The present invention relates to data communications between a central electronic control unit (ECU) in a motor vehicle and remote sensors in/on that vehicle.

With the integration of electronics within the distributed vehicle control, a need has arisen to reduce the number of connecting wires between sensors and the controlling electronics. This is particularly important when routing cables through bulkheads from the cab area since manufactures prefer to keep the number of connection ways as small as possible.

Most existing sensors require three wires, two for supplies and a signal wire. As this signal is in analogue form, it can easily be subject to external interference, possibly RFI radiation, as any injected noise could generate a valid, although-incorrect, output signal.

It is also known in some existing system for the transfer of data between sensors and the receiving electronics to be via a serial data link. This approach, while providing a more secure data transfer requires intelligence at the transmitting end as well as the receiver. A disadvantage of this system is therefore that this additional intelligence increases

the cost and complexity of the sensors without reducing the number of connections ..MT8

In other known systems, the transfer of data has been via a 4 - 20mA current loop. This approach reduces the number of connections and also provides a measure of immunity to high resistance connections as current is measured, rather than voltage. The disadvantage of this method, however, is that the data is still being transmitted as an analogue signal, making is susceptible to external interference that could corrupt the signal.

In accordance with a first aspect of the present invention, there is provided a data communications system comprising an interface circuit for linking one or more signal sources to a remote computer-based controller by a single wire which carries supply current to the interface circuit, and wherein signal data from the signal source or sources is transferred to said single wire, and hence to said remote computer-based controller in digital form by modulation of said supply curren .

Preferably pulse width modulation of the supply current is used to transmit the data.

In one embodiment a variable signal and an ON-OFF signal from two respective signal sources can be transmitted by arranging for the modulated current

signal carrying information corresponding to said variable signal to be inverted by state change of the ON-OFF signal. This can be achieved for example, by supplying to the two inputs of an exclusive-OR element a pulsed signal modulated in accordance with the signal from said variable signal source and a two- state signal corresponding to said ON-OFF signal, respectively.

Advantageously, in some embodiments, the computer-based controller is adapted to be able to change the supply voltage provided to the interface circuit via said single wire, the interface circuit being arranged to recognise a change in the supply voltage thereto and to select an alternative signal source whose signal is then transmitted in place of the signal of the first mentioned signal source. By this means at least two variable signal sources can supply data, in addition (if required) to ON-OFF signals, via said single wire to said remote computer based controller.

For transmitting signals from multiple data sources, a micro-controller can be incorporated into the interface circuit for converting the signals from said multiple data sources into coded current pulses for transmission sequentially to the remote controller.

When the present invention is applied to the transmission of a brake pedal signal, the brake pedal demand transducer would thus have integral electronics that takes the demand signal and the brake switch signal and combines them into digital current modulation. This combined information is then transferred between the transmitting device and the receiving ECU via a single wire. This single wire provides power to operate the sensor as well as the return route for the information, the information transfer being in the form of modulated current pulses.

Thus, use of the invention has the advantage of combining and transmitting both of the input signals, and the supply to the transducers, down a single wire, while using a digital signal to transfer the information.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: -

Fig. 1 shows a first embodiment in accordance with the present invention wherein the brake lamp relay of a vehicle is controlled by a brake switch connected into the supply side to the relay;

Fig. 2 shows one example of the internal design of the electronic demand transducer in the system of

Fig. 1;

Fig. 3 shows an embodiment similar to that of Fig. 1 but with the brake switch connected into the ground return side of the relay;

Fig. 4 shows an example similar to that of Fig. 2 but incorporating two demand responsive sensors; and

Fig. 5 illustrates how the system of Fig. 4 can be modified to include more than two sensors.

Referring first to Fig. l, there is shown a brake lamp relay 10 controlling a switching circuit 12 for a brake lamp (not shown) . The return connection for the relay is made via the vehicle body (system earth) at 14. The relay 10 is supplied from the vehicle ignition controlled supply 16 via switching contacts 18 of a brake switch unit 20 whose opening and closing is responsive to operation and release of the vehicle brake pedal (not shown) . As well as powering the brake lamp switch 18 and relay 10, the supply connection is used to power an electronic demand transducer (e.c.u) 22 within the brake switch unit 20, while the return path for transmitted data from the demand transducer 22 is arranged to be via a remotely located main controlling ECU 23 (e.g. the main EBS/ABS ECU) by way of a single wire 26.

Referring now to Fig. 2 one possible example of the detailed circuitry for the electronic demand ecu

22 within the brake switch unit 20 is now described.

This includes a demand sensor 24 in the form of a potentiometer whose one end is connected via a regulator 25 to the ignition-controlled power source

16 and whose other end is coupled to a junction point

28 connected to the single wire 26 leading to the controlling (remote) ECU 23. The variable slider of the potentiometer 24 is connected to one input of a comparator A^, whose second input is connected to an oscillator 30 producing a sawtooth waveform. The output of the comparator A _1; forms one input of an exclusive OR gate G-^, whose second input is coupled via a line 32 to a point between the brake switch 18 and the relay 10, ie a point that will be at B ⁺ volts when the brake switch 18 is actuated but at ground volts when the brake switch 18 is open. The output of the exclusive - OR gate G-^ is coupled firstly to the output of regulator 25 via a resister R^ and secondly to the junction point 28 via a resistor R ₂ .

Within the ECU 23, the single wire 26 provides two inputs to a comparator A ₂ via respective resistive dividers, the output of comparator A ₂ being connected to the controlling microcomputer 27 of the ECU 23.

This arrangement operates as follows:

The raw supply voltage B ⁺ is regulated by the regulator 25 to provide a stable supply voltage for

the demand transducer 24 and the other internal electronics. A quiescent current will always be flowing from the brake switch supply B ⁺ to the controlling ECU 23 (while the system is in operation) via the single wire 26. This current is arranged to be modulated digitally to transfer data from the electronic demand transducer to the controlling ECU

23, the frequency of modulation being fixed and only the pulse width being varied. For this purpose the sawtooth generator 30 provides a fixed frequency waveform that, when fed into the comparator A _χ , produces a pulse width modulated signal dependent on demand potentiometer 24 position. This pulse-width modulated signal is applied to the exclusive - OR gate

G ₁ so that, when there is zero signal from the brake switch, the pulse-width modulated signal is effectively transmitted by the exclusive - OR gate G-^ to the junction 28 via R ₂ . On the other hand, if the brake switch is actuated so that a continuous high signal is presented to one input of the exclusive - OR gate G- _j _, the latter gate will only provide a high output when the pulse-width modulated signal is low, ie the latter signal is effectively inverted/ transposed before being transmitted to the junction

28. This enables the receiving controller 23 to distinguish between the two input signals.

The application of the pulse-width modulated signal to the junction 28 changes the current being returned via the single wire 26 to the main ECU 23, and it is this change in current that is used to transfer the brake switch and demand potentiometer signals to the remote ECU 23.

The use of pulse width modulation enables the monitoring ECU 23 to detect any errors if external transients affect the normal operating frequency. The level of modulation from the demand signal is arranged to always be less than 50% so that the ratio can be inverted when the brake switch signal is present. This approach has the advantage that digitally coding the signals as a pulse width, a higher immunity to external noise is gained, while only using a single wire connection to the controlling ECU.

In other embodiments, the above-described concept can equally well be applied to a ground return connected brake switch as illustrated in Fig. 3 where equivalent components have been given the same reference numerals.

Fig. 4 shows a further modification which is adapted to operate with two brake pedal sensors and a brake switch, ie two variable output sensors S-, and S ₂ and an on/off (brake) switch.

In this case, the sensor interface circuit is

located in the ground return from the relay 10, as in

Fig. 3. The arrangement is further different to that of Figs. 1 and 2 in that the controlling ECU 40, as well as receiving the modulated current signal via a single wire 42, has the ability to change the supply voltage going to the sensor interface circuit 44.

This is done for the purpose of communicating information/instructions to the interface circuit 44.

In this example, the information is used to select between the two sensor signals as to which is to provide signals for the ECU 40.

The control ECU 40 has two voltage supply sources

Ref^ _^ and Ref ₂ , either one of which can be selected by a switch 46 controlled by a controlling microcomputer

48 to supply its voltage to the single wire 42. The latter raw supply voltage is regulated in a regulator

50 within the interface circuit 44 to provide a stable supply voltage for the demand transducers S ₁ and S ₂ and the other internal electronics. A sawtooth generator 52 provides a fixed frequency waveform that, when fed into the comparators A, and A ₂ can produce two pulse width modulated signals dependent on each of the demand potentiometer positions, respectively.

Either of these signals can be selected by a switch

SW ₂ dependent on the supply voltage being supplied by the controlling ECU 40. The latter voltage is

monitored by a comparator A ₃ and when the required change is detected, switch S ₂ is set in the appropriate position to enable the correct sensor selection. As with the embodiment shown in Fig. 2, the brake switch can again transpose the output signal via the exclusive - OR gate G-^ . Again this modulated output signal is used to change the current being returned to the main ECU, and it is this change in current that is used to transfer the brake switch and demand potentiometer signals. The controlling ECU will select either Ref _χ or Ref ₂ to request information from either sensor.

By increasing the complexity of the interface circuit it is possible to have more than two input signals. The transmission of information would then be changed from a pulse width to a pulse code. While this transmission is still via current modulation, coded information is sent as a larger quantity of data is needed. An example of such a system is shown in

Fig. 5. To provide for this coding, the interface circuit 70 contains a micro-controller 72. This micro-controller 72 receives the raw input signals from a plurality of sensor devices (S1...S4) and converts their values into coded current pulses to be transmitted over the single supply wire 74 to the controlling ECU 76.

The systems described above in accordance with the present invention have the advantages of reducing the number of connections needed between the sensor(s) and the controlling ECU, improving reliability, and also maintaining a quiescent current through the remaining connections. Furthermore, since the signal (s) are transmitted in digital form, the noise immunity of the signal (s) is increased compared to known arrangements using analog information signals.

In the systems described, using the presently described system to transfer brake demand information from an electronic pedal to the controlling ECU, only a single connection is required. In addition to the demand signal, a digital indication of the pedal position, (the brake pedal switch) , is also transmitted. This switch, as well as controlling the brake lights, provides additional information about the state of the brake demand. This additional information has always been supplied using a separate wire from this switch to the controlling ECU, but using the present system the brake switch state can also be transferred down the same demand connection.

This inclusion further reduces the wire and connector count .