16.2.4Cable types

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:

Spur cable

Spur cable

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6I have successfully built several “demonstration” FF systems using cables of questionable quality, including lamp (“zip”) cord, with no termination resistors whatsoever! If the distances involved are short, just about any cable type or condition will su ce. When planning the installation of any real Fieldbus installation, however, you should never attempt to save money by purchasing lesser-grade cable. The problems you will likely encounter as a consequence of using sub-standard cable will more than o set the initial cost saved by its purchase.

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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.

16.2.5Segment design

In addition to maximum (total) cable length and repeater count, a host of other details7 conspire to limit how any particular H1 segment is wired. To help engineers and technicians alike deal with these details, manufacturers often provide free segment design tool software to pre-validate a segment design on computer before purchasing components and installing them in the field. A screenshot taken from Emerson’s o ering shows what a typical FF segment layout might look like:

A very nice feature of these segment design packages is their built-in database of FF components. Every time you “pick” a particular component to place in your simulated segment, the program references data for that device’s current draw and other electrical parameters relevant to the performance of the segment. Of course, each manufacturer will tend to feature their own devices more prominently, and so these software tools sometimes feel like a promotional advertisement. Despite the commercial aspect of their design, however, they are extremely useful in the planning stages of a FF network, and should be used whenever possible.

Another reason to use segment design tool software is to document the wiring of each FF segment. One of the casualties of the new Fieldbus paradigm is the traditional loop diagram (or “loop sheet”), the purpose of which is to document the signal wiring dedicated for each measurement and control

7Total device current draw, spur length versus number, intrinsic safety voltage and current limitations, etc.

loop. In FOUNDATION Fieldbus, the control “loop” is virtual rather than physical, being comprised of digital data sent between field instruments, the path of which being defined by the instruments’ programming. The only physical wiring entity to document in a FF system is the segment, and each segment most likely hosts more than one measurement and/or control loop. Unless and until a standardized documentation format8 is invented for Fieldbus network segments, the graphic image provided by segment design tool software is as good as anything.

8At the time of this writing (2009), the ISA has yet to standardize new methods of FF documentation in the style of loop sheets and P&IDs. This is one of those circumstances where technology has outpaced convention.