18 - Isn’t Fieldbus too slow, 31.25 kbit/s compared to several Mbit/s for DCS?

In Fieldbus there are typically only 12 device per segment (on a pair of wires) whereas on a DCS all the device of the entire plant are indirectly connected to a single bus which therefore must be faster.

In order to transfer all link and supervisory data fast enough to get tight closed loop control and fast MMI (Man-Machine Interface, such as an operator console) screen update, Fieldbus implements a number features for communication optimisation.

• Automatic distributed alarm detection and notification
• Distributed Trending
• Parameter view object groups
• Scheduling
• Update event notification

Thanks to the mechanisms for passing of configuration, alarm and trend data this so-called background traffic has been reduced to a minimum leaving more time for operational traffic and again improving control performance. After configuration, the system resolves tags and parameter names into a format, which minimises communication.

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