EP0282559A1 - Remote accessible memory devices - Google Patents

Abstract

Access to a remotely accessible data storage device such as an integrated circuit memory (memory device) is effected by means of an arrangement allowing transmission and reception on a single channel as well as coupling to the electric field, in which the digital data and clock signals intended for said memory device are modulated with high frequency carrier oscillations of opposite polarity and multiplexed for transmission by said electric field coupling arrangement on a single channel, then demodulated and demultiplexed to obtain separate data and clock signals for supply to said memory device. The power required for the demodulation, demultiplexing and memory device circuits can be derived from clock or data signal rectification so that these circuits can be normally passive, very small, and located on a wide variety of products. , independently of the transmitter circuits, in order to allow, for example, a regenerable retrospective analysis of said products. The invention applies very particularly to the recording on the drills themselves of the data relating to their duration of use.

Description

I-SMOTE ACCESSIBLE MEMORY DEVICES This invention relates to the storage of data in integrated circuit memory devices and in particular to electronic circuits which facilitate the remote storage and reading of data concerning characteristics or parameters of objects or products to which the circuits are attached. The invention has particular application to the storage of data relating to machine tools but it is to be understood that the invention has wide application not limited the storage of data relating to any one type of object or product.

In semi-automated factories where products are manufactured using machine tools it is conventional practice to regrind or replace a machine tool bit after a specified number of hours use or after a certain number of operations have been performed with it, both of which provide a rough indication of when the bit will be sufficiently worn to justify regrinding or replacement. However it is not always practical to keep an accurate record of the hours of use or the number of times a particular bit has been used and consequently it is common practice to regrind or replace the bits after every period of use on a machine irrespective of the actual wear of the bit. This is obviously very expensive.

According to one aspect of this invention there is provided a remotely accessible data storage circuit (memory device) including electric field coupling means for coupling multiplexed data and timing signals from an appropriate independent transmitter circuit, and, demultiplexing means for demultiplexing said signals and for supplying them to a data storage device. Preferably said electric field coupling means is an electromagnetic coupling means, but an electrostatic coupling means could be devised and used. Preferably said data and said timing signals are digital and are each arranged to control a rate of change of electromagnetic field strength in a separate one of two substantially opposing electromagnetic field directions. Power for the memory device and the demultiplexing means may be derived from either of said data or said timing signals by conventional rectification means associated with said demultiplexing means. Preferably the circuit further includes non-inductively coupled means for transmitting data stored in the data storage means to a remote receiver circuit in response to a suitable cc_ι_and signal included in the coupled data signals so that the memory device may act as both a transmitter and receiver of data and clock signals (transreceiver).

The memory device may be used in a variety of applications and in particular it may be attached to and monitor the use of a machine tool bit. The independent transmitter and remote receiver circuit may be located on the machine tool so that the memory device automatically updates itself each time the bit is used. The memory device is fixed in any convenient position on the machine tool bit so that it can be read and updated with new information by an automatic read/write device incorporating the transmitter/receiver circuits whenever the bit is placed in a machine tool.

The coupling means enables the memory device to be supplied by an external power supply and for data and clocking channels to be provided externally without the need for cocrplex or high-accuracy electro-mechanical connections which require careful alignment and are subject to mechanical wear. Furthermore the novel modulation technique employed (described more fully below) whereby the data and clocking channels are derived from a transmitted signal, enables the power supply, clocking and data signals to all be received and extracted from one multiplexed signal without the need for sophisticated demodulation techniques requiring the use of complicated and expensive detection circuits. Because the power and data are supplied externally the memory device may be made small enough to be left attached to machine tool components or to other items being manufactured to provide, for example, a permanent manufacturing audit trail. The electric field coupling ensures that the memory device can operate reliably in hostile environments because there are no contacts to corrode, wear, suffer mechanical damage or provide a path for static or generate other electrical problems to damage the circuit.

According to the invention in another aspect thereof there is provided a modulator operating according to a modulation technique enabling data and clock signals to be transmitted to a remote receiver over a single transmission channel comprising the steps of:-

Converting a binary data signal, comprising a stream of '1* bits and O¹ bits representing data in serial form, to a data signal comprising high frequency oscillations of a frequency high with respect tothe bit rate of said binary data signal and of a first polarity corresponding to '1' bits in the binary data signal and a zero voltage level corresponding to '0* bits in the binary data signal. Converting a binary clock pulse signal, comprising a stream of alternate 'I¹ bits and '0* bits, to a clock signal comprising high frequency oscillations of a frequency high with respect to the bit rate of said binary clock signals and of a second polarity, opposite to said first polarity, corresponding to *1' bits and in the clock pulse signal and a zero voltage level corresponding to '0' bits inthe clock pulse signal.

Multiplexing said data and said clock signals such that the multiplexed signal for transmission has four possible states, ie, zero voltage, when both data and clock signals are simultaneously '0'; a high frequency oscillation of either the first polarity or the second polarity, when the data signal is 1 and the clock signal is simultaneously '0' or vice versa respectively; and a high frequency oscillation of alternately the first polarity and the second polarity, when both data and clock signals are simultaneously ^rl*.

The high frequency oscillations for the transmission of '1* data and -1* clock signals must be out of phase and may conveniently be derived from out of phase outputs of a single source of high frequency oscillations. However it is also possible to use high frequency oscillations of different frequencies to represent l' signals in the data and clock signals respectively and moreover the binary data and clock singals do not have to be synchronised in any way.

A demodulator operating according to a demodulation technique for demodulating and demultiplexing received clock and data signals modulated and transmitted as described above cαrprises the steps of:- Applying the received signals across a primary coil of a transformer; extracting signals at the output of a first end of a secondary coil of the transformer with respect to zero voltage to form a data channel signal extracting signals at the output of a second end of the secondary coil of the transformer with respect to zero voltage to form a clock channel signal, isolated from said data channel signal; and applying said data channel and said clock channel signals to separate ones of two Schmitt trigger or similar circuits.

The signals at the outputs of the Schmitt trigger circuits may be applied to data and clock lines rspectively of the memory device.

The first and second ends of the secondary coil of the transformer may also be applied to a full wave rectifier circuit in order to derive a direct current voltage supply for said memory device. In this way the memory device will only be pσwered-up whenever data or clock signals are transmitted to it via the electric field coupling means.

A specific embodiment of the invention will now be described by way of example only and with reference to the following figures of which:-

Figure 1 is a schematic diagram showing one part of a transceiver circuit for connection to a memory device which demodulates a single component of a multiplexed signal, in this instance it is the data component;

Figure 2 is a schematic diagram of a part of a transceiver circuit including the part shown in Figure 1 and shows circuits which detects both data and clocking components from the multiplexed signal; Figure 3 is a schematic circuit diagram of a complete transceiver circuit of which the circuit of 2 forms a part, and

Figures 4A to 4G are schematic waveform diagrams illustrating the manner in which clock, data and power signals are received and demodulated by the transceiver circuit of Figure 3.

Figure 5 is a schematic circuit diagram of a transmitter circuit for use with the circuit of Figure 3 and.

Figure 6 is a perspective schematic view of an application of the invention to machine tools.

The construction and operation of a data signal detector part of the transreceiver will now be described with reference to Figure 1. A receiver coil 1 acts as the secondary winding of a transformer and is inductively linked to a coil 50 in a separate transmitter unit (see Figure 5) acting as a primary coil. Power, clocking and data signals are transmitted electro-magnetically from the transmitter to be received by the coil 1 of the receiver.

When an electro-magnetic field generated by currents flowing in the transmitter coil 50 changes in strength or polarity a corresponding current is induced in a coil of a receiver circuit acting as a secondary coil provided the transmitter and receiver coils are substantially adjacent each other. The receiver circuit includes a detector circuit which produces output pulses in one of two channels depending upon the direction in which the current flows. The detector circuit comprises the secondary coil 1 one end A of which is connected to a resistance/capacitance (R/C) network via a diode 2 and the other end of which is connected to ground potential. The R/C network comprises a resistor 3 and capacitor 4. When the electromagnetic field is changing and is of a first polarity it induces a potential across the coil 1 in which the polarity of the point A is such as to forward bias diode 2, the point C is at or below the switching threshold of a Schmitt trigger circuit 5. The detector circuit data channel at the output of Schmitt trigger circuits then produces an output data signal of one polarity. When the electromagnetic field is changing but is of opposite polarity polarities of points A and B reverse the diode 2 is reverse biased, point C eventually falls below the switching threshold of the Schmitt trigger circuit 5 and the detector circuit data channel then produces an output data signal zero voltage. Thus, when the polarity of the coil at point A is at +5v and point B is at ground the diode 2 is reverse biased and does not conduct, the voltage across the R C network remains constant, and Schmitt trigger 5 does not produce an output or a 'O¹ data signal may be said to occur. When the magnetic field changes direction and polarity and the polarity of point A becomes ground and the polarity of point B becomes +5v, diode 2 conducts, and the potential at C point in the R/C network falls so that the Schmitt trigger circuit 5 produces a 'I¹ data signal. Consequently, only changes in magnetic field which cause point B to be +5 volt and point A to go to ground will produce a '1^! data signal output. The Schmitt trigger 5 thus produces an output signal comprising a sequence of 'l's and 'O's corresponding to the 'l's and •O's in the transmitted data which is sent to the memory chip as will be described below with reference to Figure 3.

The R/C network 3, 4 increases the duration of the drop in voltage at point C so that it is sustained, so long as point B on coil 1 is positive with respect to point A on coil 1 and smooths the high frequency oscillations at the data carrier frequency in that positive voltage. The time constant of the E/C circuit be increased by a value which is dependent upon the values of the Resistor 3 and Capacitor 4.

Referring to fFgure 2, which shows both the receiver's clock signal detector and data signal detector, it will be seen that, the receiver's clock signal detector is similar to the data signal detector and comprises a diode 6 connected to the coil 1 at the other end 'B*. Diode 6 is connected to R/C network which comprises a resistor 7 and capacitor 8 and Schmitt detector 9 which produces a *1* clock signal each time the diode 6 goes into conduction. The clock detector is eventually the same as that of the data signal detector except that it produces a *1' clock signal output when the potential at point B is at ground and the potential at A is at +5v , i.e. when the electromagnetic field is changing but is of opposite polarity to that which produces a '1' data signal output. Two diodes 10 and 11 connect the cathodes of diodes 2 and 6 respectively to provide a current return for both to ground.

Figure 3 is a diagram of the complete receiver circuit. The data and clock signals detectors discussed above with reference to Figures 1 and 2 are shown again. Ωie outputs from Schmitt triggers 5 and 9 are fed to the data and clock inputs respectively of an integrated circuit (i.e.) memory 12. Ωie power supply for the i.e. memory 12 and the detector circuits is derived from either clock signals or data signals and is provided by a full wave bridge rectifier circuit which comprises the two diodes 10 and 11 mentioned above together with diodes, 13 and 14. The bridge rectifier provides two power supply lines one at +5v and the other at ground. Voltage stabilisation is achieved by means of a Zener diode 15 placed between the positive supply rail and the ground rail. Two capacitors 16 and 17 are used to smooth the power supply. During periods when both clocking and data outputs C and D are at zero no power supply would be available, but these periods are made sufficiently short during transmission such that the interruption in power supply can be accomodated by smoothing capacitors 16 and 17 on the supply lines.

High frequency modulated power, data and clocking pulses are thus demodulated and demultiplexed and supplied to the i.e. memory 12 by electromagnetic induction as described above. The data supplied may for example, represent the number of revolutions the duriation of operation and the teπperature to whicch a tool bit to which the receiver is attached has been subjected by a tool spindle to which the transmitter is attached. Data may be read out to a remote circuit from the i.e. 12 by means of an electro-optic circuit included in the receiver shown in Figure 3. A light emitting diode (L.E.D.) 18 is used for transmitting the data stored in the i.e. memory 12. The anode of L.E.D. 18 is connected to the data output channel of the i.e. via Schmitt triggers 21, and 23 and via two resistors 19 and 20 in parallel. Whenever the output of the data channel produces negative data pulses (*1* signals) the Schmitt trigger network pulls the potential of the cathode of diode 18 to a sufficiently low level for the diode to conduct and transmit light. The pulses from the i.e. memory 12 are therefore converted to a modulated light beam and can be transmitted from the i.e. memory 12 to a suitable reading device via a photodetector such as 51 in Figure 5.

Diode 24 is provided to prevent the Schmitt gate 23 pulling the data line high and therefore allows either the memory or the transmitter to cause the data line to be low by a "wire or" arrangement with a pull up resistor. Hence the transmitted data can be stored and subsequently read out using the same data line to the i.e. memory 12 by transmitting continuous high level pulses during read operations.

Figures 4A to 4G are drawn to the same time base and further illustrate the operation of the receiver circuit of Figure 3. Figure 4A shows a typical stream of high frequency modulated multiplexed data and clock signals from a ranote transmitter impinging on the coil 1 of the receiver circuit of figure 3 and shows how corresponding voltage changes are produced in the clock and data channels of that circuit.

Each of the arrows in figure 4A represents an electromagnetic field pulse. An arrow which points down from A to B represents a magnetic pulse which induces a current to flow from points A to B through the coil 1. The induced polarity of A is then positive with respect to B which is at ground. Similarly an arrow which points upwardly from B to A represents a change of magnetic field which induces a current to flow in the reverse direction through coil 1 from point B to A. The voltage induced in the coil 1 is then of opposite polarity and point B will be positive and A will be at ground.

The clock and data channels of the receiver circuit are arranged to detect currents which flow in opposite directions through coil 1, - li ¬

the diodes 2 and 6 being oriented to conduct in opposite directions. The clock channel only detects induced currents which flow from A to B and the data channel is arranged so that it only responds to induced currents which flow from B to A.

The remote transmitter, shown in Figure 5, comprises a data encoder 52 which converts data for example from a keyboard or from a computer to a binary data stream of 'l's and '0' for example in shown in Figure 4B and applies them to one input of an AND-gate 53. A further input of the AND-gate 53 is connected to a high frequency oscillator 54 such that every '1' signal in the binary data stream allows a pulse of duration equal to the *1* signal of high frequency oscillations, frequency f, to one input of a ultipler circuit 55. Similarly a clock pulse generator 56 supplies a regular stream of binary '1' and *0' signals to be applied to one input of an AND-gate 57. The other input of the AND-gate 57 is connected to a high frequency oscillator 58 such that each *1' signal in the binary clock pulse stream such as shown in Figure 4C allows a pulse of high frequency oscillations, at f₂, to pass to another input of the multiplexer circuit 55. The high frequency oscillations Fl are of opposite polarity to the high frequency oscillations F2. The output of the multiplexer circuit 55 is connected to a transmitter coil 50 which thus transmits a multiplexed signal comprising high frequency positive or negative polarity electromagnetic field pulses which are coupled to coil 1 of the receiver circuit shown in Figure 3. The high frequency oscillations of the data signal and of the clock signal may be of identical frequency such that oscillators 54 and 58 may in fact be a single oscillator and f, equals f₂. In that case, to ensure correct demultiplexing and de_κx3ulation by the receiver the high frequency data oscillations and the high frequency clock oscillation signals must be out of phase for example 180° out of phase so that the resulting signal applied to the coupling coils 50 and 1 will be a stream of interleaved high frequency pulses in which alternate pulses are of opposite polarity as shown in Figure 4A. If the data signal is in a '1' state but the clock signal is in a '0' state the signal applied to the coupling coil 50 will be a stream of high frequency positive polarity pulses only, if the clock signal is in a '1' state but the data signal is in a *0' state the signal applied to the coupling coil 50 will be a steam of high frequency negative polarity pulses only and if both data and clock signals are in a '0* state no pulses will be applied to the coupling coil 50 at all.

Figure 4A represents a typical multiplexed stream of electromagnetic clock and data pulses coupled to the coil 1 by the remote transmitter of Figure 5 and illustrating all the^' possible combinations of data and clock signal states mentioned above. The envelope of the arrows pointing from B to A (hereinafter- "BA pulses") correspond to data *l^l signals and the envelope of the arrows pointing from A to B (Hereinafter "AB pulses") correspond to clock '1' signals. Figure 4D represents the voltage variations at the input of the Schmitt trigger circuit 5 in the data detection channel, corresponding to the data signals in the figure 4A stream. These voltage variations are smoothed by an R/C network, as will be described later so that the Schmitt trigger 5 only responds to BA pulses and does not respond to AB pulses. For the purpose of this explanation Figure 4D has been divided into different regions indicated by the letters C, D, E, F, G, H and I.

Considering the region between points C and D the first pulse in the multiplexed signal is a BA pulse which raises the Schmitt circuit 5 input voltage to a high positive voltage, say +5V. Because the pulse is of a short duration, after its application the input voltage steadily decreases towards ground (Ov) at a rate determined by the R/C network until the next BA pulse restores the Schmitt trigger voltage to +5 volts; this gives rise to the sawtooth wave form of

Figure 4C in the region C to D. The transmission of the high frequency BA pulses corresponds to the transmission of '1' data pulses and the frequency f, (or f- ) is selected to exceed the rate of decay of the input voltage so that during transmission of one or more data pulses the input voltage is maintained at a high positive level and does not fall below the Schmitt threshold. The presence of interspersed clock pulses in the opposite AB direction has no effect in the region C to D because of the smoothing effect of the R/C network.

The R/C networks in both the clock and data channels of the detector circuit prevent the alternating polarity magnetic fields of data and clock channels from interfering with each other and make it possible to multiplex both clock and data pulses. The Schmitt trigger 5 is maintained above its switching threshold by the series of BA electromagnetic pulses across coil 1. Each pulse causes the polarity of B to be positive with respect to A. The positive polarity of point B reverse biases diode 6 and prevents Schmitt trigger 5 from triggering. Each BA pulse is of a short duration and the potential at B falls as the magnetic field around coil 1 collapses. Ideally the potential at point B could be maintained by a series of BA impulses. If the frequency is sufficiently high enough each pulse would restore the Schmitt trigger back to +5 before the field and hence the input voltage decays below the Schmitt threshold; this is shown by the sawtooth in region F to G of figure 4D. However, because the signal is multiplexed the BA data pulses are interspersed with AB clock pulses. The effect of each BA pulse is to rapidly accelerate the decay of the magnetic field around coil 1 and reverse the polarity so that point B goes to ground and point A goes to +5 volts. If the point B was to suddenly go to ground, due to an electromagnetic clock pulse the diode 6 would become forward biased and conduct, the voltage at the input of Schmitt trigger 9 would fall below the threshold and cause the circuit to trigger and produce a data signal output. Effectively a clock pulse would cause the Schmitt trigger 5 to produce a data signal output and similarly a data pulse could trigger a clock signal output.

To prevent the clock and data channels from being triggered by the wrong pulses an R/C network is placed between the coil 1 and Schmitt trigger circuit. The time constant of the R/C network is chosen so that the voltage at the Schmitt circuit 9 is maintained above its threshold until the next BA pulse is incident on the coil 1 and restores the potential at point B to 5 volts. Similarly the R/C network in the clock channel prevents data pulses from putting the voltage in the clock channel below the Schmitt threshold and prevents data pulses from producing clock outputs in the clock channel.

Considering the region between D and F there are no BA pulses in the multiplexed signal to sustain the Schmitt voltage at +5 volts so the voltage falls towards ground at a rate determined by the R/C network. At point E the voltage falls below the Schmitt threshold and triggers the Schmitt circuit 5 in the data detection channel, the Schmitt input voltage remains at ground until the next electromagnetic pulse in the BA direction at point F.

The input voltage rises to and is maintained by the R/C network at +5 volts in the region F to H by the ensuing train of BA pulses. Thus a further sawtooth input voltage wave form results similar to the wave form in the region C to D. In the region G to I there are no data or clock signals and hence no electromagnetic pulses present at all in the multiplexed signal. The Schmitt input voltage thus falls until it is again below the Schmitt threshold at point H. The voltage remains at zero volts until point I when the next BA pulse representing a further data pulse in the transmitted signal restores the voltage to +5 volts and so on. When the input voltage is above the Schmitt threshold the Schmitt detector circuit 5 produces an output signal of one polarity but when the input voltage is below the Schmitt threshold the detector circuit produces an output voltage of the opposite polarity. Figure 4F shows the output signal produced by the circuit 5 in response to data signal content (BA pulses) of the multiplexed signal of Fig.4C. It will be seen that the signal wave form corresponds closely to the envelope of the BA pulses i.e. to the data pulse stream in the remote transmitter unit, ie Figure 4B. Clock signals are extracted from the multiplexed AB pulses by a similar process as for data signals. By comparing figures 4C and 4E it can be seen that AB pulses maintain the Schmitt trigger level above the threshold voltage of the Schmitt trigger 9 and an absence of AB pulses allows the R/C circuit to discharge and the voltage to fall below the Schmitt threshold. The discharge is unaffected by the presence of BA pulses, for example in the region F to G.

The presence of BA pulses maintains the Schmitt trigger circuit 5 at a positive voltage above the Schmitt threshold and prevents the circuit from producing a data output signal. By the same method the presence of AB pulses maintain the Schmitt trigger circuit 9 at a voltage above the Schmitt threshold and prevent the circuit from producing an output clock signal. In both cases an absence of pulses in the appropriate direction will cause the input voltage level to fall below the corresponding Schmitt threshold and cause the circuit of that channel to produce an output signal. A significant feature of this method is that both clock and data signals can be transmitted simultaneously and yet demodulated correctly in the transceiver. This can be seen in the region C to D projected on to Figures 4F and 4G. In effect the 'l's and *0's of both clock and data signals can be overlapped within the multiplexed signal by transmittting interleaved pulses of opposite polarity or by not transmitting any electromagnetic pulses at all respectively.

In one application of the invention shown in Figure 6 the remote memory device 12 is positioned at a suitable point on the shank 60 of a machine tool bit and becomes a permanent feature of the tool bit. A small inductive coil 1 is concentrically wound around a photo diode 18 and placed at a convenient point on the bit 60 to establish a data link with the remote transmitter/receiver such as that shown in Figure 5 used for accessing and updating the remote memory device 12. The remote transmitter/receiver device, with a transmission coil 50 wound round a photodetector 51 is placed inside the machine tool chuck 61 so that when the machine tool bit 61 is placed in the chuck 61 an optical and magnetic link can be established with the bit and the remote manory can be accessed. The inductive coil arrangement 50 inside the chuck is connected to a the transmitter/receiver and transmits power, clock and data signals to the remote memory device 12 on the bit 60. The photo detector 51 inside the chuck 61 receives data transmitted optically from the remote memory. An advantage of using separate magnetic and optical channels is that data being written into the memory device will not be confused with data being read out of the memory and both processes can be carried out simultaneously.

Every time the machine tool bit 60 is placed in a chuck 61 its manory will automatically be read and updated so that it is possible to record eg how many operations it has previously performed and the total number of hours use it has had. If the record shows the bit 60 has reached its recommended limit it can be discarded.

During use, each additional operation and period of use of the bit will be automatically read into the memory device to provide a constantly updated record which remains with the tool bit. Although the embodiment described makes use of inductive coupling with ferrite cores similar to a transformer circuit but with a large and variable coupling gap, other coupling arrangements may be devised without departing from the scope of the invention. For instance, the coupling between the remote transmitter/receiver and the manory device may be by radio wave in which case coils 1 and 50 would take the form of a suitable radio aerials.

Although the receiver circuit has been described as a combination of discrete electronic components and an off-the-shelf integrated circuit memory device it is conceivable that the whole circuit could be made in the form of a dedicated integrated circuit.

Although a particular application to the use audit of machine tool bits has been described above, many other applications of the remotely accessible memory device and the means of cσππunicating therewith according to he invention will suggest themselves to those skilled in the art. The device could for example be made part of a card of small dimensions carried by a person and containingpersonal medical details or bank account information. The card could then communicate with a transmitter/receiver as described above conveniently built into equipment at a Doctors surgery or at a bank, eg, an automatic cash dispenser.

Claims

1 A remotely accessible data storage circuit including electric field coupling means for coupling multiplexed data and timing signals from an appropriate independent transmitter circuit, and, demultiplexing means for demultiplexing said signals and for supplying them to a data storage device.

2 A remotely accessible data storage circuit as claimed in claim 1 and wherein said electric field coupling means comprises an electromagnetic coupling means.

3 A remotely accessible data storage circuit as claimed in claim 1 and wherein the electric field coupling means comprises an electrostatic coupling means.

4 A remotely accessible data storage circuit as claimed in claim 1 and wherein said data and said timing signals are digital and are arranged to control a rate of change of electric field strength in a separate one of two substantially opposing electric field directions.

5 A remotely accessible data storage circuit as claimed in any of the preceeding claims and wherein power for said circuit and for said data storage device is derived from either of said data or said timing signals by conventional rectification means associated with demultiplexing means.

6 A remotely accessible data storage circuit as claimed in any of the preceeding claims and further including non-inductively coupled means for transmitting data stored in said data storage device to a remote receiver circuit in response to a suitable command signal included in said coupled data signals so that said data storage device may act as both a transmitter and receiver of data and clock signals.

7 A remotely accessible data storage circuit as claimed in claim 6 and wherein said non-inductively coupled means comprises light emitting means responsive to data stored in said data storage means to transmit said data in the form of light intensity variations to a remote receiver circuit including photo detector means.

8 A modulation technique enabling data and clock signals to be transmitted to a remote receiver over a single transmission channel comprising the steps of:-

Converting a binary data signal, comprising a stream of '1' bits and '0' bits representing data in serial form, to a data signal comprising high frequency oscillations of a frequency high with respect to the bit rate of said binary data signal and of a first polarity corresponding to *1* bits in the binary data signal and a zero voltage level corresponding to *0* bits in the binary data signal.

Converting a binary clock pulse signal, comprising a stream of alternate '1' bits arid '0' bits, to a clock signal comprising high frequency oscillations of a frequency high with respect to the bit rate of said binary clock signals and of a second polarity, opposite to said first polarity, corresponding to *1* bits in the clock pulse signal and a zero voltage level corresponding to '0' bits in the clock pulse signal. Multiplexing said data and said clock signals such that the multiplexed signal for transmission has four possible states, ie, zero voltage, when both data and clock signals are simultaneously '0'; a high frequency oscillation of either the first polarity or the second polarity, when the data signal is '1' and the clock signal is simultaneously '0' or vice versa respectively; and a high frequency oscillation of alternately the first polarity and the second polarity, when both data and clock signals are simultaneously '1' .

9 A modulator circuit operating according to the modulation technique of claim 8.

10 A demodulation technique for demodulating and demultiplexing received clock and data signals modulated and transmitted as claimed in claim 8 above comprising the steps of :-

Applying the received signals across a primary coil of a transformer; extracting signals at the output of a first end of a secondary coil of the transformer with respect to zero voltage to form a data channel signal, extracting signals at the output of a second end of the secondary coil of the transformer with respect to zero voltage to form a clock channel signal, isolated from said data channel signal; and applying said data channel and said clock channel signals to separate ones of two Schmitt trigger or similar circuits.

11 A demodulator operating according to the demodulation technique of claim 10 above.

12 A demodulator circuit as claimed in claim 11 and wherein the outputs of the Schmitt trigger circuits or similar circuits are connected to data and clock lines respectively of a data storage device. 13 A demodulator circuit as claimed in claims 11 or 12 and wherein the first and second ends of the secondary coil of the transformer are also connected to a full wave rectifier circuit in order to derive a direct current voltage supply for the demodulator circuit and associated circuits.