Despite the market clamour for open fieldbus, hardly anyone is implementing true open systems. Omron’s Michael Juniper claims that `there is still more of an industry built on talking about them than actually using them’.
The reasons for this are manifold. For a start, everyone is waiting for a single, winning open industrial fieldbus and this is not going to happen, says Juniper. The range of potential applications is too diverse. The `bit-bus’ requirements at pure sensor level are so far removed from data transfer requirements at cell control level that we should be prepared, he insists, to let individual applications dictate which of the current standards is best to use.
There are many to choose from – Profibus, DeviceNet, Interbus/S and Modbus to name but a few – and all have inherent weaknesses and advantages for the user, meaning that, in an integrated automation project, you might find yourself using two or three of these standards in order to get the best possible system.
The whole point of open fieldbus standards, from a market perspective, was to free control equipment users from ties to particular vendors, allowing them to link `best of breed’ components simply and cost-effectively. But this has not happened. According to Juniper, it is now the norm to see an open fieldbus connecting a single vendor’s control equipment. This is because a truly open system, as well as freeing us to use any vendor’s equipment, puts the onus on the user to maintain the system, adding to complexity and making the user’s job more difficult.
However, if an open fieldbus system really is seen as desirable for an application, choice largely comes down to a matter of personal taste in styling. If a particular bus meets all of your requirements, and is likely to go on meeting them as the application develops, then, says Juniper, it will do a good job for you.