Fieldbus - Computer Busses - page 306

Speed 1 Mbps
Maximum nodes with (/without repeaters) N/A (30)
Maximum distance with (/without repeaters) N/A (40 m at 1Mbps, 1 km at 20 kbps)
Arbitration CSMA

Cable type twisted-pair

Header/data size 8 bytes fixed

Major benefits Low cost/efficient for short messages

Primary applications Automotive

19.3 FOUNDATION Fieldbus

FOUNDATION Fieldbus, is an open specification for sensors, actuators, analysers, and so
on. It allows:

• The control functionality actually resides in field devices.
• The support of other diagnostic, process operation and maintenance functions within field
devices.

In the past, 4-20 mA standards have been used to transmit plant information to controllers.
This has in some places, been replaced by transmitter vendors providing their own digital
protocol to allow bidirectional communication between the control system and smart trans-
mitters. Fieldbus has finally allowed a standardised method for process control to move from
being centralized to become distributed, and the control to actual reside in field devices, such
as transmitters, values and analysers. It provides a digital communications channel and a user
layer to provide intercommunications. Its benefits are:

• Interoperability - this allows different suppliers to be used for devices.
• Wiring cost savings - one communication channel can transmit many digital signals.
• Flexible control implementations.
• Increased field information - this includes processed data, averages, minimas, maxima,
diagnostic information and operational information.

Fieldbus was initially defined by the ISA's SP50 fieldbus standards committee, which out-
lined a two-way, multidrop, digital communications standard for the interconnection of sen-
sors, actuators, instruments and control systems. The Fieldbus Foundation has since set out
to commercialise it as the FOUNDATION Fieldbus.
19.3.1 Fieldbus topology
Most analogue transmission methods, and many digital field communications methods, re-
quire a single twisted-pair wire for the transmission of a single process variable. Fieldbus
differs from this in that it can connect using point-to-point, with buses with spurs, as a daisy
chain, as a tree, or as a combination of any of these. The methods are:

• Bus with spurs - all the devices connect to a common bus and they connect through
junction boxes, as illustrated in Figure 19.3.