14 - How is Fieldbus more distributed than a "Distributed Control System"?
Unlike the "semi-distributed" DCS system, the Fieldbus system is a completely distributed. In Fieldbus the process control functions are distributed to equipment in the field, while still allowing operation and tuning from the control room using the digital communication. The field devices are typically multi-dropped, several connected to each other and the operator console, drastically reducing wiring with subsequent savings in cost of purchase and installation.
In the legacy DCS of the 1970 the control functions for several loops was centralised to one or more "unit controllers" which contains control, input and output cards.
Since in Fieldbus communication is completely digital no input or output cards are required, and since the control functions are performed by the devices in the field, no control cards are required either. Field devices may be connected directly to the operator console, hence a data highway connecting control cards is not required either. Since there are no cards and no data highway, they need not be made redundant either. All that remains of the classic DCS architecture is the field devices and the operator console. Obviously a Fieldbus system means a tremendous hardware and subsequent cost reduction.