04 June 2008
Many automation engineers are coming face to face with real fieldbus applications for the first time. Fieldbus is a wonderful technology with many benefits, but fieldbus installation requires some additional considerations over and above normal 4-20mA projects. This article discusses some of those issues, and shows you how to deal with them.
Figure 1: Standard fieldbus segment.
Choosing a “fieldbus”
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 at the end of the project is a satisfactory and functional control system, and practically every installation will use multiple fieldbuses to accomplish the many tasks required. For example, you may use FOUNDATION Fieldbus in the process plant, DeviceNet for a PLC network, and ProfiDRIVE to run motor drives. Every DCS can easily integrate all these functional plant buses into the Ethernet-based control room bus.
In process control engineering, “fieldbus” normally means FOUNDATION Fieldbus (FF) or ProfibusPA (PA); both fieldbuses are perfectly adequate and widely used around the world in refineries and process plants as modern day enhancements to 4-20mA 2-wire devices. This article focuses on FF and PA physical layer implementation.
Fieldbus Power Supplies
A fieldbus segment (Figure 1) begins at an interface device at the control system. On a FOUNDATION fieldbus H1 system, the interface is called an H1 card; on a PROFIBUS PA system, it is a PROFIBUS DP/PA segment coupler. In terms of signal wiring and power requirements for the segment, FF and PA are identical:
• Minimum device operating voltage of 9V
• Maximum bus voltage of 32V
• Maximum cable length of 1900m (shielded twisted pair)
• Communications at 31.25 kHz, Manchester encoded
The DC power required by devices on the bus is normally sourced through a fieldbus power supply or “power conditioner” (Figure 2) which prevents the high frequency communications signal from being shorted out by the DC voltage regulators. Typical power conditioners make 350 to 500mA available on the bus and usually incorporate isolation to prevent segment-to-segment cross talk.
In FF segments, the power conditioners are separate from the H1 interface card and are often installed in redundant pairs to improve the overall reliability. For PA systems, the DP/PA segment coupler usually incorporates the power conditioning component. There is no absolute requirement for the DC source to be independent per segment, but most designs provide segment isolation via DC/DC converters.
Note that fieldbus power conditioners are NOT the same as COTS (Commercial Off The Shelf) power supplies which, if connected straight on to any segment, will immediately damp out all
Figure 2: A fieldbus power conditioner prevents the high frequency communications signal from being shorted out by the DC voltage regulators. Typical power conditioners make 350 to 500mA available on the bus.
communications. FF/PA systems carry both DC power and digital communications on the same wire pair, and a standard 24V DC power pack would effectively short-circuit the communications signal. The power supply therefore requires low pass “conditioning” to filter out that signal, and this conditioning may be “active” (notch filters, etc) or “passive” (series inductance).
Of course, fieldbus power supplies can fail while in service so it is usually a good idea to spec power supplies that are redundant (one unit can continue delivering power when the other one fails), can be “hot swapped” (a new one can be replaced without shutting down the segment), and has some sort of alarm that notifies maintenance or operations when a problem occurs. Another good feature is built-in surge protection to protect the DCS system from lightning impulses from the field.
Redundant supplies can be constructed as needed for FF segments, but ProfibusPA segments are constrained by the standard DP/PA Segment Coupler design which incorporates field power conditioning within the DP/PA protocol converter and only allows redundant power conditioning in the fault-tolerant version.
When calculating how many devices can fit on a fieldbus segment, the primary factors to be taken into account are the maximum current requirement of each device and the resistance of the segment cable (because of voltage drops along the length). The calculation is a simple Ohm’s law problem, with the aim of showing that at least 9V can be delivered at the farthest end of the segment, after taking into account all the voltage drops from the total segment current.
For example, driving 16x20mA devices requires 320mA, so if the segment is based on cable with
50 Ohms/km/loop and a 25V power conditioner, the maximum cable length is 1000m to guarantee 9V at the end.
Voltage available for cable = 25 – 9 = 16V
Allowable resistance = 16V / 0.320A = 50 Ohms; equivalent to 1000m cable
Note that many users also specify a safety margin on top of the 9V minimum operating voltage to allow for unexpected current loads and for adding additional devices in the future. Some users also allow a safety margin in case one or more fieldbus devices fail from a short circuit. We’ll discuss that below.
The calculations must be done for each segment. An engineer must add up all the power requirements of all the fieldbus transmitters, valve controllers, and other devices on the segment, and then factor in the length and resistance of all the cables to make sure that 9V can reach the farthest devices. Fieldbus devices can require anything from 10mA to 25mA, with 20mA a reasonable estimate for mental calculations.
Figure 3: A device coupler provides short circuit protection on each spur. Some device couplers have automatic segment termination
In most cases, the fieldbus device manufacturer will supply the necessary data, but be wary: sometimes they are mistaken. In one case, a customer found that valve controllers specified to draw 10mA actually required 25mA when configured in a particular way. When the plant powered up the segment, they found that discrepancy the hard way, and had to add an entire segment to accommodate the high-powered controllers.
Advice: Be certain you know the power requirements of every device you plan to install on a segment.
In ProfibusPA and FF, the communications signal is current modulated at 31.25kHz, 20mA p/p. Terminators are required at each end of the segment cable (The square “T” boxes in Figure 1) to prevent line reflections (which may otherwise result from open-ended cables) and to source/sink the communications current.
The terminator circuit is very simple: 100Ω 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 100Ω gives a nominal 50Ω load for the communications current (20mA p-p) and a signal voltage for receiving devices of 1V p-p.
If instruments worked during lab or staging tests, but don’t work in the field, in many cases it’s an installation problem. Simply put, the technicians didn’t set the segment terminators properly. Instruments can behave erratically, drop off the
segment mysteriously, and generally raise havoc—all because the terminations are not set properly.
Two terminators are required per segment, one at each end. With one terminator, the signal will be higher, and with three or four terminators, the signal will be lower. Many field devices won’t accept signals at 2V p/p and may unexpectedly reset. With three or four terminators, the signal can be so low it is unusable. The absolute minimum signal that devices must be able to recognize is 150mV p/p.
Some users may test a segment in a lab, or at the vendor site. In such a case, under carefully controlled conditions, the segment may actually work with incorrect terminators. However, they
rarely work in the field when not terminated properly.
Careful installation management to ensure the correct number of terminators is essential. It is
Figure 4: Terminators (square T boxes) must be turned on at the beginning and the end of each segment
unfortunate that many installation subcontractors pay little heed to the terminators and either forget them completely or enable them all if they are part of the device couplers, neither of which allows the segment to operate properly. Often, physical inspection of junction boxes and field enclosures is the only way to locate and correct the terminator position, which is a significant delay to the commissioning process.
Most device couplers (Figure 3) use manual on/off DIP switches to terminate couplers. In a segment, the last device coupler should contain the terminator (Figure 4), and all couplers between the last coupler and the H1 card should have their terminator switches set to off. Diagnosing the problem often requires physically examining each device coupler to determine if the switches are set properly throughout the segment.
Automatic Segment Termination simplifies commissioning and startup. It automatically activates when the device coupler determines that it is the last fieldbus device coupler in the segment; if it is, it terminates the segment correctly. If it is not the last device, it does not terminate the segment, since the downstream device coupler will assume that responsibility. No action—such as setting DIP switches—is necessary by the installation person to terminate a segment properly.
If a device coupler is disconnected from the segment accidentally or for maintenance, the Automatic Segment Termination detects the change, and terminates the segment at the proper device coupler. This allows the remaining devices on the segment to continue operation.
One of the central themes of fieldbus for process control is that it should be as practical as possible. Power and signal shall be available on the same cable, and that cable should not be fundamentally different from 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" (same as ordinary instrumentation cable but with an alternate color sheath at significantly extra cost). In general, if a cable is already in use for instrumentation and control, it is almost certainly fine for FF/PA use. Typically, 0.8mm2 cable is used, with shield on individual spurs and with overall shield if used as part of a multi-core cable. Table 1 lists the typical cables used in fieldbus applications.
Conventional instrumentation cable may not have digital communications parameters included on its datasheet (effective impedance at 31.25kHz, attenuation rate in dB/km, etc.) and so its performance in fieldbus applications cannot be guaranteed. The Fieldbus Foundation is about to release a test specification for cable which will allow manufacturers to test conformance to a proper performance specification.
Tip: if you intend to use cable glands to seal the cable entry into a device coupler or junction box, check that the fieldbus cable used is properly “round” – many cheaper two-wire cables have a distinct “lay” evident in the outer sheath of the cable and this will not seal effectively in the cable gland.
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