Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 238)

Publication 1757-UM006A-EN-P - May 2002
D-2 Fieldbus Wiring Considerations
Power Conditioning You must use a power conditioner between your Fieldbus power
supply and the Fieldbus netwrok. You can use a power supply
designed for Foundation Fieldbus operation which has the proper
power conditioning elements. If you are using an ordinary power
supply, a separate power conditioner must also be used. If an
ordinary power supply is connected directly to the Fieldbus, the
power supply would absorb signals on the cable because it would try
to maintain a constant voltage level. The power conditioner puts an
inductor between the power supply and the Fieldbus wiring. The
inductor connects the DC power to the Fieldbus wiring but prevents
signals from going into the power supply.
In practice, a real inductor is not used in the power conditioner 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.”
(1)
IMPORTANT
We suggest that you not use White/Black or
White/Red pairs since they may be mistaken for 115
volt power wiring.
(1)
RELCOM inc., Fieldbus Wiring Design and Installation Guide (2221
Yew Street, Forest Grove, OR 97116) p. 5. Reprinted by permission.