MANY automation engineers are coming face to face with real fieldbus applications for the first time. There are significant pitfalls for the unwary, who may actually believe the sales hype about plug’n’play compatibility and ease of installation and commissioning.
Firstly, don’t get “hung up” on which fieldbus to choose. Fieldbus is a generic term for a variety of communications protocols using various media, but all are simply a means to an end.
What you want is a satisfactory and functional control system. Practically every installation will use multiple fieldbusses to accomplish the many tasks required. The selection of dominant technology is driven by the relative importance of those tasks. In continuous operation process plant control engineering, “fieldbus” normally means Foundation Fieldbus or ProfibusPA. This article focuses on FF and PA physical layer implementation.
Fieldbus power supplies
FIELDBUS power supplies are not the same as COTS (Commercial Off The Shelf) power supplies. This is likely to become evident only during loop testing and pre-commissioning checks.
FF/PA systems carry both dc power and digital communications on the same wire pair. A standard 24 V dc power pack would effectively short-circuit the (31.25 kHz) communications signal. The power supply therefore requires low pass “conditioning” to filter out that signal.
This conditioning may be “active” (notch filters, etc.) or “passive” (series inductance).
There is no absolute requirement for the dc source to be independent per segment, but most designs provide segment isolation via dc/dc converters.
Redundant supplies for FF segments can be constructed as needed. However, ProfibusPA segments are somewhat constrained by the standard DP/PA segment coupler design, which incorporates field power conditioning within the DP/PA protocol converter.
Terminators
IN ProfibusPA and FF, the communications signal is current modulated at 31.25 kHz, 20 mA p/p.
Terminators are required at each end of the segment cable to prevent line reflections (which may otherwise result from open-ended cables) and also to source/sink the communications current.
The terminator circuit is very simple: 100? resistor and 1µF capacitor in series across the segment.
The end-of-line resistor provides a nominal load for the communications signal, and the capacitor stops the dc supply draining through the resistor.
Two terminators at 100? gives a nominal 50? load for the communications current (20 mA p/p) and a signal voltage for receiving devices of 1 V p/p. It is an enlightening experience to open up a commercially-produced terminator: US$75 gets a sophisticated plastic enclosure with an internal printed circuit board struggling manfully to hold a small resistor and a tiny capacitor!
One of the most common commissioning problems relates to under- or over-termination. Two terminators are required and only two.
Careful installation management to ensure the correct numbers of terminators is essential, or the issue can be completely avoided by using device couplers that automatically provide correct segment termination.
Fieldbus cable
FIELDBUS for process control should be as practical as possible. Power and signal should be available on the same cable, which should not be fundamentally different to conventional instrument cable already in common use.
Some cable manufacturers take advantage of the uninitiated by offering “fieldbus” cable in the same way as they make “intrinsically-safe cable” (ordinary instrumentation cable but with a blue sheath).
In general, if a cable is already in use for instrumentation and control, it is almost certainly fine for FF/PA use: 0.8 mm2 cable is typically used, with shield as individual spurs and with an overall shield if used as part of a multi-core.
However, field wiring is definitely different.
Fieldbus systems are simple to design because all the device wire-pairs are connected in parallel. However, in practice, any attempt to fill a box full of terminals and just “jump” between all positives and all negatives will result in a “rats nest” of cables within the enclosure.
This may be acceptable in some plants, but will lead to all sorts of maintenance problems once the installers have left site.
A better idea is to use device couplers: junction boxes specifically designed for fieldbus implementation.
These units automatically provide the necessary system interconnections without confusion and greatly speed up the process of device installation. They should incorporate the required terminator with either manual or automatic activation.
Short-circuit faults on individual spurs will drag down the entire segment. Hence device couplers also need to incorporate some form of spur short-circuit protection, which again may be active or passive in design.
Passive protection is very simple and is usually provided by series fuses per spur, which “blow” to disconnect any individual fault. This is inexpensive and very reliable, but it does require manual intervention: someone has to replace the blown fuse (hopefully after repairing the fault!).
Active spur protection comes in various forms: “current-limiting” designs fix a maximum current per spur, but clearly each fault so protected loads the segment continuously. If current-limiting designs are to be used, ensure that your segment power supply can cope with these additional loads.
An alternative design is the “fold-back” variety, where any faulty spur is switched off and that load completely removed from the segment. Both types auto-reset after fault removal, and both normally incorporate LEDs to indicate spur status.
*Commentary by Mike O’Neill, director, international sales, MooreHawke Fieldbus