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Power distribution
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
R = The resistance of the cable in ohms
For 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 "chickenfoot" 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. For example, Honeywell's smart
pressure transmitter ST3000 and smart temperature transmitter STT350 require 18mA at
9Vdc and Flowserve's Logix 1400 digital positioner/controller requires 23mA at 9Vdc.
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.
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.
The following table lists limits for some possible fieldbus cable types listed in the order
of usage preference with 1 being preferred.
R400
Experion PKS Series A Fieldbus Interface Module User's Guide
July 2010
14.1. Fieldbus Wiring Considerations
Honeywell
14. Appendix E
513
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