNet (developed by Allen-Bradley and utilized by Honeywell), clearly overlapping with some of the functionality and performance intended to be provided by Profibus-FMS and Fieldbus SP-50.

At the highest level is information and internet connectivity and EtherNet TCP/IP is clearly the standard. During the past year 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.

The overlap between the various bus-standards is evident and the details are still subject to dispute. The combination of commercial viability and practical time-to-market far outweighs theoretical considerations. The choice is dictated by point-count, physical distances, speed of response, hardware configuration, software requirements, cost and - perhaps most important - interface with other equipment and systems. Typically, a choice of one particular bus causes proliferation until limitations (speed, point-count) are exceeded.

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.

Interoperability

Who Will Benefits From Standards ?

Industrial networks bring lots of gains
But that brings with it lots of games
The fieldbus wars cause a lot of fuss
The Users want an Open bus.

The Vendors simply can’t agree
To make a fieldbus cheap, or free
The committees they just twist and turn
They argue out and then adjourn

You’ve gotta read between the lines
That Open and Closed are just designs
That Users all for Open plead
When Inter-op is what they need.

De facto standards win all debates
Ask Microsoft and Mr Gates.