How to Improve Fieldbus Wiring

Fieldbus wiring is the control backbone for many automated systems.

Fieldbus is a term used to describe computerized automation, controlled by a central network. Fundamentally, the best way to improve fieldbus wiring is stabilize and perfect system grounding as much as possible. Assuming the system is otherwise wired correctly, proper grounding improvements reduce system noise and interference and provide superior protection of connected devices.

Instructions

1. Ensure all system network cables are grounded at a sole location. Known as "star" grounding, this protocol eliminates most ground loops, and improves the performance of system current limiters.

2. Ensure no signal wires in the system are grounded, as dictated by the design of the fieldbus network. Noise immunity is dramatically reduced when some, but not all, wires are grounded, especially two wires on the same pair. Effectively, this creates a small short circuit. Noise immunity built into the design of modern network cables is bypassed when a signal wire references ground, since noise affects all wires equally.

3. Galvanically isolate the wires from ground using a current limiter. Galvanic isolation deals with smaller voltages, such as those present on signal cables. A defect in the shield of a signal cable increases the noise floor of the entire system. Additionally, a small short that is absorbed by a current limiter isolates the problem to only that cable and device, rather than the entire network.

References

• Relcom: The Importance of Isolation for Foundation Fieldbus
• Relcom: Fieldbus Wiring Guide