18 - Isnt 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
Parameter view object groups
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.