Fieldbus Wiring Question

  • Thread starter Waddell, Reginald E
  • Start date

Thread Starter

Waddell, Reginald E

In a Foundation Fieldbus network, if all of the field devices are self-powered, does the fieldbus segment need a power source for the digital
communications to take place?

Reginald Waddell
Control Engineer
Cooper River Plant

Verhappen, Ian

YES, their needs to be a way to power the network so that the signal can be altered between its 'high' and low' voltages.

Ian Verhappen
Syncrude Canada Ltd.
PO Bag 4009, MD 0032
Fort McMurray, Alberta T9H 3L1
P 780 790-4079, Cell 799-6017
F 780 790-5190
[email protected]

Chris George

You must have a conditioned 24V power supply on every segment. The Foundation fieldbus signal "rides" on a 24V DC bias. When a device is communicating '1's and '0's it is essentially varying its current (mA) draw. This creates corresponding 'low' and 'high' voltages that form the fieldbus signal. Without a separate power supply for the fieldbus signal, there is no fieldbus signal.

Chris George
Calgary, AB

Asif Khokher

Yes! You need it. Especially, in case of FISCO model, Field Devices power supplies are galvanically isolated from the fieldbus.

Asif A. Khokher
Sr. Design & Application Engr
INTECH Process Automation

Asif Khokher

FISCO is Fieldbus Intrinsically Safe Concept. It is originated in Europe and accepted by Foundation Fieldbus to increase the limit of power on the bus so as to increase the number of devices in IS applications.

Asif khokher
INTECH Process Automation
> In a Foundation Fieldbus network, if all of the field devices are self-powered, does the fieldbus segment need a power source for the digital
communications to take place?<

What do you mean by "self-powered"? Does it mean with a built-in battery? Normally for FOUNDATION Fieldbus we speak of "bus-powered" devices taking
power from the Fieldbus network and "separately powered" not taking power from the bus, instead getting power on separate wires.

Only if you have "bus-powered" devices on the network do you need to have a power supply on the network. If you only use separately powered devices it is not neccessary.

Bus-powered (active) devices communicate by changing their current consumption. If it is 12 mA quiesent they will swing the signal
approcimately between 2 mA and 22 mA where rising means 0 and fallig means 1. Separately powered (passive) devices (such as an interface or repeater) draw no power from the bus and therefore instead swing the signal -10 to +10 mA, not needing any power on the bus.

FISCO is a variation of intrinsic safety that unlike the entity concept uses a trapezoidal output providing more current.

You may want to look at this book:

Title: "Fieldbuses for Process Control - Engineering, Operation and Maintenance"
Author: Jonas Berge
Publisher: The Instrumentation, Systems and Automation Society (ISA) ISBN 1-55617-760-7
Price: US$ 95 (US$ 76 for ISA members)
Media: Hardcopy or softcopy in it entirety or as individual chapters
Language: English
Buy online: "": , "": , or

"This implementation-oriented book based on experience collected from end-users across a wide range of industries around the world provides a clear and concise presentation of how to apply fieldbuses for process control. It addresses "how-to" for all phases of the system lifecycle from engineering to device and strategy configuration, installation and commissioning, diagnostics and troubleshooting, operation, calibration and maintenance. The book covers the three leading process fieldbus technologies: HART,
FOUNDATION™ Fieldbus and PROFIBUS-PA. Both the field-level and the Ethernet-based host-level networking are covered. The operation chapter touches on software integration using OPC (OLE for Process Control). The text also addresses concerns and solutions for interoperability, integration and migration as well as availability and safety. A chapter on benefits helps engineers justify business advantages to the management. The final chapter is for "propeller heads" and covers how these fieldbus technologies work.
The book exposes similarities, differences and capabilities of these bus technologies."

# # #