It's nice way to interact through message board.
Hope, peoples are getting benefit through all the field bus question raised by me.
I have learned a lot from your answer.
At present, i am planning to implement Foundation Fieldbus in one of our Off-shore platform project.
With the limited space in offshore platform, Fieldbus has advantages over other technology.You can reduce the cable Junction which generally add the mechanical load to the platform.
Unfortunately, the use of (IS) techniques proved to place a severe limit on the number of devices that can be placed on an individual wire segment. No of devices may be limited to 3 or 4 only. In that case whole purpose will be defeated.This limit is a result of the current draw of IS components presently available. Historically, IS equipment manufacturers have provided components that are approved for all gas groups (i.e., NEC Groups A, B, C, D or IEC Groups IIC through IIA). Thus, for IS isolators designed for Fieldbus application, the maximum current limit is set from 60 to 80 mA dc by most manufacturers.
The low limit of three to four devices per segment may not be acceptable since it limited the extent of control loop span and drove up the number of H1 interface devices required. In order to address this problem, FF segment repeaters may be connected up to three wire segments into a single “logical“ FF segment that requires a single FF H1 interface device
Using IS components designed for Group C ( Ethylene) through D ( Propane) gases would raise the current limit for the individual FF segment to possibly 300 mAdc allowing up to 10 to 15 devices to be connected to a segment. We all need to motivate the IS component manufacturers to develop products that shall be "fit for purpose" and so that FF devices can be used for field control without the need of repeater.