Fieldbus is a generic-term which describes a new digital communications network which will be used in industry to replace the existing 4 - 20mA analogue signal. The network is a digital, bi-directional, multidrop, serial-bus, communications network used to link isolated field devices, such as controllers, transducers, actuators and sensors. Each field device has low cost computing power installed in it, making each device a ‘smart’ device. Each device will be able to execute simple functions on it’s own such as diagnostic, control, and maintenance functions as well as providing bi-directional communication capabilities. With these devices not only will the engineer be able to access the field devices, but they are also able to communicate with other field devices. In essence fieldbus will replace centralised control networks with distributed-control networks. Therefore fieldbus is much more than a replacement for the 4 - 20mA analogue standard.

The fieldbus technology promises to improve quality, reduce costs and boost efficiency. These promises made by the fieldbus technology are derived partly from the fact that information which a field device is required to transmit or receive can be transmitted digitally. This is a great deal more accurate than transmitting using analogue methods which were used previously. Each field device is also a ‘smart’ device and can carry out it’s own control, maintenance and diagnostic functions. As a result it can report if there is a failure of the device or manual calibration is required, this increases the efficiency of the system and reduces the amount of maintenance required.

Each field device will be more flexible as they will have computing power. One fieldbus device could be used to replace a number of devices using the 4 - 20mA analogue standard. Other major cost savings from using fieldbus are due to wiring and installation - the existing 4 - 20mA analogue signal standard requires each device to have is own set of wires and its own connection point. Fieldbus eliminates this need so only a single twisted pair wiring scheme is required.

The International Debate
Although fieldbus technology has been around for the past 8 years it is still not widely used. The reason for this delay is due to the lack of an international fieldbus protocol standard which will ensure complete interchangeability and interoperability between different suppliers. The major players in the fieldbus debate are WorldFIP and ISP. The completion of a fieldbus standard is not forecasted until 1997 meaning there is still a long time to wait. With consumers becoming impatient many companies have decided to released there own systems which work off different standards.

The WorldFIP standard is based on the Factory Information Protocol (FIP). It works on a distributed database and time-service system and has a bus manager which issues tokens on an accurate time basis which matches device requests. The InterOperable System Project (ISP) is based on Profibus, which controls messages by using a token-passing method where a token circulates through all participating stations and the station may talk while in possession of the token. The token is circulated according to a preconfigured timing.

There is a technical report which defines the full functionality of the user layer in the ISA’s SP50 version of the standard. The focus of the report is to define the complete distribution of data acquisition and control functions within the field devices. It is this layer on which a decision is needed to be reached to result in true interchangeability and interoperability. This would give users a free choice of equipment instead of having to be locked into using equipment which uses one of two totally different standards. Currently both WorldFIP and ISP organisations are both trying to implement as many high level control functions as possible in to their new fieldbus products, Therefore we may be on the path to two different standards.

Fieldbus is no longer simply a communications standard but is a complete open, integrated measurement and control system which is looking to change the face of process control forever.

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