E-Zine February 2012
In the wiring of 4-20mA analog, there is a requirement for a continuous wire pair from the field instrument to its final terminations at the analog input card in the control system. The same 4-20mA current indicating the process variable or measurement value can be measured anywhere on this wire pair. Field wiring reduction is accomplished in both FOUNDATION™ Fieldbus H1 and PROFIBUS-PA in identical ways since both use the identical wiring plan/specifications based on the Physical Layer standard of IEC 61158 and ANSI/ISA 50.02. Savings are realized because several field instruments can share the same wires. The FOUNDATION™ Fieldbus H1 and PROFIBUS- PA specifications call for a wire pair, but the signal is digital rather than an analog current. This allows the same wire pair to loop through several instruments in a multiple drop configuration that allows each instrument to communicate its values in an orderly fashion. Power is provided to each field instrument via the wire pair.
Wiring savings result because several field instruments share the same wire run from the field to the instrument termination rack. The number of devices sharing the same wiring will vary with the power that they require and the need for intrinsic safety.
There can be a cost savings from reduction in the engineering/design work to prepare detailed wiring termination drawings for each instrument/pair since the termination of H1 wires does not need to be to a specific terminal on the interface card. In practice, the design firms do not have a way to take advantage of these features to reduce costs.
It is common for individual fieldbus field instruments and control valve positioners to be wired to a common field junction box where they are joined into a single wire pair. Because of its appearance, the SP50 committee named this wiring arrangement a "Chicken Foot". Savings result from the reduction of the number of long cables and junction boxes. In addition, there are typically fewer fieldbus termination cards than analog input cards, which also reduce costs.
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Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control