PROFIBUS: Grounding Tips, Shielding, Noise, Interference, Reflections, Repeaters and more

Table 1 – Distances between digital communication cables and other types of cable to ensure EMI protection

Electromagnetic Interferences can be reduced with:

1. Twisted cable
2. Optical Insulation
3. Use of grounded metal ducts and boxes

Figure 21 – Mutual inductance between two conductors

To minimize the induction effect use the twisted pair cable that reduces the (S) area and the Vb inducted voltage in function of the B field, thereby balancing the effects (average of the effects according to distances):

The twisted pair cable is formed by two pairs of wire. The one-pair wire is wound in spiral and through the cancellation effect reduce  the noise and keep the medium electric properties constant through its whole length.

The reduction effect by using twisted wires is efficient for cancelling the flow, called Rt (in dB):

where n is the number of turns per meter  and l is the cable total length. See figure 22a and figure 22b.

The cancelling effect reduces the crosstalk between the twisted pairs and the level of electromagnetic/radiofrequency interference. The number of wire twists may vary for reducing the electric coupling. Its construction provides a capacitive coupling between the pair conductors. It works more efficiently in low frequencies (< 1 MHz). When not shielded, it has the disadvantage with common-mode noise. On low frequencies, i. e., when the cable length is smaller than 1/20 of the wave length of the noise frequency, the shield will present the same potential along its entire extension, and the shield should be connected on a single ground point. On high frequencies when the cable length is longer than 1/20 of the wave length of the noise fequency, the shield will present high sensibility to noise and the grounding on both shield ends is recommended.

On the inductive coupling we will have Vnoise = 2πBAcosα where B is the field and α is the angle where the flow crosses the area (A) vector, or still in function of the mutual M inductance: Vruído = 2πfMI, whose I is the current on the power source cable.

Figure 22 a – Inductive coupling effect in parallel cables

Figure 22 b – Minimization of the inductive coupling effect in twisted cables

Figure 22 c – Example of inductive noise

Figure 22 d – Example of Profibus Cables near the power source cable

The use of twisted pair cables is very efficient as long as the induction in each tortion area is approximately equal to the adjacent induction. Its use is efficient in differential mode, balanced circuits and has low efficiency in low frequencies on unbalanced circuits. In high frequency circuits with grounded multipoints, the efficiency is high, since the return current tends to flow through the adjacent return. However, in common mode high frequencies the cable has little efficiency.

Figure 23 details the Profibus-DP and the ground loops situation.

Figure 23 – Profibus-DP and the ground loops

Protection with metal ducts

Following is the use of metal ducts to minimize Foucault currents.

The spacing between ducts induces the magnetic field to generate disturbances. In addition, this discontinuity may help ease the difference of potential between each duct segment, and, in the event of current surge generated by an atmosphere discharge or a short circuit, the lack of continuity will not allow the current to circulate along the aluminum duct and consequently will not protect the Profibus cable.

The ideal is to connect each segment with the largest possible contact area  to  provide more protection against electromagnetic induction and also a conductor between each segment on each duct side, with the shortest possible length to ensure an alternative path to the currents, in case of the increase of resistance at the segment joints.

If the aluminum duct is properly mounted, when the magnetic field penetrates on the aluminum plate will produce inside it  a a magnetic flow that varies in function of the time [f = a.sen(w.t)], creating an induced f.e.m. [ E = - df/dt = a.w.cos(w.t)].

In high frequencies the f.e.m induced on the aluminum plate will be higher, originating a larger magnetic field and will cancel almost completely the magnetic field generated by the power source cable. This cancellation effect is smaller in low frequencies. In high frequencies the cancellation is more efficient.

This is the effect of metal plates and screens on the incidence of electromagnetic waves; they generate their own fields that minimize or even nullify the field through them, therefore working as true shields against electromagnetic waves. They work as a Faraday cage.

Make sure that the plates and the coupling joints are made with the same material as the cable ducts/boxes. Protect the connecting points against corrosion after mounting with zinc paint or varnish.

Although the cables are shielded, the shielding against magnetic field is not as efficient as against electrical cables. In low frequencies, the twisted pair absorbs most parts of the effect from electromagnetic interference. In high frequencies these effects are absorbed by the cable shield. Whenever possible, connect the cable boxes on the equipotential line system.

Figure 24 – Protection of transients with the use of metal ducts

Transient and effective distances protectors

In regard to projects and installations we must be aware when dealing with concepts and techniques for the protection of PROFIBUS DP and PROFIBUS PA field equipment in terms of high voltage signals and induced lightning or other sources.

It is public knowledge that control systems installations may include of air  distribution and underground cables, wire boxes, cables near to high voltage cables subject to the exposition to lightning, electrostatic discharges and electromagnetic interference (EMI). EMI may be radiated (via air), conducted (via conductors), induced (above 30 MHz) or a combination of these means. For an idea of the voltage generated by electrostatic discharge, if we consider a conductor with 50nH of inductance, it may include voltage peaks around 200V (V=L*dj/dt) or more, since the current pulse generated by the electric discharge has a very short upward time, approximately 4A/ns.

This exposition can affect the behavior of the signals and damage the equipment, as long as they have the same low power components and may easily burn with the overvoltage.

What is a transient protector?

The transient protector is a protection hardware that, properly positioned (as we will see next) and installed, protects the equipment by limiting the level of the transients that could reach it. It works almost instantaneously and “deviate” the transient to the ground and controls the voltage at a level that does not damage the equipment connected to it. When the current reaches an acceptable level, the normal operation is automatically reestablished.

A large variety of models is sold in the market. These protection devices are based on a combination of components like gas discharge tubes (GDTs, or surge arresters, cutting diodes (clamping voltage) and metal-oxide varistors (MOVs) whose features are fast operation, precise voltage control and automatic return, as soon as the overvoltage stops.

Figure 25 – Surge Arrester

How to protect PROFIBUS PA networks and equipment

In PROFIBUS PA installations, the voltages exceeding normal operation conditions are known as ‘surges’. They appear transitorily and may affect the network behavior. It is worth mentioning that as every fieldbus network, they exchange data and, most importantly, they guarantee the integrity of the data and the plant operational safety.

The longest the PROFIBUS PA network trunk and derivations, the larger will be the magnitude of transients due to the ground potential difference. Significant damage can be caused in equipment connected by short cables if the circuits or components are particularly sensitive. In some situations, there may be serious damage on the installation and the equipment, depending on the energy. 

The standard PROFIBUS PA network cable is the twisted pair type, whose twisted conductors minimize the voltage between lines, although, as mentioned before, the ground potential difference may damage the components and affect their work, thereby turning the system sensitive. Also notice that the cable and its distribution are factors to be considered to minimize noises and transients. The use of shield is recommended, as it acts as a Faraday cage, and when grounded at the source of signal it maximizes its efficiency against noises in common mode. Furthermore, it provides better EMI protection.

Figure 26 – PROFIBUS PA twisted pair cable

Concerning the transient protector, the limit voltage must not be higher than the equipment work voltage and in practice this voltage is used as twice that of the equipment.  In terms of lightning, studies show that the discharges may generate currents from 2 kA to 200 kA on the peak currents lasting less than 10μs.

The choice of the transient protector must be judicious, as it can degrade the PROFIBUS PA signal and limit the quantity of equipment. Depending on the manufacturer, this device may add capacity and resistance to the PROFIBUS PA network and these may affect the communication wave signal. Furthermore, some cutting diodes could not be transparent on the network and could also affect the signal levels. In practice, the user should choose devices that comply with the IEC 61643-21 standard and provide high surge currents (around 10 kA), while adding less  than 1? and less than 40pF to the wiring.

Figure 27 – Distances recommended in PROFIBUS wiring

Figure 28 – Degree of interference on a Profibus signal.

The degree of cable interference will depend on a series of factors such as project, construction and characteristics, in addition to their interaction with other elements on the PROFIBUS network (connectors, equipment, shielding, other cables,  etc.) besides certain system parameters and environment properties. A variety of factors limit the performance of the digital signals transmitted related to the cables, which should be considered in the project and their use, such as:

• attenuation;
• noise, the following types:

o    differential noise (circuit characteristics);

o    longitudinal noise (interference due to electric power supply);

o    impulse noise;

o    diaphony (crosstalk);

o    distortions by propagation delay;

o    jitter (phase noise).

What is effective distance?

Effective distance is the physical separation of two devices grounded during the network installation. Whenever the effective distance is bigger than 100 m on the horizontal or 10 m on the vertical between two grounded points, transient protectors are recommended at the initial and the final distance points. In practice, the use is recommended between 50 and 100 m.

Figure 29 – Use of transient protector and the effective distance.

Figure 30 – Example of PROFIBUS PA protector transient. 

How to protect PROFIBUS DP networks and equipment

The rule for effective distance also applies to the PROFIBUS DP network and equipment.

According to figure 31, protection will be provided in case of tension drop or when a surge or even any  differential surge should exceed the breakdown voltage. According to figure 32, this protection is recommended when grounding is not possible and any differential surge will be converted into common mode.

 Figure 31 – Protection with ground insulation.

Figure 32 – Protection with common-mode insulation.

Interferences

The interferences on digital network cables and the instrumentation may be due to:

• Capacitive coupling
• Electromagnetic coupling  

Electrostatic interferences may be reduced by:

• Adequate grounding and shielding
• Optical insulation
• Use of grounded metal conduits and boxes

 Electromagnetic Inferences can be reduced with:

• Twisted cables
• Optical insulation
• Use of grounded metal conduits and boxes
• CCM repeaters with galvanic insulation, by insulating the groundings.

Basically there are 4 types of interferences:

• Line noise

o    Generated by an electromagnetic source

o    Corona Effect, Line Noise and Sparking

• Overload

o    Generated by the signal fundamental

• Spurious emissions

o    Signal harmonics or undesirable signals

• Generated by the Architecture and the Project

 Let us now  comment on the Corona effect. Inverter cables load the energy of the speed control motor into the AC motor. These cables should support not only the high power from the MLP signals (Modulation of Pulse Width), but also the voltage that occurs when the stationary waves develop in the conductors because the cable impedance does not match the motor impedance and that from modern switching inverters. The high voltage may cause Effect Corona discharges and damage the cables and the inverter variable startup.

Currently, special cables use a thicker, electrically more stable insulation. The insulation increases the distance between the conductors and decreases the danger of a Corona Effect discharge. In addition, by reducing the capacitance, the amplitude of the stationary waves is reduced, as well as the transference of noises to the ground loop. 

Profibus grounding, shielding and equipotentialization

The ideal grounding condition in a plant and its installations is when the same potential is found in any point. This may be reached by connecting all its grounding systems through a potential equalization conductor. This condition is known as equipotentialization.

So, any person inside a facility will be free from electric shock, even if the existing voltages increase, because every element will have the same ground potential.

When referring to shielding and grounding, in practice there are other ways of dealing with this subject - a very controversial one - such as, the shield grounding can be done in each station through the 9-pin sub D connector, whose housing touches the shield at this point and it is grounded when connected. This situation, however, should be analyzed timely and the potential graduation of the groundings checked at each point and equalized. The equipotential line system is used to balance the ground potential in different plant locations so that no current circulates over the cable shield. 

♦    Copper: 6 mm²
♦    Aluminum: 16 mm²
♦    Steel: 50 mm²

   In hazardous areas always observe the recommendation from the certifying organs and the installation techniques required by the area classification. An intrinsically safe system must have components to be grounded and others not. Grounding has the function to avoid unsafe voltages from appearing in classified areas. Avoid to ground  intrinsically safe components unless required for functional purposes when using galvanic insulation. The minimum insulation established by standards is 500 Vca. The resistance between the ground terminal and ground system must be below 1Ω. In Brazil, the installation in potentially explosive atmospheres is regulated by the NBR-5418 standard.

For grounding, it is recommended grouping circuits and equipment together with similar noise characteristics in serial distribution and uniting these points on a parallel reference. Always ground conduits and boxes.

• Use copper cables or galvanized grounding tapes on the equipotential line in the system and between the system components.
• Connect the equipotential line to the grounding terminal or to the bus with a wide surface area.
• Connect all ground and shield connections to the equipotential line system.
• Connect the mounting surface (e.g. the cabinet panel or mounting rails) to the equipotential line system.
• Whenever possible, connect the network equipotential line system to that of the facilities. 
• If the parts are painted, remove the paint on the connection point before connecting.
• Protect the connection point against corrosion after mounting, using zinc paint or varnish.
• Protect the equipotential line against corrosion. An option is paint the points of contact.
• Use safety screws or terminal connections for every ground and surface connection. Use pressure washers to prevent the connections from getting loose due to vibration or movement.
• Use terminals on the equipotential line flexible cables. The cable ends should never be covered with stain (no longer permitted by regulations).
•  Locate the equipotential line router the closest possible to the cable.
• Connect the individual metal parts of the boxes to one another. Use special bonding links or specific jumpers. Make sure the bonding links are made from the same material as the cable boxes. The cable box manufacturers may supply the due links.
• Whenever possible, connect the metal cable boxes to the equipotential line system.
• Use flexible bonding links to expand the joints. The bonding links are supplied by the cable makers.
• For connections between buildings or between building sections, the equipotential line route must be directed parallel to the cable. Keep the following minimum transversal sections, compliant to IEC 60364-5-54:

In hazardous areas always observe the recommendation from the certifying organs and the installation techniques required by the area classification. An intrinsically safe system must have components to be grounded and others not. Grounding has the function to avoid unsafe voltages from appearing in classified areas. Avoid to ground  intrinsically safe components unless required for functional purposes when using galvanic insulation. The minimum insulation established by standards is 500 Vca. The resistance between the ground terminal and ground system must be below 1Ω. In Brazil, the installation in potentially explosive atmospheres is regulated by the NBR-5418 standard.

For grounding, it is recommended grouping circuits and equipment together with similar noise characteristics in serial distribution and uniting these points on a parallel reference. Always ground conduits and boxes.

A common error is the use o protection ground as signal ground. Remember that this ground is very noisy and may present high impedance. Recommended is the use of grounding loops, as they have low impedance. Common high frequency conductors present the disadvantage of having high impedance. Current loops should be avoided. The ground system must be regarded as a circuit that supplies the current flow with the least possible impedance. The minimum ground value recommended is below 10 Ω.

The shield (the loop as well as the aluminum blade) should be connected to the functional ground system via the PROFIBUS-DP connector, so that it provides a wide connection area with the conductive surface grounded.

When extending the cable, be careful so that the shield finishing is well done and is not contacting other points unless the grounding points. The maximum protection is at the grounded points, which provide a low impedance route to the high frequency signals.

In cases with a voltage differential between the grounding points, such as different areas in separated facilities, locate  a potential equalization line next to the wiring, by using the proper metal conduit or an AWG 10012 cable. (See Figure 33).

This will provide the most effective protection for a wide frequency band.

Figure 33 – Equipotential line

Figure 33 – Detail of wiring in different areas with equalized ground potentials

Figure 34 shows wiring, shield and ground details in different areas.

Figure 33 – Detail of wiring in different areas with equalized ground potentials

Profibus-PA network

When considering the question of shield and grounding on fied buses, take into account:

• The electromagnetic compatibility (EMD).
• Protection against explosion.
• Protection of people.

According to IEC 61158-2, to ground means to be permanently connected to the ground by a sufficiently low impedance and with enough conductive capacity to prevent any voltage from causing damages to equipment or persons. Voltage lines with 0 Volts must be connected to ground and galvanically insulated from the fieldbus bus. The purpose of grounding the shield is to avoid high frequency noises.

Preferably, the shield must be grounded on two points, a the beginning and the end of the bus, provided there is no difference potential between these points and allows the existence and paths to loop currents. In practice, when this difference exists, ground the shield only at a single point, i.e., the power source or the intrinsic safety barrier. Make sure the continuity of the shielded cable is longer than 90% of the cable total length.

The shield must cover entirely the electric circuits through the connectors, couplings, splices and junction and distribution boxes.

 Never use the shield as a signal conductor. Always verify the shield continuity until the last PA segment in the segment, with due analysis of the connection and finishing, as it should not be grounded on the equipment housing.

In classified areas, if the equalization potential between the safe and the hazardous areas is not possible, the shield must be connected directly to ground (Equipotential Bonding System) only at the hazardous area side. At the safe area, the shield must be connected by a capacitive coupling, preferably a dielectric solid ceramic capacitor, C<= 10nF, isolation voltage >= 1.5kV).

Figure 35 – Shield and Ground  Ideal Combination

Figure 36 – Capacitive Grounding

The IEC 61158-2 recommends the complete insulation. This method is used mainly in the U.S. and U.K. In this instance, the shield is insulated on all grounds, with exception to the negative ground point of the power source or the intrinsic safety barrier on the safe side. The shield has continuity from the DP/PA coupler output, passes along the junction and distribution boxes and reaches the equipment. The equipment housings are grounded individually on the non-safe side. This method has the disadvantage that it does not protect totally the high frequency signals and, depending on the topology and cable length, may generate occasional communication intermittence. In these cases, the use of metal ducts is recommended.

Another additional manner is to ground the equipment junction boxes and housings on a ground equipotential line on the non-safe side. The grounds on the non-safe and safe sides are separated.

Multiple grounding is also usual and provides more effective protection on situations of high frequency and electromagnetic noises. This method is preferably adopted in Germany and some Europeran countries. In this method, the shield is grounded on the ground point of the power source or the safe side of the intrinsic safety barrier, besides the ground on the equipment junction boxes and housings, and is also grounded on the non-safe side. An additional an complementary situation is one whose grounds would be grounded in a set on an equipotential ground line, connecting the non-safe side to the safe side.

For more details, always refer to the local safety standards. The IEC 60079-14 is recommended as a reference for applications on classified areas.

Figure 37 – Grounding and Shield– Several types

Cautions and recommendations with grounding and shield on the PROFIBUS-DP bus

The shield (the loop and the aluminum blade) must be connected to the system functional ground on all stations through the connector and DP cable) to provide a large connection area with the grounded conductive surface.

Maximum protection is provided with all points grounded through a low impedance path to the high frequency signals.

In cases with voltage diferential between the grounding points, pass close to the wiring an equalization potential line (the metal duct or an AWG 10-12 cable can be used). See figure 33.

Figure 38 – Equipotential Line

In terms of wiring use the twisted pair of wires with 100% of the shield covered. The best conditions for the shield work are met with at least 80% covered.

When referring to shield and grounding, in practice there are other ways of handling this subject, one with much controversy, as for example, the shield grounding can be made in each station with the sub D 9-pin connector (figure 34), where the connector housing  makes contact with the shield and is grounded when connecting with the station. In this case, though, annalyze and verify punctually the ground graduation potential and, if necessary, equalize it.

In hazardous areas always use the recommendations from the certifying bodies and the installation techniques demanded by the area classification. An intrinsically safe system should have components that must be grounded and others not. The purpose of grounding is to avoid occurring unsafe voltages on the classified area. On classified areas, avoid grounding intrinsically safe components, unless it is necessary for functional reasons, when employing galvanic insulation. The standards establish the minimum insulation of 500V resistance between the ground terminal, while the system ground must be lower than 1Ω. In Brazil, the installation on potentially explosive atmospheres is regulated by the NBR-5418.

An extra caution should be taken against excessive termination. Some devices have on-board termination.

Figure 39 – Detail of a typical Sub D 9-Pin connector

Figure 33 presents wiring, shield and grounding details on multiple areas.

For grounding, group circuits and equipment with similar noise characteristics in serial distribution and connect these points on a parallel reference and also ground ducts and boxes.

A common error is the use of protection ground as signal ground. It is worth noting that this ground is too noisy and may present high impedance. An interesting alternative is the use of grounding loops, as they present low impedance. Common high frequency conductors have the disadvantage of having high impedance and current loops should be avoided. The grounding system should be seen as a circuit that favors the flow of current under the minimum possible inductance. The ground value should be lower than 10 Ω.

Use of shielded cables to minimize noises

In order to provide better efficiency against noises, the double shield (twist and foil) has been applied with considerable improvement in the signal/noise relationship. The following can be added:

·         Double protection most certainly provides more efficiency, in some cases even three-fold. The more closed the loop, the better the protection.

·         The twisted shield and the foil can be applied in different ways, for low and high frequencies.

For low frequencies the cable is grounded in only one of the ends and in these frequencies the shield is expected to present the same potential. This would provide more protection in low-frequency noises.

For high frequencies the shield will present high susceptibility to noise and both ends should be grounded, although some precaution should be taken due to equipotentiality and safety.

This double protection alternative would protect both low and high frequencies, providing better EMI protection.

The efficacy of the loop (shield) is generally better in low frequencies, while the foil is more efficient in higher frequencies.

Concerning inverters, which normally are a source of noises, an important point is that most of them have commutation frequency ranging from 1kHz to 30 kHz. In addition, some inverter manufacturers state they comply with the CE standards, but that in adequate installations mandatory is:

1. Properly ground, according to manuals, i.e., the shield is grounded on both ends and the motor housing is also grounded (per manufacturers´ recommendation).

2. Output power, control wiring (I/O) and signal must use shielded cable, twisted with original cover equal to or above 75%, with metal conduit or equivalent attenuation.

3. All shielded cables must have their ends in an adequate shielded connector.

4. The control and signal cables must be separated at least 0.3 m from power wiring.

How to understand the Profibus signal reflection

RS485 physical means is the standard with two independent channels known as A and B that transmit  equal voltage levels, however with opposed polarity (VOA and VOB) or simply VA and VB).

For this reason, the network must be activated with the correct polarity. Although with opposed signals, one does not work as return for the other, i.e. there is no current loop.

Each signal returns through ground or a third return conductor, but the signal must be read by the receptor in a differential way without reference to the ground or the return conductor.

This is the great advantage of the differential signal concerning ground in this communication system: note on figure 41 that  the signal is running with inverted phases on the cable conductors, while the noise runs with the same phase.

On the input terminals of the differential amplifier the Profibus communication signal reaches in differential mode and the noise reaches in common mode, hence rejecting it. Therefore, all noise induced in the cable, generally from electromagnetic origin, will be mostly rejected.

   Figure 41 – RS485 Profibus-DP signal

  Figure 42 – RS485 Profibus-DP network

Differential transmission lines use as information only the difference of potential that exists between the two twisted pair conductors, regardless of the potential difference presented in relation to voltage referential (common or ground referential).

What is signal reflection?

Signal reflection occurs when a signal is transmitted along a means of transmission such as a copper cable or optics fiber and part of the signal energy may be reflected back to its origin. It may be due to a cable imperfection, change of impedance along the communication line (splices), lack of terminator spur beyond permitted, total length beyond permitted, etc. 

Figure 43 – Profibus signal without reflection (left) and with reflection (right) due to lack of terminator 

Figure 44- Profibus signal with reflection by splices (left) and without  reflection (right).

Note on figure 45 that the largest the communicaton rate, the largest the reflection influence will be, as the bit time is smaller.

Figure 45 – Profibus signal with reflections in different baud rates.

 Figure 47 shows an example of installation whose minimum curve was violated and makes the Profibus signal act  like that on figure 48.

Minimum curvature

Flexion, stretching, twisting, crushing while installing the Profibus cable may force the conductors or even alter their transversal sections. This disturbs the common axis of the conductors and their shield, and fakes a change of impedance on the cable stress points. By capturing the signals, these points are easily identified by the reflections on the signals. Nonetheless, the minimum specified radius refers to the cable internal surface and not the cable axis.

Figure 46 – Minimum Curvature  Radius 

Frequently the damages are not visible and the cable insulation and integrity may be impaired.

Figure 47 – Examples of inadequate minimum curvatures and damaged cables

Figure 48 – Profibus signal with reflection due to the violation of the cable minimum curvature

  Figure 49 shows the diagram of a basic single-ended transmission line. A voltage source (VS) generates a digital signal with a Zs impedance. The transmission line has the AC impedance ZO in relation to the ground and at the cable end there is the ZT impedance, for impedance matching. Profibus has a terminator at the beginning and the end of each segment that guarantees the best signal condition.

Figure 49 – Diagram of a basic (single ended) transmission line 

What is a network terminator?

The terminator is an impedance added to the Profibus line with the function of matching the network impedance. The longer the network length is, the bigger the signal distortion. The terminator eliminates communication errors caused by the distorted signals. If the terminator is not installed, the wiring will work as an antenna, hence facilitating signal distortions and increasing the susceptibility to noises. The characteristic impedance is the load value, which placed at the end of this line does not reflect any energy. In other words, it is the load value that provides the zero reflection coefficient, or still, a relation of stationary waves equal to one.

Without any terminators at the Profibus segment, the resulting signal in the load is distorted in time (jitter) and amplitude (oscillations). Every time the cable geometry is altered it will cause unbalanced impedance and resulting reflections.

Both Profibus-DP and Profibus-PA networks require terminators. The use of a bus terminator is mandatory where its absence may cause unbalance, with the resulting propagation delay, as well as the damped resonance oscillations cause the transposition of the logical level (thresholds). In addition, the static noise margin improves. In the Profibus-DP, the terminators are active ones, i.e. they are powered. See figure 50.

Figure 50 – Profibus-DP bus terminator.

The active termination is needed at the beginning and the end of each bus segment in order to keep the communication signal integrity and both terminator should be powered. See figure 51.

Figure 51 – Profibus-DP active bus terminator.

The Profibus-DP must have bus terminators (a 100 Ohm resistor and a serial 1 uF) one at the beginning and other at the end. The shield must not be connected to the terminator and the impedance must be 100 ohms +/-20% from 7.8 to 39 kHz. This value is approximately the average value of the characteristic cable impedance on the work frequencies and it is chosen to minimize the transmission line reflections, as well as to convert the signal in acceptable 750 to 1000 mV levels.

Figure 52a – Typical PA network wave form and the influence of the terminators.

Figure 52b – PA terminator with signals of humidity: wrong termination.

Necessary care with the Profibus-DP network terminators

As terminators are active devices, a common error is to place the work stations as a DP slave where an energy drop or a microcomputer is reset, causing  unbalance on the power supply lines, intermittence and undesirable timeouts.

Shielding

Grounding and shielding are mandatory requirements to guarantee the integrity of a plant data. In practice, it is very common to watch intermittent work and gross errors in measurements due to the bad installations.

Noise effects can be minimized with adequate techniques of projects, installation, cable distribution, grounding and shielding. Inadequate grounding may be the source of undesirable and dangerous potentials that may endanger the effective operation of an equipment or the work system itself.

The shield must be connected to the signal reference potential of what is being protected (see figure 53).

Figure 53 – Shielding connected to the signal reference potential it protects

When there are multiple segments keep them connected, ensuring the same reference potential, according to figure 54.

 

Figure 54 – Multiple-segment shielding connected to the signal reference potential it is protecting