Slope control device for an electric data transmission system

The invention relates to a slope control device for an electric data transmission system comprising a first line and a second line for differentially transmitting binary data pulses in such a manner that a first logic value of the data pulses has a high potential on the first line and a low potential on the second line associated therewith, and a second logic value of the data pulses has a low potential on the first line and a high potential on the second line associated therewith.

Such a data transmission system usually includes a larger number of data locations that are interconnected by means of the two lines and of which at least part is capable of operating both as transmitter and as receiver.

A first advantage of such differential transmission via two lines consists in that interference pulses reaching both lines in equal manner are eliminated in the differential assessment or decoding of the data pulses transmitted via the lines. A second advantage consists in that such a differentially operating data transmission system displays redundancy with respect to a number of line errors, so that error-free transmission thus is still possible if the two lines are shorted to each other if one of the two lines is open or if one of the two lines is shorted to a ground potential or a supply potential.

More detailed explanations in this respect can be found in DE 195 23 031 A1.

Such a differentially operating data transmission system can be a CAN system. The term CAN stands for Controller Area Network. Closer details in this respect can be found in the book “Controller Area Network: CAN” by Konrad Etschberger, Carl Hanser Publishing House 1994, ISBN No. 3-446-17596-2.

Such CAN systems are employed for example in the field of motor vehicles. With such differentially operating data transmission systems, an error detection and error processing circuit may be employed through which, in evaluating or decoding received data, switching over to different signal selection is possible depending on the line condition. In case both lines of the data transmission system are deemed error-free, differential signal evaluation takes place such that the potentials on both lines are compared to each other. In case a line error is found for one of the two lines, it is switched over to a mode of operation in which the potential patterns or curves of the line deemed error-free only are evaluated. One may proceed in corresponding manner if it is found out that the two lines are shorted to each other.

Such circuit arrangements for error recognition, error processing and switching over to different evaluation modes are known from the afore-mentioned DE 195 23 031 A1 and from the applicant's own German patent applications 198 26 388 and 198 50 672.

With such a differentially operating data transmission system, the potential transitions on both lines, which occur simultaneously, should have the same slope steepness in order to keep electromagnetic interference radiation low and to be able to obtain good error elimination in the receiving data locations. Conventional attempts to obtain this consist in associating a slope regulating circuit in each data location of each line, which regulates the slopes on the particular line to a predetermined desired slope steepness value, with the desired slope steepness values for both lines being identical. Each control loop, however, has its own amplification, its own phase compensation and its own offset behavior. Due to the fact that the control loops of different slope control circuits differ from each other in these parameters in practical embodiments, there are corresponding slope steepness deviations arising in the output signals.

It is the object of the invention to overcome the foregoing disadvantages and to provide for slope steepness conformity on both lines.

According to the invention, this is achieved by way of a slope control device according to claim 1, which may be developed further in accordance with the dependent claims.

In accordance with an embodiment of the invention, in the normal case, i.e., with correct line conditions, slope steepness regulation is effected only with respect to the potential curve of one of the two lines. The potential curves on the two lines are compared with each other with respect to the slope steepness thereof and the slope steepness of the potential curve of each line is controlled without slope steepness regulation in accordance with the comparison result.

Due to these measures, the slope steepness of the potential curve of one of the two lines is regulated to the desired nominal value, and the slope steepness of the potential curve of the other line is subjected to corresponding adjustment control.

In practical embodiments, each line in each data location has a slope control circuit, with the slope control circuit of a line carrying out slope regulation and the slope control circuit of the other line carrying out adjustment control to the regulated slope steepness of the other line. In this respect, the slope-steepness regulating slope control circuit constitutes a master circuit whereas the adjustment-controlling slope control circuit constitutes a slave circuit. As regards the allocation of master circuit and slave circuit, it is possible to use either a fixed assignment or a switchable assignment in which each of the two slope control circuits can be selectively switched to a regulating mode or an adjustment control mode of operation, and in doing so, it may be selected freely which one of the two slope control circuits is switched to the regulating mode and which one of the two slope control circuits is switched to the adjustment control mode.

The latter alternative is to be preferred if the slope control device according to the invention cooperates with a line error checking circuit by means of which the two lines can be checked with respect to the presence of correct line conditions or erroneous line conditions. In case an error-free condition has been ascertained for both lines, it is possible then to switch an arbitrary or predetermined one of the two slope control circuits to the regulating mode and the other slope control circuit to the adjustment control mode. If the line checking circuit ascertains a line error for one of the two lines, the slope control circuit of the intact, other line is switched to the regulating mode.

In a preferred embodiment of the slope control device according to the invention, each of the two lines comprises at least one switchable potential switching means in the form of a controllable analog switch means for data pulse generation, and a slope control circuit for controlling the slope steepness of the data pulses. The analog switch means operates, in the on-state and thus in the range of potential curve slopes in an analog mode, for example as analog amplifier, and in states in which it is turned off, it represents a pure switch behavior. Employed as such an analog switch means is, for example, a transistor, preferably in the form of a MOS transistor which, in the on-state, can be controlled in analog manner via its control electrode.

For comparison of the slope steepness of the potential curve of one line to the slope steepness of the potential curve of the other line, there may be used a differentiating circuit by means of which the potential curves on both lines can be differentiated each, as well as a comparison circuit by means of which the resulting differential signals may be compared with each other. In doing so, the slope control circuit that is in the adjustment control mode is controlled by the output signal of the comparison circuit.

Due to the fact that, in a slope control device according to an embodiment of the invention, only the slopes of the potential curve on one line are still regulated and the slope steepness of the potential curve on the other line is adjustment-controlled in accordance with the actual deviation state of the slope steepnesses on both lines, there is always an identical slope steepness obtained on both lines, at least as long as both lines are error-free.

The embodiments of the invention as well as further objectives and advantages of the invention will be elucidated hereinafter in more detail by way of embodiments shown in the drawings in which:

FIG. 1 shows an embodiment of a slope control device according to the invention, partly in a block diagram;

FIG. 2 shows details of the slope control device shown in FIG. 1; and

FIGS. 3 and 4 show signal patterns resulting from a slope control circuit of the type shown in FIGS. 1 and 2 .

The circuit arrangement show in FIG. 1 comprises two lines A and B of a differentially operating data transmission system, which connect a plurality of different data locations to each other. FIG. 1 shows circuit parts of such a data location.

Associated with line A is a first series connection including a first resistor RA and a first analog switch ASA, while line B has a second series connection associated therewith including a second resistor RB and a second analog switch ASB. Line A is connected to a connecting point PA between RA and ASA, while line B is connected to a connecting point PB between RB and ASB. In FIG. 1, the two analog switches ASA and ASB are each shown as controllable current source QA and QB, respectively.

Each of the two series connections is connected between a first supply voltage terminal Vd (for example +5V) and a second supply voltage terminal GND (for example 0V). In the first series connection, the resistor RA is located on the side of Vd and the current source QA is located on the side of GND. In the second series connection, in opposite manner, the current source QB is located on the side of Vd and the resistor RB is located on the side of GND.

Each of the two analog switches ASA and ASB comprises a switching control input SEA and SEB, respectively. Both switching control inputs SEA and SEB are connected to a common switching control signal input SE via which ON/OFF control signals are supplied through which the two analog switches ASA and ASB are switched on and off for generating the data pulses on both lines A and B.

Each of the two analog switches ASA and ASB is under the influence of a slope control circuit FAS and FSB, respectively.

Each of the two slope control circuits FSA and FSB has four inputs, namely E 1 A, E 2 A, E 3 A, E 4 A and E 1 B, E 2 B, E 3 B, E 4 B, respectively. Inputs E 1 A and E 1 B are fed with switching control signals USA and USB, respectively, by means of which the respective slope control circuit FSA and FSB, respectively, can be switched over between regulating mode and adjustment control mode. Inputs E 2 A and E 2 B are connected to the associated lines A and B, respectively, via differentiating capacitors CA 1 and CA 2 , respectively. Inputs E 3 A and E 3 B are each fed with a reference signal Ref delivering a desired slope steepness value for each of the two slope control circuits FSA and FSB. Inputs E 4 A and E 4 B are each fed with a comparison signal VS delivered by a comparator output CA of a comparator COMP.

Lines A and B, furthermore, are each connected to a capacitor CA 2 and CB 2 , respectively, via a low-pass filter LPA and LPB, respectively. The sides of CA 2 and CB 2 remote from low-pass filters LPA and LPB are connected to both ends of a series connection comprising a resistor RdA and a resistor RdB. A central point MP between RdA and RdB is connected via an additional transistor Rt to a reference voltage source Vct which, on the side remote from Rt, is connected to GND. The two sides of resistor Rt are connected to a first comparator input CI 1 and a second comparator input CI 2 , respectively, of comparator COMP.

The two capacitors CA 2 and CB 2 are used to differentiate the potential curves on lines A and B, respectively. The resulting differential signals are summed by means of the resistor circuit RdA, RdB and Rt and compared to the reference voltage Vct. Due to the differentiating operations, there are formed differential currents IdA and IdB, respectively, the amount and sign of which is dependent on the direction and the steepness of the potential curve slopes on lines A and B. For example, an ascending slope leads to a differential current causing a current to flow from the particular capacitor CA 2 or CB 2 , respectively, to reference voltage source Vct via the associated resistor RdA or RdB, respectively, whereas a descending slope leads to a differential current through which current is withdrawn from reference voltage source Vct via Rt and RdA or RdB, respectively.

In the ideal case, slopes of identical steepness but different slope direction are occurring on the two lines A and B at the same time. In this case, differential currents IdA and IdB of the same amount but different signs result. The current delivered by one of the two capacitors CA 2 and CB 2 through Rt in the direction towards Vct in this case compensates the current withdrawn from Vct by the other one of these two capacitors via Rt. Thus, there is no voltage drop arising across Rt, whereby the same potentials are applied to both inputs CI 1 and CI 2 of comparator COMP, so that a comparison signal VS equal to zero results at the output CA thereof. This is illustrated by way of the signal patterns in FIG. 3 . In accordance with the protocol for differentially operating data transmission systems, the two potential curves PVA and PVB on both lines A and B are opposite to each other in the manner shown, so that the slopes thereof each have opposite directions as well. Differential currents IdA and IdB, during slope times f 1 and f 2 , have the same amount but opposite signs. Due to the fact that the differential current curves IdA and IdB compensate each other in resistor Rt, a resulting current flow Ires equal to zero is formed in resistor Rt. As the voltage drop across Rt is correspondingly equal to zero, a differential voltage Vdiff equal to zero is created between comparator inputs CI 1 and CI 2 . This leads to a comparison signal VS of zero as well.

By way of FIG. 4, a case shall be considered now in which the two potential curves PVA and PVB on lines A and B have different amplitudes which, with the same slope duration, leads to different slope steepnesses. It is assumed in the example that the slope steepness of PVB is less than the slope steepness of PVA. The result thereof is that the amount of the differential current IdB is less than the amount of the differential current IdA and that a resulting current Ires through Rt is formed which is different from zero in the region of the slopes. The differential voltage Vdiff between comparator inputs CI 1 and CI 2 accordingly is different during the slope times, which results in a comparison signal VS different from zero during the slope times.

The voltages arising in the range of the differentiating and summing circuit are checked and evaluated by a line checking circuit LPS delivering at its output the afore-mentioned switching control signals USA and USB supplied to the inputs E 1 A and E 1 B of slope control circuits FSA and FSB, respectively. Line checking circuit, for example, may be formed in accordance with the teaching of the already mentioned Applicant's own German patent application 198 50 672 and/or in accordance with the already mentioned Applicant's own German patent applications 198 26 388. As regards the details of such a line checking circuit, express reference is made herewith to the afore-mentioned patent applications 198 26 388 and 198 50 672 of the applicant, and the teachings thereof are expressly incorporated herein by reference thereto.

Before the function of analog switches QA, QB and of slope control circuits FSA and FSB will be elucidated in more detail, an exemplary realization for an analog switch and for a slope control circuit will be considered first by way of FIG. 2 .

FIG. 2 shows a slope control circuit FSB and an analog switch ASB for line B only. Slope control circuit FSA may be identical to FSB. As regards analog switch QSA, there is a deviation from QSB only in so far as resistor RA connected to line A is connected to supply voltage terminal Vd whereas analog switch QSB is connected to GND itself.

In the embodiment shown in FIG. 2 slope control circuit FSB comprises a slope steepness comparator CompB with two inputs, which are inputs E 2 B and E 3 B, and with an output AB. As already elucidated in connection with FIG. 1, E 2 B is connected to line B via capacitor CB 1 , so that input E 2 B is fed with a differential signal that is a time derivative of the potential curve on line B, corresponding to differential signal IdB coming from capacitor CB 2 in FIG. 1 . Input E 3 B is fed with reference signal Ref which constitutes a desired slope steepness value. Connected downstream of comparator CompB in the direction of signal flow is a summing circuit comprising resistors R 1 B and R 2 B, which on the output side thereof as seen in the direction of signal flow, are commonly connected in a summing point SP. The input sides of resistors R 1 B and R 2 B are connected to a change-over switch UB 1 and, via a reference voltage source Ur, to ground GND, respectively. Through change-over switch UB 1 , the input side of resistor R 1 B is connected, in accordance with switching control signal USB, either to output AB of comparator CompB or to input E 4 B to which the comparison signal VS is supplied.

Summing point SP is connected to gate G of a p-channel MOS transistor TB serving as analog switch, with the drain B of said transistor being connected to resistor RB and the source S thereof being connected to supply voltage terminal Vd. Gate G and source S of this transistor are connected to each other via an additional change-over switch UB 2 which, under the control of the data control signal DS, is adapted to be controlled to a conducting or a non-conducting state. DS controls the generation of the data pulses to be transmitted via the data transmission system. If change-over switch UB 2 is in the conducting state, transistor TB is in the off-state. If change-over switch UB 2 is switched to the non-conducting state, transistor TB acts as analog amplifier element the output currentof which is dependent on the voltage values supplied to gate G thereof from summing point SP.

Reference voltage source Ur delivers a basic steepness signal to summing point SP via R 2 B, while a slope error signal FFS is supplied to summing point SP via resistor R 1 B.

If change-over switch UB 1 is in the switching state shown in solid line in FIG. 2, slope control circuit FSB is switched to the regulating mode. That is, differential current IdB as actual value is compared by means of comparator CompB to reference value Ref as desired value, and in case of a deviation between these two values, i.e., a deviation between actual value and desired value, a slope error signal FSS different from zero is delivered to summing point P. If, in contrast thereto, change-over switch UN 1 is in the switching state shown in broken lines in FIG. 2, in which slope control circuit FSB is in the control mode, comparison signal VS is fed from comparator COMP to summing point SP via R 1 B as slope error signal FFS.

In the following, the operation of a slope control device according to the invention in the design shown in FIGS. 1 and 2 shall be considered.

In a differentially operating data transmission system, the two analog switches ASA and ASB, whose switching states serve to generate the data pulses, are always switched simultaneously either to the non-conducting state or to the conducting state. In the non-conducting state, having e.g., the logic data value “1” associated therewith, the potential values of lines A and B are at Vd (e.g., 5V) and GND (e.g., 0V). If ASA and ASB are switched to the conducting state, which corresponds to logic data value “0”, the potentials of lines A and B are at GND and Vd, respectively.

Assuming that both switches ASA and ASB are first in the non-conducting state, lines A and B have a high potential “H” and a low potential “L”, respectively, as shown in FIGS. 3 and 4 in time section t 0 . If a logic value change of the data pulse to be transmitted is controlled by means of data control signal DS, both switches ASA and ASB are each switched to the conducting state which, following slope duration f 1 , leads to the potential curves shown in FIGS. 3 and 4 in time sections t 1 . What happens between slope durations f 1 , shall be elucidated by way of FIG. 2 . If, at the beginning of time section f 1 , change-over switch UW 2 in FIG. 2 is switched to the non-conducting state by data control signal DS, the basic steepness signal generated by reference voltage source UR first acts at gate G of transistor TB, thereby switching this transistor to the conducting state. Capacitances of transistor TB and line capacitances of line B cause a gradual potential increase on line B with a specific slope steepness. This slope steepness is differentiated by means of capacitor CB 1 and leads to differential current value IdB during slope time f 1 . Depending on whether or not the slope steepness produced by the basic steepness signal is identical with the desired slope steepness signal preset by Ref, an error signal FSS equal to zero or unequal to zero is created at output AB of comparator CompB. In case of an error signal FSS unequal to zero, a corresponding correction of the voltage value supplied to gate G of transistor TB is carried out, whereby transistor TB, in accordance with the error signal, is controlled to the conducting state either to a stronger or to a lesser extent, resulting in a lower or higher source or drain current of transistor TB and thus in a corresponding change in slope steepness. If the capacitances of transistor TB and the line capacitances of line B are charged up at the end of slope duration f 1 , potential PVB of line B changes over to the constant high value “H”. Differential current value IdB then becomes zero again.

In analog switch ASA in FIG. 1, there are corresponding processes created, however, with decreasing slope and accordingly with negative differential current value IdA, as shown in FIGS. 3 and 4.

In the ideal case, as shown in FIG. 3, potential curves PVA and PVB on lines A and B have identical high potential values “H”, identical low potential values “L” and identical slope steepnesses. In this case, differential currents IdA and IdB of opposite sign and the same amount are created at center point MP of the summing circuit, so that the resulting differential current value Ires and thus the differential voltage Vdiff between inputs CI 1 and CI 2 of comparator COMP equal zero. The comparison voltage VS arising at output CA of comparator COMP accordingly equals zero.

In the embodiment considered, slope control circuit FSB has the master function, i.e., is switched to the regulating mode in that its change-over switch UB 1 is driven to the switching position shown in FIG. 1 in solid line. In this case, slope control circuit FSA has the slave function, i.e., its switch (not shown) corresponding to change-over switch UB 1 is driven to the switching state illustrated in broken lines in FIG. 2 .

Under the conditions depicted in FIG. 3 and resulting in a comparison signal VS equal to zero, switch ASA is controlled by slope control circuit to the basic slope steepness, as there is no slope error signal superimposed on the basic steepness signal. Reference voltage Ur is dimensioned such that it leads to a basic slope steepness corresponding to the desired slope steepness preset by reference signal Ref.

In the following, the case shown in FIG. 4 shall be considered in which asymmetries result with respect to potential curves PVA and PVB and with respect to the slope steepness on both lines A and B. The different slope steepness leads to differential currents IdA and IdB of different amount, which in turn causes a resulting differential current Ires and a differential voltage Vdiff that are different from zero. Thus, a comparison signal VS results at the output CA of comparator COMP that is also different from zero. The latter is superimposed in slope control circuit FSA on the basic steepness signal as slope error signal FSS, which results in a corresponding correction of the slope steepness of the current delivered by analog switch ASA. In this manner, the current delivered by analog switch ASA is changed until comparison signal VS has become zero, which means that the slope steepness of potential curve PVA has become equal to the slope steepness of potential curve PVB.

In the case considered above it was assumed that both lines A and B are o.k. and that one of the slope control circuits, namely that of line B, takes over the master function and carries out slope regulation, whereas the slope control circuit of line A takes over the slave function and controls a slope steepness as a function of comparison signal VS. This is the case if it was ascertained by means of line checking circuit LPS and evaluation of differential current signals IdA and IdB, that both lines are free from errors, whereupon slope control circuits FSB and FSA, via switching control signals USA and USB, have been switched over to the regulating mode and to the control mode, respectively.

In case line checking circuit LPS detects a line error on one of both lines A and B, this leads to the effect that only the potential curve of the other line is still evaluated. To bring both lines to the same slope steepness does no longer make sense then. This is why both slope control circuits FSA and FSB are switched to the regulating mode by the switching control signals USA and USB delivered by line checking circuit LPS, so that the comparison signal VS delivered from comparator COMP no longer has an effect in any of the two slope control circuits FSA and FSB. Due to the fact that anyway only the potential curve of the one line that is not defect is evaluated still, it is immaterial if the two slope control circuits provide regulation to different slope steepnesses.

The same holds for line errors in which both lines are defect or the two lines are shorted to each other. If both lines are defective, safe decoding of the data pulses transmitted is no longer possible. If the two lines are shorted to each other, equal potential curves arise on both lines and, by means of line checking circuit LPS, switching over to evaluation of the potential curve on only one of the two lines is effected. In this case, too, both slope control circuits FSA and FSB are each switched to the regulating mode, but only the regulation result of one of the two slope control circuits is utilized.

FIG. 1 shows an embodiment in which two capacitors each are used for CA 1 and CA 2 on the one hand and CB 1 and CB 2 on the other hand. However, it is possible as well to use, for example, capacitors CA 1 and CB 1 also for the functions performed by capacitors CA 2 and CB 2 .

While a preferred embodiment of the invention has been illustrated and described, it is to be understood that various changes may be made therein without departing from the spirit and scope of the invention. Thus, the invention is to be limited only by the scope of the claims that follow and the equivalent thereof.