A DATA COMMUNICATION SYSTEM OF THE FIELD BUS TYPE WITH A TWIN LEAD FOR POWER SUPPLY TO CONNECTED UNITS AS WELL AS FOR DATA TRANSMISSION BETWEEN THE UNITS

Abstract: A field bus control system uses electronic units having a high impedance which appear as constant current consumers except for any particular unit (2) during transmission which creates a signal by a varying current consumption causing detectable and hence demodulable voltage variations at the other units. Due to the invention safety is obtained in a very simple manner against errors and accidents caused by 1) common mode interference in the bus cable, 2) incorrect termination impedance of the bus cable at connected units, 3) discharge of capacitive energy from the individual units in case of short circuit of the system, and 4) discharge of energy stored in the self-inductance in sections of the cable in case of breakage in order to avoid discharges causing fires.

Description: A data communication system of the field bus type with a twin lead for power supply to connected units as well as for data transmission between the units. The present invention regards a data communi¬ cation system of the so-called field bus type, i.e. having a long twin lead cable to which are connec¬ ted a number of electronic units which may inter- communicate in that they are supplied with current or voltage from the bus cable and furthermore use this for the data transmission by means of pulse techniques. Such systems would find wide applica¬ tion, e.g. for process control in factories, where signalling may take place selectively directly between the connected units via the bus cable, whereby individual cables between the units and the central control units may be avoided to a large extent, however the systems developed to date have a number of disadvantages. In a system of moderate size the lenght of the bus cable may well be in the order of one kilometre, while a conventional cen¬ trally connected system may demonstrate a much larger cable consumption with the large installa- tion costs it entails. The centrally connected systems have the advantage that the connection between the control unit and the individual units "in the field" may be provided in an individually adjusted manner, and generally individual connections will be shorter, many very much shorter, than the corresponding bus cable. It is among other things this relatively greater lenght of a bus cable which has given rise to noticeable problems in the known field bus cable systems. Hence there may in particular in long cables be noticeable disturbances caused by common mode inteference, i.e. where a current is induced in the cable because of external influences which has the same direction in both the conductors in the cable. Those units which are receptive in the system continuously check the cable for signals with special address to the individual unit, and in case of error signals serious errors may occur, both as regards the address and the meaning of the subsequent information signal. It is known that a transformer may be used at each unit for suppres- sing these error signals, viz. a balanced transfor¬ mer with two primary coils in series as regards high frequencies which normally cooperate but which in case of common mode interference will give rise to oppositely directed magnetic fields from the two coils whereby the error signal is suppressed. The method may be efficient as such, but according to the invention a much cheaper and much more appro¬ priate solution will be provided. According to the invention consistent use is made of connection units, so-called nodes, which appear with constant current consumption, i.e. with a high load impedan¬ ce across a similarly high curremt supply impedan¬ ce, whereby this pattern is only broken during signal transmission from a unit. " The transmission signal is provided by controlled variation of the current consumption, e.g. of + . 5 mA whereby corre¬ sponding voltage variations, dependent on the impe¬ dance of the cable, are impressed on the cable, e.g. in the order of ± 250 mV. The data signals are made of these differential voltage pulses which are deteced in the detector parts of the units. The signal transmitting units may themselves be supplied with means for measuring the provoked voltage change and to control the current change in reaction thereto, in case the voltage change is less than a desired value, which may in particular be relevant by comparatively short bus cables. In transmission cables it is usual to termi¬ nate free cable ends by a resistor having the same value as the characteristic impedance which is done in order to prevent disturbing signal reflections from the ends of the cable. This function is im¬ portant, but when the cable as in the present case is also used for the supply of power to the units connected to it and there is a noticeable voltage of e.g. 24 V between the two conductors, the said resistors will appear as little heating elements, which are constantly consuming unneccesary energy. It is already known that this calamity may be avoi¬ ded by connecting a large capacitor in series with the resistor to disconnect the DC component, but in practice it turns out that such a cable termination by many installation technicians is regarded as incomprehensible and as an unneccesary expense, and in consequence it must be envisaged that possibili- tes for error are already created during installa- tion. According to the invention it is desrcibed below how this disadvantage may be conteracted in a simple way, which will be an important contribution towards a system which will work well in practice. It is a condition for the practical useful- ness of a system af the kind considered that it will not give rise to discharges causing a fire when a cable is cut, and in this respect the con¬ siderable lenght of a field bus cable comes into the picture. A cable constitutes an inductance which is proportional to to the lenght of the cable, and it contains an energy which is equal to the square of the current in the self inductance times its value. At a sudden cutting of the cable a high voltage may be induced which may rise'"to a discharge and an electric arc, i.e. to ignition of the surrounding material. It could be stated that a system of the kind discussed would be useless in practice if it carried this risk, and it is hence an integrated part of the invention that a simple solution also to this problem is given. Just as by a cut cable a short circuit may give rise to a flash which may be dangerous, in particular in an explosive environment. Electronic equipment for use in explosive environments may either be mounted in an explosion-proof enclosure or may be made in a self-protective manner. By this expression is meant that the equipment is incapable of producing a flash with sufficient energy to ignite an explosive atmosphere. The self-protective equipment is fed from a power supply fitted in a non-explosive area, and the voltage and current of the power supply are limited so that an occuring short circuit or dis¬ connection will not cause an explosion. However, it must be taken into account that the maximum short circuit current is equal to the sum of the current that the various connected units may contribute, because these units via their capacitances may represent a considerable current potential. Also in this very important area the invention provides an advantageous solution as described in the follow- ing. Usually the problem in self-protective sy¬ stems of the field bus type is solved by fitting resistors in connection with the capacitor which establishes the AC connection to the primary of the transformer in the measuring unit (the transformer which suppresses common mode interference). These resistors discharge the capacitor so that its ener¬ gy cannot constitute any security risk. Provided there is impedance matching between units and the field bus cable, a construction in the form of a constant current system and signalling by means of modulation of the current consumption of each in- dividual unit, it is according to the invention possible to obtain complete safety by just a series connected diode in the connection. This is possible because the diodes will during normal use always be biased for conduction. With a view to fulfil parti¬ cular standards which require safety even if a simple component should fail, two diodes may be connected in series. Viewed as a whole the inven¬ tion will have provided all the required counter- measures against the disadvantages that may occur by using the comparatively long bus cables which often have to pass via areas which are liable to catch fire or which are explosive, and by means of the invention one may talk about an overall solu- tion which may give the systems of the kind dis¬ cussed the practical wide acceptance which they would be destined to in principle. The invention which is defined in the claims will be described in detail in the following with reference to the drawing, in which Fig. 1 shows an electronic unit according to the invention with connection to the bus cable, Fig. 2 shows a corresponding diagram for connection of an automatic impedance matching, Fig. 3 shows a diagram for illustrating the self-protective bus feature, Fig. 4 shows a schematic diagram for illu¬ stration of how to obtain a "balanced bus" accor¬ ding to the invention, and Fig. 5 is a schematic diagram illustrating a damping of the self-inductance of the bus cable. In Fig. 1 is seen a schematic reproduction of an electronic unit according to the invention. The electronic unit 2 is connected to the field bus cable A, B via the leads a, b. It is a so-called "multidrop syatem" which means that apart from a constant current unit which provides an essentially constant current to the field bus cable (i.e. with a high generator impedance), a number of electronic units are connected.which may communicate with each other. This occurs by modulation of the current consumption of each individual unit in a suitable code and at a high frequency by means of a current generator G built into the unit 2 being controlled by an encoder E. The encoder receives digitized signals from an A/D converter A which represents measuring data from a measuring unit or sensor 4. The electronic components are supplied from the field bus cable via a capacitor C. This capacitor would be able to be discharged by short-circuiting the field bus cable A, B but this is prevented by the diodes 12 which block in case the voltage on the lead a becomes less than the voltage of the capacitor. Transmission cables for units having a high impedance should normally be terminated by a resistor of the same value as the characteristic impedance of the cable. For transmission technici¬ ans this is obvious, but it has to be taken into account that in this case it is a system which is at the same time a power supply system which is typically installed by power technicians, i.e. ordinary electricians. These may not always be counted upon to have an understanding for such special requirements for termination which are furthermore complicated by the fact that many kinds of terminal unit may be present in a particular system, and that termination should occur, not only be means of the said resistor but also a larger capacitor in order to counteract energy losses in the resistor occuring already from a supply voltage of about 24 Volts. The invention has in view to provide an ap¬ propriate termination which is built in beforehand in the individual electronic units and which will only function when needed. This may be obtained in the invention by designing the current supply generator with a high output impedance near the transmission frequency and by each electronic unit being supplied with a dynamic clamp circuit which will counteract sudden voltage variations which are larger than the plus or minus variations for which the clamp circuit is adjusted. On a transmission cable which is injected with a voltage step of e.g. 1 Volt a wave will travel with a velocity close to that of light. The voltage pulse will be damped somewhat because of the ohmic resistance of the cable. When the wave arrives at the end of a non-terminated cable the wave will be reflected 100%, and in any point of the cable, at the passage of the reflected wave the voltage will be the summed voltage of a transmitted wave and a reflected wave which generally may give rise to problems because of so-called multiple re- flections. If it is assumed that the damping by the transmission of the first wave to the termination area is 10%, the voltage at arrival of the wave will be 0.9 V, but following reflection the voltage will double, i.e. 1.8 V. If, however, the cable is terminated with a clamp circut which prevents the voltage from rising above 1 V, the reflected wave will only be 0.1 V (i.e. equal to the damping), and when the reflection arrives back at the generator the voltage rise will be reduced to almost nil, whereby the disturbing multiple reflections will be completely avoided. This principle is illustrated schematically in Fig. 2, where it is shown that each of the elec- tronic units 2 is supplied at the input with the simplest form of a clamp circuit, namely a pair of diodes 8 wired in antiparallel which via a capaci¬ tor discharge any voltage which exceeds the nominal signalling voltage. A wave of 0.1 V may well be reflected, but it will arrive at the generator designated 9 virtually damped to nil, so that the very disturbing multiple reflections will be avoi- ded. Similarly it is shown in Fig. 1 how the coup¬ ling capacitor Cc in series with the capacitor C which has a low impedance at the specified frequen¬ cy together with the diodes 8 constitute the cir¬ cuit described. During installation of the elec- tronic units 2, 4 there is hence no need to take into account any kind of correct cable termination, and it is an insignificant complication that extra diodes or even a more complex but still standardi¬ zed clamp circuit have to be fitted in each unit. This aspect of the invention will obviously be of importance also for other kinds of transmis¬ sion cables where the operational requirements may be different but where it is still relevant to terminate the cables using the principles here outlined. ' It may be foreseen that error conditions or particular conditions may provoke voltage spikes which in a more or less fatal manner may manifest themselves in a flash discharge in the system. This has to absolutely avoided, in particular because many of the systems or parts of systems will be installed in environments which are either a fire risk or explosive. Here use is made of a self-pro¬ tective system which is typically supplied from a power supply unit which is placed in a non-explosi¬ ve area, the voltage and current of which are limi¬ ted so that a short circuit or a break will not give rise to ignition. The maximum short circuit current will be equal to the sum of the currents which each of the connected units will be able to provide to a cable which is short-circuited, based on the capacitive storage of the individual units. According to the invention one or more diodes wired in series with the connectors in each of the units may ensure that the units are unable to con¬ tribute a short circuit current. This will hence only be provided by the power supply, and when the extra contributions are removed it may work with a higher permitted current which makes it feasible to connect an increased number of electronic units. The principle is shown in Fig. 3 where the current supply unit is designated 10 and the diodes are designated 12. In Fig. 4 it is shown that such a diode may be fitted as a standard component in the unit 2. Since the transmission is based upon a varia- tion in current consumption, but always with the current in the same direction, the diodes will not inhibit this function but only reduce the voltage to the individual units by typically ca. 0.5 V per diode. In Fig. 4 is shown an electronic unit 2 which is connected via wires a and b to a field bus cable A, B and which is designed to transmit and/or re¬ ceive impulse modulated signals through the wires a and b. This may be a transmitting unit for the sensor 4, the output signal of which has to be translated to the code used in the system and to be transmitted by means of a particular address to one or more acting units in the system, possibly follo¬ wing a query transmitted from one of these or from a control unit with particular address to the unit 2. Most of the units will be constantly in stand-by in order to react on a relevant address signal, and there should not be disturbances which could mani¬ fest themselves as false signals. In order to counteract disturbances caused by the mentioned common mode interference, i.e. volta¬ ges in the same direction in the two wires a and b, and A and B respectively the unit 2 is mounted in an electrically conductive housing 6, however with¬ out galvanic connection between them, except for a discharge resistor. Between the local ground which is connected to the wire b and this housing 6 there will be a capacitance which is represented by Cl, while there between the same housing and the wire a will be a capacitance C2 which is normally much smaller than Cl. According to the invention there is interposed between the wire a and the housing 6 a capacitor C3 which of a value Cl minus C2, in order that Cl = C2 + C3. Thereby there will be the same capacitance with respect to the housing 6 for the two wires a and b, whereby a common mode inter- ference will be balanced out. It should be noted that according to circumstances it may be relevant to fit the supplementary capacitor between local ground and the housing 6 which is suggested in Fig. 4 by a capacitor C4. This very simple measure is based on that the invention uses electronic units which exclusively appear as constant current consu¬ mers, i.e. have a high input impedance. Hence it is possible to make do without the special transformer mentioned above and to use the extra capacitor whereby it becomes possible to integrate the cir¬ cuit on a semiconductor chip. Even if the electronic units may be self- protecting in themselves it has, however, to be taken into account that the cable itself constitu- tes an inductance so that a break may induce a very high voltage, the higher, the longer the cable is. The permissible cable lenght will be limited to such a value that a break cannot provoke an effi- cent arc. In particular when use is made of a maxi- mum current the permissible lenght may be determi¬ ned fairly precisely, and it has been determined that this problem is felt already in semi-large installations. According to the invention the problem is counteracted by dividing a fairly long bus cable inductancewise into two or more partial cable lenghts, each being permissible, by fitting zener diodes or a similar voltage limiter at correspon¬ ding distances along the cable between the conduc¬ tors, with suitable account taken of the normal voltages present. By a break there can only appear an energy discharge corresponding to the self-in¬ ductance of the stretch of cable between neigh¬ bouring diodes, because these will limit the volta¬ ge rise on both sides of the affected partial lenght. There will be a current peak in each of the partial stretches, but the voltage contributions will not be summed, because they will be discharged through the diodes. The principle is illustrated in Fig. 5 where the self-inductances of the partial stretches of cable are marked as LI, L2, and L3, whereas the diodes are marked 14. As indicated their cut-in voltage may be somewhat higher than the operation voltage.

Claims: P A T E N T C L A I M S 1. A data communication system of the field bus type, with a long twin lead cable to which are connected a power supply and a number of electronic units which are supplied with power from the cable and furthermore communicate selectively between themselves via the same cable, based on transmis¬ sion and reception of pulse trains provided with addresses, c h a r a c t e r i s t i c i n that the electronic units are driven as constant current consumers, i.e. with a high input impedan¬ ce, in that the the impulse transmission takes place by modulation of the current consumption, producing variations of the voltage in the cable, and that each electronic unit contains dynamic clamp circuits with respect to AC signals. 2. A data communication system according to claim 1, c h a r a c t e r i s t i c i n that the transmitting electronic units detect the voltage variations produced and control the current consumption in such a manner that a pre-determined maximum value for the voltage variations is not exceeded. 3. A data communication system according to claim 1 or claim 2, c h a r a c t e r i s t i c i n that one or more diodes are wired into the connection from the twin lead cable to each of the electronic units which prevent the feeding of capacitive energy from the individual unit to the twin lead cable. 4. A data communication system according to any of the claims 1 through 3, c h a r a c t e r i s t i c i n that the elec¬ tronic units are each fitted into an electrically conductive housing, and that capacitor means are connected between the housing and its electronic unit of a value such that differences in capacitan¬ ce between the housing and the respective terminals of the electronic unit are to a large degree balan¬ ced out. 5. A data communication system according to claim 1, c h a r a c t e r i s t i c i n that voltage limiting circuits, such as simple Zener diodes, are connected between the conductors of the twin lead cable at comparatively large di¬ stance from each other in order to self-inductan- cewise isolate stretches of cable for eliminating voltage pulse reinforcement due to reflection phe- nomena.

Inventors: NIELSEN OLE CRAMER, NIELSEN, OLE, CRAMER
Applicants: NIELSEN OLE CRAMER, NIELSEN, OLE, CRAMER
Application year: 1992
Main classification: H04L12
IPC codes: H04B3, H04L12