Keeping Life Cycle in Mind: Infrastructure System for Foundation Fieldbus H1 and Profibus PA

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Typical System Configuration

Every bus segment opens with either a Profibus DP/PA coupler or link, or with an FF power conditioner/power supply. Both devices, which are generally installed close to the DCS, establish the characteristic two-wire transfer type using current-modulated communication at a voltage of up to 32 V. The DP/PA coupler connects to a higher-level DP segment, while the FF power supply is directly connected to the FF-H1 card of the control system. In so doing, attention must be paid to correctly connecting the bus, as the most frequent problems in fieldbus networks are caused by termination errors.

Standard supply units electrically isolate the output signal from the main supply. When faults occur in different segments, this feature ensures that cross-currents cannot flow. The power supplies represent the weakest point in the fieldbus segment, and to increase reliability they are optionally available in redundant versions.

Field devices can be up to several hundred meters away and surge voltage protection modules are typically included in the cable link. In the field, up to 31 devices can be theoretically coupled in parallel to each segment over a distance of 1,900 m. The permissible spur line lengths depend, however, on the number of devices to be supplied, and reserves also have to be taken into account. In practice, between 8 and 18 transmitters are planned for each segment in order to achieve spur line lengths of between 60 and 120 m.

Costly Reserves

Field devices are only rarely connected in parallel with one another. Instead, they are generally connected to the segment via field distributors, known as junction boxes, which include segment protectors (device couplers). A device coupler protects the segment, which must not completely fail if a short circuit occurs on a spur line. In addition, it ensures approved energy limitation or intrinsically safe separation as required by the Atex Directive.

In recent years, these device couplers have been available almost exclusively in a block-type design. The 4, 8, or 12 blocks have the disadvantage that at the time of installation, the plant operator must keep unused reserves in each segment for possible subsequent expansion (Figure 1). As a consequence, the operator pays for more connections than are required in the first phase of the project.

These surplus electronic devices gather dust in control cabinets over the years, and their components age, to the point where they may not even function when they are required for a plant expansion. Furthermore, it is not possible to exchange individual channels. If a complete block has to be replaced, its design means that the complete segment fails. This is something that no operator can afford.

Can be Flexibly Expanded

In view of this, Phoenix Contact provides a scalable infrastructure system that starts with optionally redundant Foundation Fieldbus power supplies. These devices already contain the terminating resistors, so that the installation technician cannot forget them. Thanks to the modular design of the electrically isolated power supplies, the socket and plug-in modules allow the required number of segments to be assembled in accordance with the actual requirements while still permitting subsequent expansions.

If the system is expanded, then an additional module is simply inserted. Individual supplies or sockets can also be replaced without influencing the other segments. The redundant FF power supplies also feature Auto Current Balancing (ACB) technology, which helps to increase service life by using both power supplies to provide the current required by the segment. If a fault occurs, this is signaled to the maintenance team using a diagnostics relay.

The field cabling starts with matching surge protection modules, which Phoenix Contact offers for different applications. The field distribution elements are based on the well-proven rail-mounted T bus connector: the FB-ET main cable coupler connects the bus signal to the T bus, from where it is accepted by either the FB-2SP device coupler or FB-ISO isolated device coupler (Figure 2).

Depending on whether the process instruments are installed in zone 0, 1 or 2, the two devices then supply the energy-limited (FNICO/FISCO ic) or intrinsically safe (FISCO) fieldbus spur lines. If the system is to be expanded, the required number of couplers can now be easily connected using the T bus, without having to interrupt the segment operation. It is also possible to replace the coupler while still retaining the integrity of the remaining segments (Figure 1, left). The field distributors are cost-effectively installed in zone 2.

Conclusion

Fieldbus components from Phoenix Contact allow a process plant to be planned, installed, expanded, and components replaced in the same user-friendly fashion as for conventional signal transfer. Maintenance is simplified, as the system can be scaled channel by channel and the user can access all the diagnostics data. In addition to fieldbus solutions, the product portfolio includes point-to-point and wireless solutions with and without Hart support.

* The author works with Phoenix Contact Electronics GmbH, Business Development Interface, Bad Pyrmont/Germany.

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