Power Conditioning - Honeywell Series A User Manual

14. Appendix E
14.1. Fieldbus Wiring Considerations

Power Conditioning

If an ordinary power supply were to be used to power the Fieldbus, the power supply
would absorb signals on the cable because it would try to maintain a constant voltage
level. For this reason, an ordinary power supply has to be conditioned for Fieldbus. This
is done by putting an inductor between the power supply and the Fieldbus wiring. The
inductor lets the DC power onto the wiring but prevents signals from going into the
power supply.
The inductor together with the capacitors in the terminators forms a circuit that can "ring"
and disrupt the signals. A resistor is placed in series with the inductor to stop this ringing.
This combination of components is a power conditioner.
In practice, a real inductor is not used but an electronic equivalent. The electronic
inductor circuit has the added advantage of limiting the current provided to the network
segment if the cable is shorted.
The voltage supplied to the Fieldbus cable can be as high as 32 V. The voltage at any
device can be as low as 9 V for the device to operate correctly. A typical Fieldbus device
takes about 20 mA of current from the cable. The Fieldbus is configured so that one of
the wires has a (+) voltage, the other wire has a (-) voltage and the shield is grounded.
A cable with the orange wire as plus and the blue wire as minus is shown above. This
type of cable is available from Fieldbus cable manufacturers. Other cables or existing
plant wiring conventions may be different. Regardless of the color convention, keep the
sense of Fieldbus polarity consistent throughout the plant.
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RELCOM inc., Fieldbus Wiring Design and Installation Guide (2221 Yew Street,
Forest Grove, OR 97116) p. 5. Reprinted by permission.
512 Experion PKS Series A Fieldbus Interface Module User's Guide
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Honeywell
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R400
July 2010