(*Fieldbus is defined in ISA standard 5o.o2, Section 24. There are several types of Fieldbusses described in that standard. The subject of this guide is referred to as H1 low power signaling and is commonly known as Foundation Fieldbus.)

Fieldbus is a process control network used for interconnecting sensors, actuators and control devices to one another. A common type of Fieldbus configuration is shown below:

A twisted pair cable, called the home run connects the control room equipment with a number of devices in the field, sensors such as pressure transducers and actuators such as valves. The field devices can be connected to a common terminal block called the chickenfoot, or crowsfoot at a field junction box. A terminator (T) is needed at each end of the Fieldbus cable to allow the twisted pair to carry digital signals. The Fieldbus cable provides power to the attached devices. A power conditioner (C) is needed to separate a conventional power supply from the Fieldbus wiring. The devices use the shared wiring system to get their power and to send signals to one another.

Devices can also be connected along the home run cable with spurs (S). This is sometimes called daisy-chain wiring.

For control systems that are limited in size, all the wiring components, power conditioner and terminators, can be in a single wiring block to form a star configuration.

The diagrams above show only three of the many possible Fieldbus configurations. The power supply and conditioner could be in the field or in a marshaling panel. The control device could be in the field and only a display terminal could be in the control room. All these configurations are possible so long as the basic signal transmission capabilities are provided - a twisted pair cable, two terminators and a conditioned power supply.

While many devices can be on a Fieldbus, not all devices in a plant need to be on a single network. Usually, a control device has connections to several Fieldbus networks called segments. If the distance to a field device is longer than can be spanned by a single segment, a repeater is used to boost the signals to and from the further segment.

Wiring

Fieldbus uses twisted pair wires. A twisted pair is used, rather than a pair of parallel wires, to reduce external noise from getting onto the wires. A shield over the twisted pairs reduces the noise further. The twisted pair and the shield combination plus any covering is called a cable. Fieldbus cable is similar to the type used for existing 4-20 mA device wiring and, in most cases, existing cable can be used for Fieldbus. For new installations or to get maximum performance from Fieldbus, twisted-pair cable designed especially for Fieldbus should be used. The important twisted-pair cable characteristics are:

Another item to check is the colors of the twisted pair wires. If possible, to avoid confusion, the wire colors should match the color convention of existing wiring in the plant. If installing new cable, the suggested convention is to use blue for the (-) wire and orange for the (+).

Signal Termination

When a signal travels on a cable and encounters a discontinuity, such as a wire short or open, it produces a reflection. This portion of the signal that echoes from the discontinuity travels in the opposite direction. The reflection is a form of noise that distorts the signal A terminator* is used to prevent a reflection at the ends of a Fieldbus cable. In most networks, the terminator is simply a resistor whose value is the same as the characteristic impedance of the wire. Since the Fieldbus cable also carries power, a simple resistor cannot be used because it would use up the power intended for the devices. A Fieldbus terminator has a capacitor in series with the resistor to block the DC voltage but lets the signal through to the resistor.

*A terminator used to prevent reflections is different than a wire screw terminal block that is used to connect wires to each other.

Power Conditioning

If an ordinary power supply were to be used to power the Fieldbus, the power supply would absorb signals on the cable because it would try to maintain a constant voltage level. For this reason, an ordinary power supply has to be conditioned for Fieldbus. This is done by putting an inductor between the power supply and the Fieldbus wiring. The inductor lets the DC power onto the wiring but prevents signals from going into the power supply.

The inductor together with the capacitors in the terminators forms a circuit that can "ring" and disrupt the signals. A resistor is placed in series with the inductor to stop this ringing. This combination of components is a power conditioner.

In practice, a real inductor is not used but an electronic equivalent. The electronic inductor circuit has the added advantage of limiting the current provided to the network segment if the cable is shorted.

The voltage supplied to the Fieldbus cable can be as high as 32 V. The voltage at any device can be as low as 9 V for the device to operate correctly. A typical Fieldbus device takes about 20 mA of current from the cable. The Fieldbus is configured so that one of the wires has a (+) voltage, the other wire has a (-) voltage and the shield is grounded.

A cable with the orange wire as plus and the blue wire as minus is shown above. This type of cable is available from Fieldbus cable manufacturers. Other cables or existing plant wiring conventions may be different. Regardless of the color convention, keep the sense of Fieldbus polarity consistent throughout the plant.

Signals

The twisted pair cables, the terminators and the power conditioner work together as a wiring system to carry signals between Fieldbus devices. Now let's look at how the signals are transmitted.

There are two ways for a device to transmit signals onto the cable, the bipolar method and the low power, unipolar method. Both types of signals can be received by all devices so there are no compatibility issues. At this writing (1998), the bipolar signaling appears to be universally used. It works like this:

A bipolar signaling device draws power from the cable for its internal operation and it also draws an additional 10 milli-amps that it "wastes". When this device transmits a high signal, it turns off this 10 mA. This results in the signal on the wires going high. When the device transmits a low signal, it draws 20 mA from the wires. The resulting signal waveform is shown below. Note that the signal is above and below the non-transmitting voltage level.

A unipolar signaling device does not draw any more power from the cable than is necessary for its internal operation. When this device transmits a low signal, it draws 2o mA from the wires. When the device transmits a high signal, it draws no power from the wires. The resulting signal waveform is shown below. Note that the signal is below the non-transmitting voltage level of the cable.

The peak-to-peak signal voltage is the same as for bipolar signaling. The advantage of the unipolar signaling is that it takes 1o mA less current from the cable. The disadvantage of the unipolar signaling is that it may cause a cable system that is not properly terminated to ring.

Digital data is sent on the Fieldbus at a rate of 31.25 kbits/second. Thus, each bit cell is 32 microseconds long. The digital data, ones and zeros, is represented as a Manchester signal. A zero is a positive signal transition in the middle of a bit cell; a one is a negative transition in the middle of a bit cell. A sequence of Manchester encoded ones and zeros would look like this:

When a device begins transmitting, it puts out a preamble, a sequence of 8 bits with alternating ones and zeros.

This pattern is used by the receiving devices to get synchronized to bit cell boundaries.

There are also two non-data symbols. These are N+ that is a high level during the whole bit cell and N- that is a low during the whole bit cell. These symbols are used to make an 8-bit start delimiter that shows where real data starts and an 8-bit end delimiter that shows where data transmission stops.

Combining the different parts, a single transmission from a device, a frame, looks like this:

The Data portion of the frame contains information such as the address of the device for which the frame is intended, identification of the type of frame, measurement values, etc. The Data portion of a frame can be up to 266 bytes long.

The delimiters are very different from any signal pattern that might be in the Data portion of the frame. This allows the Data portion of the frame to be unambiguously identified and allows Data corrupted by noise to be detected. This feature makes Fieldbus much more robust than other control networks.

Because all devices share the cable, a way must be established for only one device to transmit at a time. Otherwise, there would be chaos on the cable with all the transmitted signals interfering with one another. Selecting which device can transmit is performed by a special device called the Link Active Scheduler, LAS. The LAS sends out a special frame to each device in turn to allow it to transmit. If an oscilloscope were used to observe the signals on the Fieldbus, the display would show frames with gaps of silence between them. A frame might be one from the LAS asking a device to transmit data, a device broadcasting its data to other devices, a device reporting an error condition, etc.

The discussion about how Fieldbus is used for conveying specific types of information is beyond the scope of this Wiring Design and Installation Guide.

Now that the basic characteristics of Fieldbus wiring are known, let's look at what happens to power and signals on the cables.

Wiring Limitations

The size of a Fieldbus wiring system and the number of devices on a network segment are limited by power distribution, attenuation and signal distortion.

Power

The number of devices on a Fieldbus segment is limited depending on the voltage of the power supply, the resistance of the cable and the amount of current drawn by each device. Consider this example:

• The power supply and power conditioner output is 20 volts.

• The cable used is 18 GA and has a resistance of 22 Ohms/km for each conductor. The home run is 1 km long. Therefore, the combined resistance of both wires is 44 Ohms.

• Each device at the chickenfoot draws 20 mA.

Since the minimum voltage needed by a device is 9 Volts, there are 20 - 9 = 11 Volts that can be used up by the cable. The total current that can be supplied at the chickenfoot is

Voltage / Resistance = Current

11 Volts / 44 Ohms = 250 mA

Since each device draws 20 mA, the maximum number of devices at the chickenfoot of this example is:

250 / 20 = 12 devices

The Fieldbus cable can be tested for power carrying capability by simply shorting out the wires at one end of the cable and measuring the resistance of both wires with an ohmmeter at the other end.

The power used by Fieldbus devices varies by device type and manufacturer. Check the device specifications to determine the device power requirements. One of the gray areas of the power specifications is the initial inrush current and the lift-off voltage. Some devices may use a great deal more current when they are first turned on and may require more than the 9V minimum voltage to "lift off" and begin operating. The network power distribution calculation should be based on the worst-case inrush current and liftoff voltage numbers. Otherwise the network may not start up when power is first turned on.

Normally Fieldbus is powered by 24 volt supplies. The maximum voltage that can be on the Fieldbus is 32 Volts. Devices can withstand up to +/- 35 Volts without damage. To keep the maximum voltage on the wiring below this limit, some Field bus wiring blocks have built-in voltage limiters.

When a number of devices are on the cable at different places, the power distribution calculation becomes more involved. Following is an example:

A network is shown with four devices designated 1 through 4. The network wiring has segments a through g. The junctions of the segments are at A, B and C. Here are the facts about the network:

From this, the amount of current in each segment can be calculated. Starting at the devices furthest from the power source:

Because voltage equals resistance times current, the voltage drop in each segment can be calculated.

From this, the voltage drop at each node can be calculated:

As the table above indicates, the largest voltage drop is 2.815 volts at device 2. The current flowing in segment g is 90 mA. Therefore, the power supply and conditioner must be able to deliver at least 90 mA. The lowest voltage that can be at the power supply/conditioner is the 9 volt minimum required by the devices plus the 2.815 volt drop of the cable segments plus the 1 volt needed for signaling plus a safety margin of, say, 1 volt for a total of about 14 volts.

Intrinsic safety barriers can be considered segments in the network with a resistance specified by their manufacturer. This will be covered in more detail later.

Signal Degradation Limitations

The length of a Fieldbus network is limited by what happens to the signals as they travel on the cable.

Attenuation

As signals travel on a cable, they become attenuated, that is, get smaller. Attenuation is measured in units called dB or deciBell. This is calculated:

dB = 20 log (transmitted signal amplitude / received signal amplitude)

Cables have attenuation ratings for a given frequency. The frequency of interest for Fieldbus is 39 kHz. Standard Fieldbus cable has an attenuation of 3 dB/km at 39 kHz or about 70% of the original signal after 1 Km. If a shorter cable is used, the attenuation is less. For example, a 500 meter standard Fieldbus cable would have an attenuation of 1.5 dB.

A Fieldbus transmitter can have a signal as low as 0.75 volts peak-to-peak. A receiver must be able to detect a signal as little as 0.15 volts peak-to-peak. This means that the cable can attenuate the signal by

20 log (0.75 / 0.15) = 14 dB

Since the standard Fieldbus cable has an attenuation of 3 dB/km, this indicates that the Fieldbus can be as long as

14 dB / (3 dB/km) = 4.6 km

This distance may be theoretically possible, but there are other factors that have to be considered. Signals also become distorted as they travel on the cable.

Distortion Effects on Network Size

Shown below are a transmitted signal and a received signal at the end of a long cable.

The top signal is ideal in that the signal fits within the exact bit boundaries, the rise and fall time of the signal is within the Fieldbus specification and the signal tops are nearly fiat. At the other end of a cable, the signal is distorted. Besides being attenuated, the signal does not fit nicely within the bit boundaries, the rise and fall times are longer and the signal top is not fiat. This signal distortion is caused by varying characteristic impedance, spur connection reflections, etc. For this reason, Fieldbus cable cannot be as long as theoretically possible if only attenuation is a consideration.

There are many causes for signal distortion. Spurs on the cable are one source. Although it is not possible to provide a definitive analysis of the effects of spurs, here are some guidelines that will help estimate if a particular network will work or is close to having problems. Consider the network below where the lengths of the cable segments are shown in meters.

It is not clear in this network which is the home run cable and which are the spurs. In such a network, place the terminators as far away from one another as possible. In this example, they are shown as "T". Consider the cable between the terminators to be the home run cable. Consider ail other cables segments as spurs.

Testing of various cable configurations for signal distortion has shown that spurs up to 300 meters in length do not present a problem. The issue is to determine the allowable number of spurs. A way to estimate this is as follows:

The effect of a spur on the signal is very similar to that of a capacitor (providing the spur is less than 30o meters long). As an estimate, the capacitance of Fieldbus cable is about o.15 nF/meter. In this example, the network can be modeled as the home-run cable with attached capacitors.

In this model, the home run cable length is 1o35 meters. The capacitance is calculated from the total cable length of each spur.

From the measurements of actual cable, it has been determined that the worst case signal distortion occurs if all the capacitors are on one end of the home run cable. In this example this would be modeled as

Again, from cable measurements it was determined that signal attenuation due to capacitance is 0.035 dB/nF. In this example, attenuation can be calculated as that caused by the cable plus that caused by the capacitance:

1035 meters x 3 dB/meter + 55 nF x 0.035 dB/nF = 5 dB

This is well within the 14 dB allowed between the lowest level transmitter and the least sensitive receiver. There are other signal distortions caused by spurs, but these are insignificant.

Please note that the results of measurements of signals on long cables and the analysis of spurs in the above discussion are different from the recommended network size and spur lengths in the Fieldbus standard. The standard's recommendations are much more restrictive and are summarized below. In case there is a question about signal levels or fidelity, use a Fieldbus Tester to examine the signals before the network begins operation or during operation.

The fieldbus standard contains estimates of how long a Fieldbus cable can be and still get adequate signal quality. For the standard Fieldbus cable, and some types of existing cables used for control applications, the limits are:

There are also estimates in the standard for the length of spurs and how many devices can be on various lengths of cable.

These are only estimates. The quality of existing cable may vary a great deal. Some existing cable may be very good while other cable of the same type may be waterlogged, have deteriorated insulation or be mechanically damaged. The only real way to determine if existing cable is suitable for Fieldbus or if new cable has been installed correctly is to use a Fieldbus Tester.

Cable Testing

Existing or newly installed cable should be tested to see that it is capable of carrying Fieldbus signals.

An ordinary digital voltmeter can be used to test the resistance between the wire pairs and the resistance from each wire to the shield. Good cable will have resistances of 10K Ohms or greater. The resistance of the two wires should also be measured and noted so that this information can be used in network design calculations.

The ability of the wire pair to carry Fieldbus signals can be tested using a Fieldbus Wire Tester. This consists of two parts - a Transmitter and a Receiver. These are attached to the opposite ends of the cable to be tested. Lights on the Receiver indicate if the wire pair is able to carry Fieldbus signals.

Wiring Polarity

Wiring polarity is important because some Fieldbus devices are polarity sensitive and have to be attached to the wiring in the right way. Wired with the wrong polarity, a device may short out the network or simply not operate.

Currently, the Fieldbus standard does not specify the colors of the conductors of the twisted pair wires nor which wire color should be positive and which should be negative. However, it has been suggested for new installations that the (+) wire be orange and the (-) wire be blue and that the fieldbus cable carry an orange jacket. Existing cables may use many different colors. Whatever the local choice of polarity colors, it is a good idea that this be consistent throughout the plant. To make the job of keeping polarities consistent in the plant, some Fieldbus wiring blocks are clearly labeled with (+) and (-) polarity designations.

Shield

The purpose of the shield over the twisted pair wires is to keep out noise that might interfere with the signals. The shield is most effective if it is connected to ground or earth at only one place. Otherwise, the shield can carry ground currents that introduce noise into the twisted pair cable. The cable shield is generally grounded at the power conditioner or at the intrinsic safety barrier.

Surge Protection

The Fieldbus is expected to be used outdoors. This exposes the wiring to possible lightning strikes or large currents or voltage surges induced by nearby lightning strikes. Since the shield is grounded at only one end, it can become a good lightning conductor into the control room. A way to overcome this problem is to use a surge suppressor.

The surge suppressor is a small gas-filled tube that has a very high resistance when the voltage across it is below 75 Volts. At higher voltages, the gas in the tube ionizes and produces a very low resistance path to ground. This can carry the very large currents as long as the voltage surge lasts. Surge suppressors are built into some Field bus terminators.

Even with surge suppressors, lightning may induce a large voltage between the wire pair. To prevent this from damaging the attached devices, a voltage limiter is placed between the two wires in some Fieldbus terminators.

Intrinsic Safety Barriers

In some plants, the atmosphere may be explosive because of volatile gasses or liquids, grain dust, coal dust, etc. In these situations, all equipment has to be such that it is not hot enough to ignite the atmosphere and that electrical equipment is such that under no condition can it produce sparks that can ignite the atmosphere. These requirements also apply to Fieldbus if it is used in a hazardous area. An Intrinsic Safety barrier limits the available power to the Fieldbus to provide this protection. It should be noted that generally no more than 6 devices can be used on a segment with an IS barrier and the segment will be reduced in length because the IS barrier reduces the available power and attenuates the signal.

A detailed discussion of Intrinsic Safety and the requirements for barriers, cabling and devices is beyond the scope of this Guide. For more information, contact the Fieldbus Foundation at 512-794-8890 for Document AG-163, 31.25 kbit/s Intrinsically Safe Systems.

Wire Connections

Segments of the wires that make up the wiring system for a Fieldbus network need to be connected together. traditionally, this has been done by using terminal strips. For example, to connect two segments of a home run cable and one device on a spur, the following connections would have to be made:

While this type of wire termination works, it has some disadvantages. For example, it is easy to get mixed up and reverse the polarity of the wires. Also, multiple wires are fastened under the same screw. This has questionable reliability.

There are wire termination blocks that are designed specifically for Fieldbus. These blocks have the connections between corresponding wire terminations made internally. There are several methods of terminating the wire to the block - spring clamps, screw terminals and pluggable connectors. The use of any particular type depends on how permanent the wiring installation is to be, on preferences of the installers, and on plant standards.

Preparing the Wiring System

Test the installed cable before connecting any other wiring system components or Fieldbus devices to be sure it meets Fieldbus requirements. Install the terminators, power supply and power conditioner, any spurs and, if required, the intrinsic safety barrier.

Provide a substantial connection, such as a AWG #6 to #10 wire, between the chickenfoot terminator's ground stud and a good ground. The terminator has surge protectors built into it. If a nearby lightning strike induces large voltages on the cable, the surge protectors shunt the unwanted energy to ground and protect the Fieldbus devices attached to the network. Under normal conditions, the surge protector does not affect the Fieldbus operation in any way.

Before connecting the shield to ground, use an ohmmeter to check that the shield is not connected to ground or to one of the wires in the cable.

Wiring Practices

The best designed Fieldbus wiring system, even one that uses high quality cable and components, will not be reliable if some care is not taken during installation.

• If multiple homerun cables go to a field junction box, do not attach the cable shield wires from different network segments together. This can cause ground loops and induce noise into the wires.

• Do not ground the shield of any cable in more than one place.

• At a device, do not connect the cable shield to the device ground or chassis.

• Use wire strippers that do not nick the wire as they strip the insulation.

• Use crimp ferrules or tin the wire ends to prevent stranded wires from getting loose and short to other wires. There is an added benefit to using crimp ferrules: The ferrules provide a gas-tight connection between the wire and the ferrule that is corrosion resistant. The ferrule material is the same as the wire terminal on the wiring blocks. Similar metals are much more corrosion resistant than a bare wire in wire terminal.

• Use wiring terminals that hold the wire ferrule securely and are vibration resistant.

The cable shield should be grounded at only one point. This is usually at the control room end of the cable. If an intrinsic safety barrier is used, the cable shield is grounded at the barrier.

Testing an Operating Network

Once the Fieldbus network starts operation, there are several types of network tests that can be performed.

• The simplest test is to determine if there is sufficient power on the wiring at each of the devices. This can be done with an ordinary digital voltmeter. Another simple test is to use the Receiver part of the Wire Tester to verify the general signal levels on the network.

• The next level of testing is to determine what devices are on the network and measure the signal amplitude of each device. Measuring the noise on the network is also useful. This is done by using a Network Tester.

• Beyond determining the health of the signals on the wiring, the information sent by the different devices to each other can be examined. This is done by using a computer that runs an analysis program. The computer acquires all frames on the network and shows their sequence on the screen. Discussion of protocol testing is beyond the scope of this Guide.

Generally, once a control network is operating properly, the Fieldbus communications protocols do not break. Wiring, on the other hand, can deteriorate or be damaged in a number of ways. Having the capability to identify a wiring problem and to determine its location is very useful.

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