Fieldbus cable is rated according to a four-level code (A, B, C, or D), each successive letter representing a cable of lower quality6. The following table gives minimum specifications for each FF cable type:
Bear in mind that the maximum length given for each cable type is the total length of all cables in a segment, trunk length plus all spur lengths. As a general rule, spur lengths should be kept as short as possible. It is better to route the trunk cable in a serpentine fashion to locate coupling devices close to their respective instruments than it is to streamline the trunk cable routing. The following illustrations contrast the two approaches:
If greater lengths are required for a network segment, devices known as repeaters may be added which sense and re-broadcast the Manchester-encoded FF signal between trunk cables. A maximum of four repeaters may be used to extend any H1 segment.
As always, neat wiring practices help make an instrument system easier to maintain and to diagnose when things go wrong. The following photograph shows a triad of FOUNDATION Fieldbus junction boxes and (orange) network cables. Coupling devices located inside each enclosure link each spur cable to the trunk:
