The promise of standards-based digital fieldbus technology which was first demonstrated by Foxboro’s I/A Series system included intelligent field devices and remote intelligent I/O designed to communicate with the system via a standard digital fieldbus (IEEE 1118). Benefits include wiring & installation saving; faster start-ups; improved automation accuracy & quality and minimised maintenance costs.
The basic architecture has remained reasonably constant since the 1980s in that it is defined in two levels. In Fieldbus Foundation the lowest level in the architecture (H1) is designed to communicate from field I/O or communication bridges to field devices, while the upper level (currently HSE) communicates between controllers and field I/O or communication bridges.
The lower level bus powers field devices, whilst the upper level bus does not. The upper level requires numerous connections while the lower level will typically support few connections. The lower level must also support intrinsic safety while the upper level does not. Also, the upper level must provide high-speed communication while the lower level is much slower.
The specifications for the lower level (H1) fieldbus are complete through the application layer, but only the physical and data link layer specifications have been accepted by IEC as international standards. The current specification does not include some key functionality such as field device or I/O redundancy.
Advanced Manufacturing Research (AMR) have conducted a review of fieldbus standardisation activities and made the following suggestions:
For users they suggest:
* Consider fieldbus technologies for new projects of plant additions. Project implementation savings alone justifies the small additional investment. Reduced maintenance cost and increased plant up-time are bonuses.
* Only consider a fieldbus instrument retrofit if there is another reason to replace existing instrumentation (such as instrument obsolescence or poor instrument wiring).
* Ignore politics and choose your fieldbus technology based on its ability to deliver the functionality you need.
For suppliers they suggest:
* Be prepared to support different standards. You cannot dictate a digital standard to your customer.
* Work towards a standard everyone can endorse that benefits from economies of scale. There is more to be gained by economies of scale than lost giving up captive markets.
* Focus on ‘communication’ rather than control.
Many control system suppliers have been trying to mould an individual fieldbus standard to meet their specific needs in order to gain competitive advantage. Foxboro believes that there will be different standards that need to be integrated. Only in this way, will customers maximise plant performance with freedom of choice.