Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 239)

Publication 1757-UM006A-EN-P - May 2002
Fieldbus Wiring Considerations D-3
Power Distribution The design of a fieldbus network requires understanding of the
electrical requirements of the devices (current/voltage) and the
properties of the cable use (resistance). The number of devices on a
fieldbus segment is limited by Ohm's law:
E = I x R
Where:
E = The voltage of the dc power supply
I = The amount of current in amperes drawn by each device
(typically 20mA)
R = The resistance of the cable in ohms
Signal Degradation
Limitations One approach to minimizing signal distortion is to apply “rules of
thumb” that suggest how long a fieldbus cable can be and still get
adequate signal quality. This is based on published Fieldbus
Foundation wiring guidelines.
EXAMPLE
Assume that the characteristics of the segment are a
power supply output of 20Vdc, 18 AWG cable with a
resistance of 22 ohms per kilometer (3281 feet), and
a “home run” (trunk) cable that is 1 km (3,281 ft.)
long. This results in a combined resistance of 44
ohms for both wires.
If each device at the branch draws 20mA and needs
a minimum of 9Vdc, the cable can use up to 11Vdc
(20 - 9 = 11) and the total current that can be
supplied at the chickenfoot is 250mA (11Vdc/44
ohms = 250mA). This means the maximum number
of bus powered devices that can be used at this
example chickenfoot is 12 (250mA/20mA = 12
devices).
Be sure to check the fieldbus device specifications to
determine the power requirements, since they can
vary by device type and manufacturer. A physical
test can (should) be performed on each H1 segment
prior to power up to check for shorts and verify
resistance. Test the power carrying capability of the
fieldbus cable by connecting the wires together at
one end of the cable and measuring the resistance
across the wires at the other end with an ohmmeter.