Abstract:
An identification system is provided for identifying a plurality of object-based transponders ( 14, 16, 18, 20 ) using an interrogator ( 10 ). The interrogator ( 10 ) includes a transmitter for transmitting an interrogation signal to the transponders, a receiver for receiving identification signals from the transponders, and processor means ( 21 ) for determining the individual and correct receipt of an identification signal. The transponders operate on the basis of backscatter modulation, and include a detector for detecting the presence of an interrupt signal from the interrogator. Control logic responsive to the detector is arranged to cease signal transmission from the transponder if the transponder completes transmission of the identification signal without receiving an interrupt signal during such transmission. The interrogator ( 10 ) is arranged to receive the identification signals from the transponders ( 14, 16, 18, 20 ) and substantially contemporaneously to determine if any identification signal has been individually and correctly received. In the event of any identification signal is being individually and correctly received, the interrogator ( 10 ), at substantially the same time, transmits a common interrupt signal for temporarily suspending signal transmission from a transponder ( 14 ) if the transponder is transmitting its identification signal at the time it receives the interrupt signal. The transponder ( 14 ) independently ceases signal transmission if it completes transmission of an identification signal without receiving an interrupt signal.

Description:
BACKGROUND OF THE INVENTION 
     THIS invention relates to a method and apparatus for the identification of objects, and in particular for the identification of a plurality of remote electronic identification tags by a reader using electromagnetic communication means. 
     Numerous different types of electronic tags, which are typically in the form of transponders, are attached to physical objects such as goods, equipment, people, animals and the like. These tags are programmed to contain identity data, which is used to electronically identify the tagged objects via interrogation by a reader. The read data is typically arranged to be transferred to a computer system for tallying and reading the identity of those objects which are in the interrogation area of the reader. 
     A large number of patents already cover the situation of a reader and a plurality of transponders with the communication between the reader and the transponders being based on acoustic or electromagnetic radiation principles. In most cases, the prior art protocol requires the transponders to have a unique identification number and the ability to receive and decode a transmission containing a data stream from the reader and match it to all or components of its unique identification number. 
     Another class of prior art is an identification system that needs neither a unique identity nor a multi-bit receiver and decoder for the transponder, but which relies on the reader communicating with the transponder at the correct time after it has finished its transmission, if the transponder was successfully identified. The communication can be single bit in format, and the message is communicated by the timing of the communication after the completion of transmission, the communication being as simple as disturbing the energising field from the reader at the correct instant. Such a system is described in South African patent 92/0039, which requires the reader and the transponder whose identity is being determined to be synchronised and to remain synchronised after communication has been completed so that the reader may respond at the correct time after the transmission has been completed. 
     A further category of prior art covers an identification system that uses a selection process to isolate a single transponder so that the transponder&#39;s data can be received without being corrupted by transmissions from other transponders. U.S. Pat. No. 5,751,570 describes such a system. A collision signal is initially sent by the reader to place all transponders in to a dead state. Each transponder calculates a random dead state after receiving the collision signal, during which they do not transmit their data. At the end of the dead state each transponder will transmit its data again. If the reader receives data from only one transponder it sends an occupied or busy signal which place, all the transponders except the one busy transmitting into an idle state. The transponders in the idle state no longer transmit data. After the reader has received the complete data from the individual transponders the reader transmits an acknowledge signal which places the identified transponder into a passive or idle state in which it ceases transmission of its data. The rest of the transponders in idle state are reactivated and they recalculate a random dead time. The steps above are repeated until all the transponders have been identified. The drawback of this system is that if the electromagnetic field is changed to send the busy signal, the changes in the electromagnetic field may cause the receiver to misread the data from the transponder because the transponder signal is very much smaller than the changes in the electromagnetic field 
     A similar system is described in U.S. Pat. No. 5,124,699, where changes in frequency are used to send signals to the transponders, Initially a frequency shift is sent to the transponders to start a selection procedure that will isolate an individual transponder. The transponders calculate a random delay after which they transmit a starting block. Due to the random delay, only a few transponders might transmit a starting block at the same time. When the reader receives a starting block it changes the transmitter frequency which causes the transponders not transmitting a starting block to go into a passive state. The remaining transponders transmit their unique code. If the reader detects that more than one transponder is transmitting its code then the reader signals an error by changing the transmitter frequency which causes these transponders to recalculate a random delay again and to follow the steps above. The transponders in the passive state remain so. This selection process will eventually yield an individual transponder. Once the reader has identified the individual transponder it is placed into a passive state by another frequency change which also causes the remaining unidentified transponders to restart the selection process again. This system requires complex tuned circuitry in the transponder so that the different frequencies can be detected 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a method of identifying objects by an interrogator, comprising the steps of: 
     transmitting an interrogation signal from the interrogator to the objects; 
     transmitting from each object to the interrogator an identification signal having predetermined indicator characteristics in response to the interrogation signal; 
     receiving the identification signals from the objects at the interrogator and substantially contemporaneously determining at the reader if any identification signal has been individually and correctly received on the basis of the indicator characteristics; 
     substantially contemporaneously transmitting from the interrogator a common re-transmit or interrupt signal in the event of any identification signal not being individually and correctly received; and 
     ceasing signal transmission from an object if that object completes its signal transmission without receiving an interrupt signal front the interrogator during such transmission. 
     Preferably, the method includes the step of temporarily suspending signal transmission from an object if that object is transmitting its identification signal at the time it receives the interrupt signal from the interrogator. 
     In a preferred form of the invention, the method comprises the further steps of continuing so to receive all identification signals at the interrogator, and to transmit the interrupt signals, until no further identification signal is individually and correctly received for a time period sufficient to ensure that all identification signals from objects have been individually and correctly received by the interrogator. 
     In one form of the invention, the method may comprise the steps of continuously transmitting the interrogation signal and transmitting the interrupt signals at a time interval which defaults to less than the average transmission time of a valid identification signal in the absence of the receipt of an identification signal at the interrogator. 
     Conveniently, the method includes the step of transmitting at random time intervals an identification signal from each object which has not ceased its signal transmission and allowing each object which has ceased its transmission to recommence transmission in response to a reset event. 
     The reset event may include the absence of or variation in the interrogation signal for a predetermined minimum time period. 
     Advantageously, the method further includes the steps of transmitting a disable signal from the interrogator, receiving the disable signal at at least one of the objects, and setting a memory element in the object preventing it, only after the object has ceased signal transmission, from responding to any subsequent interrogation signal for a minimum predetermined stand-off time period. 
     The method may include the still further steps of transmitting an enable signal from the interrogator, receiving the enable signal at at least one of the objects, and resetting the memory element in the object to enable the object and allow it to respond to the same or subsequent interrogation signal in the manner described above. 
     Advantageously, the enable and disable signals are at least initially transmitted prior to the possible transmission from any object of an identification signal, and the enable and disable signals also serve as interrupt signals 
     Typically, the predetermined indicator characteristics of the identification signals are identical in form and have a predetermined duty cycle, and include a fixed length data stream preceded by an initial header and including a data component and a checksum component, with the transmission of a signal from an object commencing with the same header, and the interrogator being arranged to accept the start of an identification signal only if there has been no received transmission immediately prior to the receipt of such header. 
     The indicator characteristics may further include the transmission of the identification signal in a Manchester form modified to combine transmission clock rate and the data stream to produce a 50% duty cycle at a minimum operational bandwidth. 
     The invention extends to an identification system comprising an interrogator and a plurality of object-based responders, the interrogator including transmitter means for transmitting an interrogation signal to the responders, receiver means for receiving identification signals from the responders, and processor means for determining the individual and correct receipt of an identification signal, each responder comprising a receiver for receiving the interrogation signal, generator means for generating the identification signal, a transmitter for transmitting the identification signal back to the interrogator, a detector for detecting the presence of an interrupt signal from the interrogator, and control means responsive to the detector and being arranged to cease signal transmission from the responder if the responder completes transmission of the identification signal without receiving an interrupt signal during such. transmission. 
     Preferably, the control means is arranged temporarily to suspend transmission of an identification signal from the responder if the responder is transmitting its identification signal at the time it receives the interrupt signal. 
     Advantageously, the generator means comprises a first memory means for storing identification data, an oscillator, a Manchester encoder for deriving encoded identification data from the identification data and the oscillator, and a modulator driven by the Manchester encoder to derive an identification signal, and the transmitter and receiver comprise an antenna coupled to an RF module for performing backscatter modulation of the identification signal. 
     Typically, the interrogator is arranged to continue receiving identification signals, and transmitting the interrupt signal, until no further identification signal is individually and correctly received for a time period sufficient to ensure that all identification signals have individually and correctly been received by the interrogator. 
     The identification system may include processor means for generating the interrupt signals and for generating enable and disable signals for respectively enabling and disabling the responders, and each of the responders include second memory means arranged to be set by a disable signal for preventing the responder from responding to any subsequent interrogation signal from the interrogator for a minimum predetermined time period, only after it has ceased signal transmission, and to be reset by all enable signal for allowing the responder to respond immediately to an interrogation signal. 
     According to a further aspect of the invention there is provided a responder for an identification system of the type comprising an interrogator and a plurality of responders, each responder comprising a receiver for receiving an interrogation signal, first memory means for storing identification data, an oscillator a modulator for deriving a modulated identification signal from the identification data and the oscillator, and a transmitter for transmitting the identification signal back to the interrogator, the responder further comprising a detector for detecting the presence of an interrupt signal from the interrogator, and signal ceasing means arranged to cease signal transmission from the responder if the responder completes transmission of the identification signal without receiving an interrupt signal from the interrogator during such transmission. 
     The responder advantageously includes signal suspending means responsive to the detector and being arranged temporarily to suspend transmission of an identification signal from the responder if the responder is transmitting its identification signal at the time it receives the interrupt signal. 
     Typically, the signal suspending means and the signal ceasing means are incorporated in control logic circuitry, the responder further including a random timer connected to the control logic circuitry for enabling identification signals to be repeatedly re-transmitted at varying random time intervals from the responder until such time as it has completed the transmission of an identification signal without being interrupted by an interrupt signal. 
     Conveniently, the responder includes second memory means arranged to be set by a disable signal for preventing the responder, only after the responder has ceased signal transmission, from responding to any subsequent interrogation signal for a minimum predetermined time period. 
     The second memory means may be arranged to be reset by an enable signal for allowing the responder to respond to a subsequent interrogation signal after the responder has ceased signal transmission. 
     Typically, the second memory means comprises a register responsive to disable signals and a memory module responsive both to the register and to the signal ceasing means for allowing the responder to complete the transmission of an identification signal prior to being disabled. 
     Advantageously, the responder further includes a Manchester encoder for deriving encoded identification data from the identification data and the oscillator, for receipt at the modulator, and the transmitter and receiver comprise an antenna coupled to an RF module for performing backscatter modulation in response to an interrogation signal, the Manchester encoder being responsive to an output enable signal signifying completion of the transmission of the identification signal. 
     According to a still further aspect of the invention there is provided an interrogator for an identification system of the type comprising an interrogator and a plurality of responders, the interrogator including transmitter means for transmitting an interrogation signal to the responders, receiver means for receiving identification signals from the responders in response to the interrogation signal, first processor means for substantially contemporaneously determining the individual and correct receipt of an identification signal, signal generator means responsive to the processor means for generating substantially contemporaneously an interrupt signal in the event of any identification signal not being individually and correctly received from any one or more objects, and for causing the interrupt signal to be transmitted sufficiently quickly to suspend signal transmission from any such object whilst it was not individually and correctly transmitting its identification signal. 
     Preferably, the first processor means is arranged to continue receiving and processing all identification signals, and the signal generator means is arranged to continue generating interrupt signals, until no further identification signal is individually and correctly received, for a time period sufficient to ensure that all identification signals from objects have been individually and correctly received. 
     Typically, the transmitter is arranged to transmit the interrogation signal continuously, and the signal generator means is arranged to transmit the interrupt signal at a time period which defaults to less than the average receipt time of a valid identification signal in the absence of the receipt of an identification signal at the interrogator. 
     Conveniently, the signal generator means is arranged to generate enable and disable signals for transmission via the transmitter, the disable signal being arranged to set memory elements in the objects preventing them, only once they have ceased signal transmission, from responding to any subsequent interrogation signal for a minimum predetermined stand-off time period, and the enable signal being arranged to reset the memory elements to allow the objects to respond to a subsequent interrogation signal. 
     The receiver may include a receiving antenna and a quadrature receiver, and the first processor means may include a signal processor for processing baseband components of the identification signal derived from the quadrature receiver into a reconstructed Manchester data signal, and an error checking microprocessor for decoding and error checking the Manchester signal at least on the basis of duty cycle, clock rate, data stream length and checksum calculations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a reader and four transponders of the invention in the electromagnetic field of the reader; 
     FIG. 2 shows a schematic block diagram of the reader of the invention; 
     FIG. 3 shows a schematic block diagram of a transponder of the invention; 
     FIG. 4 shows a series of signal waveforms illustrating the communication protocols between the reader and transponders; 
     FIG. 5 shows a schematic diagram of a typical successfully transmitted identification signal, and 
     FIG. 6 shows a flowchart illustrative of the operation of the control logic of the transponder 
    
    
     DESCRIPTION OF EMBODIMENTS 
     In FIG. 1 a reader or interrogator  10  is shown which transmits an electromagnetic energising field depicted by an oval footprint  12 . The oval footprint  12  represents the effective reading range of the reader, which is typically between 4 to 6 meters. Four transponders  14 ,  16 ,  18  and  20  are within the field. Any transponder within the footprint  12  will derive sufficient energy from the energising field so as to power its circuitry. In this situation where the transponders are using energy from an energising field to power up their electronic circuitry, and due to the fact that the field needed for energising is typically much stronger than the field strength needed for communication using the backscatter modulation method, it can be accepted that all transponders that are being energised by a reader will have quality communications with the reader and that their reflected backscatter signal will be received by the reader. When leaving the zone, the transponders lose energy and stop operating before their communication signals degrade to such a level that the reader is no longer able to receive all the backscatter signals correctly. 
     Initially the energising field is off. The mode of operation is set by a computer  21  connected to the reader. The modes of operation are “normal read” with no electronic article surveillance (EAS) after the transponders are successfully identified, “set EAS on” to disable each transponder once it has been successfully identified, and “clear EAS” to enable each transponder before reading. Electronic article surveillance is commonly used as an anti-shoplifting and anti-“shrinkage” measure. The energising field is turned on, the transponders are identified and then it is turned off again. 
     FIG. 2 shows a schematic block diagram of reader  10  An oscillator  22  provides a carrier wave signal at a typical operating frequency of 915 MHz. A modulator  24  is controlled by a microprocessor  26 . This allows the microprocessor  26  to send signals to the transponders by modulating the carrier wave signal. The signals are sent by the microprocessor  26  switching the modulator  24  to 100% modulation depth for a short period (100 μsec) which has the effect of sending a negative going pulse to the transponders. Another method might be to transmit an independent signal on another frequency, to increase the power level of the energising field, or to transmit the energising field briefly with another polarisation. 
     The different types of interrupt signals sent to the transponders are a “normal interrupt” signal consisting of a single pulse, a “set BAS interrupt” signal consisting of two pulses and an “FAS clear” signal consisting of five pulses, and which must be sent within 16 ms after the energising field has been turned 
     The interrupt signals are communicated by some means to all of the transponders, regardless of whether or not they are transmitting at that instant These signals are interpreted as the failed transmission signal for those transponders that are currently in a transmission state. The interrupt signal is transmitted at the instant that the reader detects a corrupted transmission state, and it is not necessarily in synchronism with any data clock in any transponder. 
     A power amplifier  28  boosts the modulated carrier wave signal to sufficient levels to give the effective reading range  12 . A transmit antenna  30  radiates the carrier wave signal to give the energising field, and the transponders  14 ,  16 ,  18  and  20  derive power from the energising field and transmit their codes by means of backscatter modulation. 
     A receive antenna  32  receives the reflected backscatter modulation signals from the transponders. The received backscatter modulation signals are mixed with the local oscillator signal  34  in a quadrature receiver  36 . A local oscillator signal  34  is derived from the oscillator  22 . The output of the quadrature receiver  36  is a baseband I signal  38  and a baseband Q signal  40  which represents the transponder data. The I signal  38  and Q signal  40  are fed into a signal processing module  42 . The signal processing module  42  amplifies the signals, and combines them together to give a reconstructed Manchester data signal  44  which represents the Manchester data sent by the transponders. The combining of the I signal  38  and the Q signal  40  allow the transponder code to be detected irrespective of the path length between the reader  10  and the transponders  14  to  20 . 
     The reconstructed Manchester data signal  44  is input into the microprocessor  26  for decoding and error checking. The transponder data is encoded in a modified Manchester form whereby the transponder&#39;s clock rate, typically 10 kHz, and the serial data are combined to give a data stream having a 50% duty cycle at minimum operational bandwidth. From this data stream it is possible for the microprocessor  26  to extract the data and the clock rate of the transponder. 
     The microprocessor  26  performs error checking to determine first, if there is more than one transponder transmitting at the same time and second, if only one transponder is transmitting, if there are errors in the received transponder data. In both cases the transponders or transponder respectively are sent an interrupt signal if an error is encountered by the microprocessor  26  so that the transponders can retransmit their data again. The clock rate of the transponders is typically 10 kHz with a tolerance of 20% i.e. the clock rate of a transponder can vary from 8 kHz to 12 kHz. The microprocessor  26  checks that the clock rate of the transponder is between these two predetermined limits, failing which an interrupt signal is transmitted. The Manchester encoded data stream, which is shown at  45  in FIG. 5, is a fixed length data stream, which in this embodiment it is 81 bits. Any lengthening of the code means that more than one transponder is communicating at the same time and that the reader is detecting a corrupted signal. 
     The microprocessor  26  checks the length of the data stream and if more than 8) bits are received, then an interrupt signal is transmitted. All transponder transmissions will start with the same 1 bit header  45 A at the front of the Manchester data. The microprocessor  26  requires that no transmissions are received immediately before the header transmission to clearly define the start of transmission, which is signalled by 3 clock periods  45 B of 10 kHz i.e. 300 μsec. if there are transmissions with in this period then an interrupt signal is transmitted. After the header  45 A, the transponder data is in the form of a 72 bit string  45 C followed by a checksum or parity component  45 D allowing the microprocessor  26  to verify the correctness of the data received by verifying that the parity or checksum calculations provide the correct answer and that the data has not been contaminated by a simultaneous transmission by a second transponder having the same start time and the same transponder clock rate. If the checksum or parity calculations provide an incorrect answer then an interrupt signal is transmitted. During transmission the clock rate of any transponder will be stable and the microprocessor will be able to monitor this stability as transferred in the data stream, and determine as soon as there is any sudden change in the clock rate in the data stream such as might occur when another transponder starts its simultaneous transmission. The microprocessor monitors the clock rate of the data stream and any sudden change in the received clock rate will cause an interrupt signal to be transmitted. If the data stream passes all the above tests then the microprocessor  26  has decoded valid transponder data from a single transponder. This data is sent to the computer  21  via a communication link  46  to he processed further. In this embodiment, the communication link is a RS232 serial link. This could also be a parallel link or a network link. 
     The incorporation of a unique identity number in the transponder data is not one of the required properties to successfully receive data from the transponders. The system will operate when many transponders have identical numbers, in which case it will be able to count how many instances of such numbers were encountered. 
     FIG. 3 shows a schematic block diagram of a transponder  48 . The transponder data or code is stored in a data memory module  50 . in this embodiment the transponder data consists of 80 bits and a leading 1 bit header. The data memory is preferably PROM memory, but EPROM or EEPROM memory may also be used. It is preferably programmed when the transponder is applied to the object it is tagging. The programming is performed by a programmer which has two direct contact pins on an antenna  52  to provide power and a third direct contact pin onto a programming pad  54  for the transponder data. 
     The antenna  52  collects energy from the energising field  12 . The antenna is a wire dipole antenna. A foil patch antenna may also be used. A radio frequency (RF) module  56  rectifies the collected energy and charges an energy store in the form of a capacitor  57  to provide the operating voltage for the transponder circuitry. A small battery can also be used to power the transponder. The closer the transponder is to the reader, the stronger the energising field and the greater the collected energy. The RF module  56  also has overvoltage protection in the form of a zener diode-type component  57 A to limit the operating voltage when the transponder is close to the reader. When the energising field is turned on, the transponder circuitry powers up. An oscillator  58  provides an imprecise but stable clock signal  60  for the transponder circuitry in the range of 8 kHz to 12 kHz, typically 10 kHz. 
     A control logic unit  62  divides the clock by eighty one to provide a frame clock signal  64 . A frame is defined as the number of clock cycles to transmit the entire transponder data of 81 bits, so the frame clock occurs every  81  clock cycles or 8.1 ms for a 10 kHz oscillator. The frame clock signal  64  is used to clock a random delay timer  66  The random delay timer consists of a pseudorandom number generator  68  and a counter  70 . On power up of the transponder, the counter  70  is always loaded with the number two so that the transponder will always transmit its data in the third frame after power up so that if it is the only transponder present it can quickly be identified. The counter  70  counts down and when it reaches 0 it generates a trigger signal  72 . The trigger signal  72  causes the pseudo-random generator  68  to calculate a new random number which is then loaded into the counter  70  to time the next random delay period 
     The trigger signal  72  also causes the transponder to start transmission of its data which occurs in the following manner. The control logic unit  62  sends a shin clock signal  74  to the data memory module  50  which serially shifts data  76 , starting with the start bit, out of the data memory module  50 . The data  76  is exclusive or&#39;ed with the clock  60  in a Manchester encoder  78 . The control logic also enables the output of the Manchester encoder  78  by means of an output enable signal  80  The output of the Manchester encoder  78  drives a modulator  82 . The modulator  82  varies the loading on the antenna and so modulates the backscatter from the antenna with Manchester encoded transponder data. On the next frame clock signal  64  the transmission of the data is terminated and the output of the Manchester encoder  78  is disabled by means of the output enable signal  80 . 
     When the reader  10  sends a signal to the transponder it momentarily removes the RF energy i.e. it pulses the RF energy off and then back on again. A pulse detector  84  continuously monitors the incoming RF energy on the antenna  52  for pulses from the reader  10 . The number of pulses are counted. If a single pulse is received then a “normal interrupt” signal  86  is pulsed. If a double pulse is received then the “set EAS interrupt” signal  88  is pulsed into a memory device in the form of a set EAS register  89 . If five pulses are received then an “EAS clear” signal  90  is pulsed to clear the register  89 . The “EAS clear” signal is only monitored during the first two frames after power on, after which it is ignored. 
     When any pulse is received from the reader i.e. if either of the interrupt signals  86  or  88  is pulsed whilst the transponder is busy transmitting its data, then the output of the Manchester encoder  78  is immediately disabled by means of the output enable signal  80 . This immediately stops the modulation of the antenna and so stops the backscatter modulation. If, however, the transponder is able to transmit its entire data without receiving a pulse or pulses from the reader  10 . then the transponder “knows” it has been successfully identified by the reader  10 . The control logic  62  places the transponder into a passive state in which it ceases any further transmission of data until it has been reset by having the energising field removed. Another method of resetting the transponder might be a tone modulated on the carrier. 
     If “set EAS interrupt” signals  88  are received prior to the transponder successfully transmitting its data, then a set EAS register  89  in the control logic Unit  62  is set so as to remember that the EAS mode must be enabled when the transponder successfully transmits its data. Thus, when the transponder does successfully transmit its data and the set BAS register is set, then an EAS memory module  92  is set and so the transponder is placed in EAS mode. The PAS memory module  92  keeps its state even if power is removed from the transponder If the EAS memory  92  is set then the transponder is disabled from transmitting its data. A set EAS interrupt signal has to be received by the transponder prior to the transponder transmitting its identification signal for the first time so that the set EAS register can be set prior to the first transmission, thereby ensuring that in the event of only one transponder being located in the footprint of the interrogator, the identification signal from this transponder is picked up. More particularly, should the identification signal from a single transponder be successfully transmitted without generating an interrupt signal, the set EAS register would not be activated. As a result a “dummy” interrupt signal is initially sent to ensure that the set EAS register is activated regardless, as is shown by interrupt signal  88 A in FIG.  4 . 
     The BAS memory module  92  is a medium term memory element with a typical time constant of several hours, which operates to block any backscatter modulation transmissions as long as the memory is set. This memory element has leakage and will reset after its charge has leaked away, which is typically from four to eight hours, depending on ambient temperature. This operation functions whether or not the transponder has power from its presence in an energising, field. If an BAS clear signal  90  is received during the first two frames after power up, then the contents of the EAS memory module  92  is cleared whether it is set or not and so the transponder is re-enabled to transmit its data again. 
     FIG. 4 shows signal waveform interaction between the reader  10  and the transponders  14  to  18 . Waveforms  10 A,  10 B and  10 C show the energising field of the reader  10  for three different scenarios. Waveform  10 A is for the scenario when “normal interrupt” signals are used, waveform  10 B is for the scenario when “set EAS interrupt” signals are used and waveform  10 C is for the scenario when an “EAS clear” signal is initially sent to clear the EAS memory modules  92 , followed by “normal interrupt” signals. Waveforms  14 A,  16 A,  18 A and  20 A are the Manchester encoder  78  outputs i.e. the modulation of the antennae  52  of the respective transponders  14 ,  16 ,  18  and  20 . 
     At time t 0  the reader  10  turns on the energising field  12 . The transponders are powered up and perform a reset. At time t 1 , typically 4 ms, the reader sends an initial interrupt signal prior to the transponder&#39;s first transmission. For waveform  10 A a normal interrupt signal  86 A is sent which has no effect on the transponders. For waveform  10 B a “set EAS” signal  88 A is sent which causes the set EAS signal  88  in the transponders to be pulsed and the set EAS register in the control logic  62  to be set. For waveform  10 C an “BAS clear” signal  90 A is sent which causes the EAS clear signal  90  in the transponders to be pulsed and the control logic  62  to clear the EAS memory  92 , thus allowing the transponders to transmit data again. 
     At time t 2 , typically 16 ms, all the transponders have waited two frames after power up and they start transmitting their respective data. The random delay timers  66  in the transponders are seeded with new random delays. The reader  10  receives the backscatter modulation  14 B to  20 B from the transponders and attempts to rebuild the Manchester data. As there is more than one transponder  14  to  20  transmitting, the microprocessor  26  will detect an error in the reconstructed Manchester data and so at time t 3  sends an interrupt signal by modulating the energising field. The transponders&#39; pulse detectors  84  detect the pulse/s on the energising field and pulse the relevant signal. In the case of waveform  10 A and  10 C the normal interrupt signal  86  is pulsed and in the case of waveform  1013 , the “set EAS” signal  88  is pulsed. The control logic  62  immediately disables the output of the Manchester encoder  78  and so data transmission from the transponders ceases. The transponders then wait their respective delay times. 
     At time t 4  the random delay timer  66  of transponder  14  generates the trigger signal  72  which causes transponder  14  to start transmitting its data  94 . The reader  10  receives the backscatter modulation  94  and attempts to rebuild the Manchester data. The microprocessor  26  decodes the Manchester data while checking it for errors. As there is only a single transponder transmitting, the data will contain no errors and so the microprocessor does not send an interrupt signal. Once transponder  14  has finished transmitting its data and no pulses were received by the transponder, the control logic  62  of transponder  14  places transponder  14  into a passive state and it ceases any further transmissions for the duration of the interrogation signal. The microprocessor  26  sends the successfully received transponder data to the computer  21  via the communication link  46 . 
     At time t 5  the random delay timer  66  of transponder  18  generates a trigger signal  72  which causes transponder  18  to stat transmitting its data  96 . The reader  10  receives the backscatter modulation and attempts to rebuild the Manchester data. The microprocessor  26  decodes the Manchester data, whilst checking it for errors. At time t 6  the random delay timer  66  of transponder  20  also generates a trigger signal  72  which causes transponder  20  also to start transmitting its data  98 . The microprocessor  26  detects an error in the reconstructed Manchester data and so at time t 7  sends interrupt signals  86  or  88  by modulating the energising field. The transponders&#39; pulse detectors  84  detect the pulses on the energising field and the relevant interrupt signal  86  or  88  is pulsed. The control logic units  62  of transponders  18  and  20  immediately disable the output of the Manchester encoder  78  of the transponders  18  and  20  and so data transmission from the transponders  18  and  20  ceases. Transponders  18  and  20  then wait their respective delay times determined by their reset random delay timers  66 . 
     At times t 8 , t 9  and t 10 , transponders  16 ,  20  and  18  respectively transmit their respective data  100 ,  102  and  104  without interruption and so are successfully received by the reader  10  and sent to the computer  21 . Transponders  16 ,  18  and  20  are placed into a passive state and cease further transmissions. Once the receiver  36  has not received any further transmissions for a period equal to the longest random delay timer of the transponders (in this case 2 seconds) it indicates to the computer  21  that all transponders are read and shuts down the energising field  12 . 
     Referring now to FIG. 6, a self-explanatory flowchart indicating the operation of a control logic unit  62  is shown. In particular, the flowchart is illustrative of the manner in which the various EAS modes operate. 
     For modelling purposes, for transponders with a 10 kHz clock rate, and a 128 transmission frame time maximum random period, for 400 transponders to be read, more than 30000 clashes (or interrupts) would occur in reading the transponders. 
     Should a reader be in the process of radiating an energising field and for some reason does not want to accept further identities, say while waiting for the computer backlog to clear, the reader can generate interrupt signals at a period less than the message transmission time for the fastest allowable transponder clock and thereby prevent the transponders not already in passive mode from going into passive mode until the blockage has been cleared. 
     The above embodiment is suited to applications where a number of transponders are placed with in the reading range of a reader, the energising field is turned on, the transponders are read and the energising field is turned off again. In situations where a reader is mounted, say, on a door frame for access control or above a conveyor belt, the energising field would be permanently on and the transponders would be moved into the energising field, they would power up, transmit their data and then move out of the energising field again. In these situations the reader would continuously transmit interrupt signals at an interval less than the message transmission time for the fastest allowable transponder clock, typically 5 ms. The reader stops sending interrupt pulses only when it starts to receive uncorrupted transmission data from a particular transponder and continues to suspend transmission of such interrupt pulses for as long the data remains uncorrupted until the transponder successfully finishes its transmission. Thereafter, it continues to send interrupt signals at 5 ms intervals until it starts to receive uncorrupted transmission data from another transponder. In this manner transponders can be moved passed a permanently powered reader, which will be able to read them and still be able to process multiple transponders. Also, since the transponder only transmits after a period of 2 frames (typically 16 ms) then the transponder would receive at least two interrupts of the “sct EAS interrupt” type, if EAS mode is required, prior to its first transmission, thus fulfilling the requirement for the enabling of EAS.