FOUNDATION™ Fieldbus Segment Wiring Design Requirements

Following points we are going to discuss Wiring Design Philosophy of FOUNDATION™ Fieldbus.

The trunk cable should be a maximum of 8 pair overall and individually screened cable.
The field junction box shall therefore accommodate a total maximum of 8 H1 segments with each accommodating up to 12 FF devices including installed spares with an execution time of one second. Segment loading shall allow for the 20% spare devices requirement.
Each FF device shall be connected to the Fieldbus junction box via a single pair screened spur cable.
Intermediate JB which contains pass-through terminal blocks may be provided where necessary to connect maximum 8 H1 segments to the trunk cable.

FOUNDATION™ Fieldbus Topology

The Tree (Chicken-Foot) topology consists of a Fieldbus segment connected to a common JB via the trunk cable from the marshalling cabinet as detailed in Figure.

In order to simplify the Fieldbus implementation effort, this topology which mirrors a traditional implementation method, can be used.

Only Tree, Spur, and combination of tree and spur topology can be used.

FOUNDATION™ Fieldbus Cable Type

To obtain maximum performance for Fieldbus network, individually shielded twisted-pair cable designed especially for FF should be used. The following FF Cable Characteristics per IEC Physical Layer Standard

Wire Size ———————————-18 AWG (0.8 mm²)

Shield ———————————— 90% coverage

Maximum Attenuation —————- db/km at 39 kHz

Maximum Capacitance —————-150 pF/m

Characteristic Impedance————-100 Ohms +/- 20% at 31.25 kHz

Maximum DC Resistance————–24 Ohms / km (per conductor)

All trunk cables and spur cables must follow Specification for Instrument Cabling IEC- 61158-2.

FOUNDATION™ Fieldbus Cable Length

Fieldbus segment is limited to a maximum of 1900 meters, this total segment length is computed by adding the length of the main trunk line and all the spurs that extend from it.

Total Segment Length = Trunk + All Spurs

In practice the length of a segment is also limited by voltage drop and signal quality (i.e. attenuation and distortion).

In order to minimize the need to calculate the physical loading of each segment and to reduce the validation requirement, the following limits is normally used.

Trunk cable length ≤800 metres

Spur cable length ≤75 metres

FOUNDATION™ Fieldbus Power Consumption

The maximum segment current is limited to 350 mA by the segment power conditioners.

FOUNDATION™ Fieldbus Attenuation

FF operates at a frequency of 31.25 kHz where a standard FF cable has an attenuation of 3 dB/km at 39 kHz or about 70% of the original signal after 1 km.

A Fieldbus ‘transmitter’ can have a signal as low as 0.75 volts peak-to-peak and a Fieldbus ‘receiver’ must be able to detect a signal as little as 0.15 volts peak-to-peak.

Based on 3 dB/km attenuation, the Fieldbus signals can be attenuated by 14 dB. This normally does not provide a problem for Fieldbus installation especially based on the cable limits detailed in this article.