The fieldbus specifications call for a power supply on the segment to provide power to all devices that are not self-powered. A standard DC power supply connected directly to the segment would attempt to regulate out the digital communications between devices on the segment. To prevent this, a power supply with power conditioning that contains a specific impedance profile must be installed between the bulk power supply and the fieldbus segment. For H1 cards, an external fieldbus power supply provides the required conditioning and powers the individual field devices. Fieldbus power supplies are available for simplex, redundant, Intrinsically Safe, and Non-Incendive applications. The fieldbus power supply is integrated into the H1 with Integrated Power card.
Emerson recommends the use of redundant fieldbus power in conjunction with device short circuit protection for high availability, Intrinsically Safe, and Non-Incendive applications. When redundant fieldbus power is used, the process remains in control if one supply fails because the other supply continues to provide power to the devices on the segment.
The power requirements for fieldbus devices differ, but the average is 20 mA per device. The DeltaV system supports 16 field devices on a segment.
In addition to the conditioning requirement, neither signal on the segment can be connected directly to ground. The power supplied to the segment must be isolated from ground and from any other uses.