Abstract:
Information is communicated between an RFID tag and first and second readers. A first transceiver of the RFID tag is controlled so that the first transceiver communicates with the first reader and so that the first transceiver has substantially longer periods during which the first transceiver is not in communication with the first reader than when the first transceiver is in communication with the first reader. A second transceiver of the RFID tag is controlled so that the second transceiver communicates with the second reader at least during the periods when the first transceiver is not in communication with the first reader. The RFID tag may also have a battery, a switch coupling the battery to at least the first transceiver, and a controller that operates the switch in a duty cycle such that power is provided by the battery to the first transceiver during ON times of the duty cycle and such that power from the battery to the first transceiver is interrupted during OFF times of the duty cycle.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to a tag that can be suitably attached to an article and that can be used for RF communications with a tag reader.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various labels have been attached to articles so that the articles can be distinguished one from the other. For example, bar code labels are attached to articles of grocery and are scanned at a check-out counter in order to automatically identify the articles and to register the price of the articles as they are purchased.  
           [0003]    Bar code labels have also been used in inventory control and monitoring. Accordingly, these bar codes may be scanned in order to track articles as they move into, through, and out of a storage area. It is also known to read the bar codes attached to articles in order to access various computer records regarding the articles.  
           [0004]    Bar code labels, however, have several drawbacks. For example, computer stored records that are accessed when a bar code is read do not move with the corresponding article. Therefore, if the article to which the bar code label is attached is remote from the computer, the records concerning that article cannot be immediately accessed if necessary.  
           [0005]    Moreover, bar code labels cannot be read remotely. Thus, if it is desired to take an inventory of articles currently in the storage area, personnel must physically scan each label on each article one at a time in order to determine which articles are presently in the storage area. Such scanning requires the physical presence of the personnel at the location of the articles and is extremely time consuming. Additionally, because bar code labels cannot be read remotely, they cannot be used as security devices that can be detected if the articles to which they are attached are improperly removed from a secured area.  
           [0006]    Instead of bar coded labels, it is known to attach radio frequency identification (RFID) tags to the articles to be monitored. The RFID tags can be read, as can bar code labels. However, unlike bar code labels, reading RFID tags does not require the physical presence of personnel because the RFID tags can instead be read remotely. Thus, inventory can be taken more quickly because personnel are not required to walk around a storage area or other area in order to read the RFID tags. Moreover, because RFID tags can be read remotely, they can be used as security devices. Thus, if someone attempts to surreptitiously remove an article to which an RFID tag is attached from a secured area, a remote reader can sense the RFID tag and provide an appropriate alarm.  
           [0007]    RFID tags can be read one at a time or in groups. When multiple RFID tags in a group are read at the same time, the information transmitted by the multiple tags frequently collide. Accordingly, spread spectrum techniques, such as either direct sequence spread spectrum (DSSS) or frequency hopping, in the communications between the reader and the tags have been suggested in order to reduce the impact of such collisions. It is also known to interrogate a tag using either a direct sequence spread spectrum (DSSS) signal or a frequency hopping signal.  
           [0008]    An RFID tag requires a power source in order to permit the transmission of information from the tag to a reader. Traditionally, an RFID tag is powered either locally or remotely. In remote powering of an RFID tag, the RFID tag typically derives its power from the signal transmitted by the reader. A capacitor or other similar storage device stores the power and supplies the stored power to the processing, memory, and transceiver of the RFID tag. One disadvantage of this powering technique is that, if the RFID tag is operated too far away from the reader, the RFID tag cannot derive sufficient energy from the reader&#39;s signal to effectively power its components.  
           [0009]    Local powering of the RFID tag usually involves using a battery on the RFID tag in order to power the processing, memory, and transceiver. While the use of a local battery overcomes the disadvantage of operating the RFID tag too far away from the reader when the RFID tag is deriving its power from the reader&#39;s signal, the use of a local battery has the disadvantage that it requires frequent replacement or re-charging due to the amount of power consumed by the processing, memory, and transceiver of the RFID tag.  
           [0010]    It is known to duty cycle a portion of a receiver of a tag in order to conserve battery power. An example of such a receiver is a super-regenerative receiver. The receiver includes an amplification stage, a local oscillator, a quench frequency source, and a detector stage. The tag also includes a microcontroller, an RF transmitter, and an antenna. The tag remains in a low-power quiescent stand-by state until it is activated by a signal from the reader. Following transmission of an activation signal by the reader, the reader sends a request for information. Specifically, the receiver is duty cycled in order to provide quiescent operation with a low current draw. Thus, some elements of the receiver are completely shut down to save tag power while the tag is operating in a quiescent state. Amplifiers of the receiver utilize forward-biased transistor stages whose forward biasing is provided at the duty cycle rate such that power draw is limited due to the duty cycle of the bias. A duty cycle of 1 to 5% is thought to provide sufficient time for reception of the activation signal and to reduce total current draw.  
           [0011]    However, such an arrangement has a number of problems. For example, power consuming elements of the tag and even of the receiver may not be shut down during the OFF times of the duty cycle. Accordingly, power is still wasted. Also, if the transmitter of the tag is capable of using several frequencies specified by the reader, the tag described above cannot quickly acquire the desired frequency for its transmitter and must remain on for a sufficient period of time in order to permit acquisition of that frequency.  
           [0012]    Moreover, when a tag is turned off and on in order to conserve battery power, or when the tag is infrequently interrogated by a tag reader, or otherwise, it has periods of OFF time during which it is not or cannot be interrogated by a tag reader. Therefore, interrogations by the tag reader can occur only at widely separated interrogation intervals, thereby allowing an article to which the tag is attached to be removed following one interrogation without anyone realizing it until a succeeding interrogation.  
           [0013]    The present invention overcomes one or more of these or other problems.  
         SUMMARY OF THE INVENTION  
         [0014]    In accordance with one aspect of the present invention, a method of communicating information between an RFID tag and first and second readers comprises the following: controlling a first transceiver of the RFID tag so that the first transceiver communicates with the first reader and so that the first transceiver has substantially longer periods during which the first transceiver is not in communication with the first reader than when the first transceiver is in communication with the first reader; and, controlling a second transceiver of the RFID tag so that the second transceiver communicates with the second reader at least during the periods when the first transceiver is not in communication with the first reader.  
           [0015]    In accordance with another aspect of the present invention, an RFID tag comprises first and second transceivers. The first transceiver transmits and receives first signals to and from a first reader. The second transceiver transmits and receives second signals to and from a second reader.  
           [0016]    In accordance with still another aspect of the present invention, a method is providing to conserve battery power in an RFID tag having a battery, a receiver, and a transmitter. The method comprises the following: duty cycling the receiver so that the receiver is turned on during ON times of duty cycles and so that the receiver is turned off during OFF times of the duty cycles; during the ON times of the receiver, receiving a frequency from a tag reader; and, transmitting data to the reader at the frequency.  
           [0017]    In accordance with yet another aspect of the present invention, an RFID tag comprises a transmitter, a receiver, a battery, a switch, and a controller. The transmitter transmits first data to a tag reader. The receiver receives second data from the tag reader. The switch couples the battery to the receiver. The controller operates the switch in a duty cycle such that power is provided by the battery to the receiver during ON times of the duty cycle and such that power from the battery to the receiver is interrupted during OFF times of the duty cycle.  
           [0018]    In accordance with a further aspect of the present invention, an RFID tag comprises a transceiver and a receiver. The transceiver transmits and receives first signals to and from a first reader. The receiver receives second signals from a second reader and activates the transceiver thereby causing the transceiver to transmit and receive the first signals to and from the first reader. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which:  
         [0020]    [0020]FIG. 1 illustrates a tagging system in accordance with one embodiment of the present invention;  
         [0021]    [0021]FIG. 2 illustrates additional detail of a tag that can be used with the tagging system of FIG. 1;  
         [0022]    [0022]FIG. 3 illustrates additional detail of a long range reader that can be used with the tagging system of FIG. 1;  
         [0023]    [0023]FIG. 4 illustrates a message format useful in supporting communications between the tag and the reader of FIG. 1;  
         [0024]    [0024]FIG. 5 illustrates an exemplary composition of a frame of the message format shown in FIG. 4;  
         [0025]    [0025]FIG. 6 illustrates an exemplary composition of the header of the frame shown in FIG. 5;  
         [0026]    [0026]FIG. 7 illustrates an exemplary composition of a time slot of the frame shown in FIG. 5;  
         [0027]    [0027]FIG. 8 illustrates an exemplary composition of the header of the time slot shown in FIG. 6; and,  
         [0028]    [0028]FIGS. 9, 10, and  11  are flow charts showing an exemplary operation of the tag illustrated in FIGS. 1 and 2. 
     
    
     DETAILED DESCRIPTION  
       [0029]    As illustrated in FIG. 1, a tagging system  10  includes a long range reader  12 , a short range reader  14 , and an RFID tag  16 . The long range reader  12  includes an antenna  18 , and the RFID tag  16  similarly includes an antenna  20 . The antennas  18  and  20  establish a long range RF link between the long range reader  12  and the RFID tag  16  so that the long range reader  12  can remotely read the identification stored in a memory of the RFID tag  16 . The range of the long range reader  12  can be as high as several hundred feet or more. For example, the long range reader  12  can have an expected range of approximately 500 feet.  
         [0030]    A short range link  22  between the short range reader  14  and the RFID tag  16  permits the short range reader  14  to read information from the RFID tag  16  over a shorter range and/or in a more secure manner. For example, it may not be desirable for the long range reader  12  to read certain information stored in the RFID tag  16  because long range RF communications can be intercepted by a strategically placed surreptitious reader similar to the long range reader  12 . Accordingly, the short range link  22  increases the difficulty in illicitly acquiring the more sensitive information that may be stored on the RFID tag  16 .  
         [0031]    The short range link  22  is shown in FIG. 1 as a hard wire link between the short range reader  14  and the RFID tag  16 . Accordingly, the more sensitive information stored on the RFID tag  16  can be read by establishing a physical interconnection between the short range reader  14  and the RFID tag  16 .  
         [0032]    Alternatively, the short range link  22  may be a limited range magnetic link such as those provided by contact-free smart cards. As a still further alternative, the short range link  22  may be a very limited range RF link. Other alternatives will occur to those skilled in the art. One advantage of using one of these non-hardwired alternatives for the short range link  22  is that then the RFID tag  16  can be more readily used as a security device. Accordingly, when an attempt is made to remove an article to which the RFID tag  16  is attached, the short range reader  14  located at a portal of a secured area or otherwise can pick up a signal from the RFID tag  16  indicating that an attempt is being made to remove the article from the secured area.  
         [0033]    The expected maximum range of the short range reader  14  over the short range link  22 , for example, may be less than four feet, and is expected, in typical usage, to be between six inches and eighteen inches.  
         [0034]    An embodiment of the RFID tag  16  is shown in additional detail in FIG. 2. The RFID tag  16  includes a first transceiver  30  comprising a frequency agile (frequency hopping) RF transmitter  32  and a direct sequence spread spectrum RF receiver  34 . The frequency agile RF transmitter  32  and the direct sequence spread spectrum RF receiver  34  are coupled between the antenna  20  and a microprocessor  36 . Accordingly, the frequency agile RF transmitter  32  of the RFID tag  16  implements frequency hopping in transmitting information to the long range reader  12 , and the direct sequence spread spectrum RF receiver  34  of the RFID tag  16  implements direct sequence spread spectrum synchronization and decoding in receiving communications from the long range reader  12 .  
         [0035]    The RFID tag  16  also includes a second transceiver  38  between the microprocessor  36  and the short range reader  14 . Accordingly, the RFID tag  16  can transmit and/or receive communications to and/or from the short range reader  14 . In the case where the short range link  22  is a hardwire link, the second transceiver  38  may simply be a plug that is connectible to a corresponding plug of the short range reader  14 . In the case where the short range link  22  is an RF link, the second transceiver  38  may be an RF transceiver of any known type provided that this RF transceiver preferably has a much shorter range than the frequency agile RF transmitter  32  and the direct sequence spread spectrum RF receiver  34 . In the case where the short range link  22  is a magnetic link, the second transceiver  38  may simply be a magnetic emitter (and/or sensor) capable of magnetically interfacing with the short range reader  14 .  
         [0036]    The RFID tag  16  further comprises a memory  40  coupled to the microprocessor  36 . The memory  40  stores the ID of the RFID tag  16  that can be read by the long range reader  12  through the antennas  18  and  20 , the frequency agile RF transmitter  32 , the direct sequence spread spectrum RF receiver  34 , and the microprocessor  36 . The memory  40  may also store information supplied to it by the short range reader  14  through the short range link  22 , the second transceiver  38 , and the microprocessor  36 . The memory  40  can additionally store information supplied by the long range reader  12 .  
         [0037]    This information can include, for example, the inventory history of the article to which the RFID tag  16  is attached. Accordingly, the date that the article entered inventory, the date that the article left inventory, the length of time that the article has been in inventory, any movement within inventory, and similar information may be stored in the memory  40 .  
         [0038]    The information stored in the memory  40  may also include shipping manifests that indicate when and to whom the article is to be shipped. Moreover, in the case where individual articles with differing destinations are shipped in the same container, an RFID tag attached to the container, hereafter called a container tag, can be attached to the container. This container tag may be arranged to store the identity and destination of each article in the container. As articles are removed from the container, the information stored in the container tag can be updated to indicate which articles have been removed, the location at which the articles were removed, and the identity of the personnel who removed the articles.  
         [0039]    The information stored in the memory  40  may further include maintenance, repair, and date of service records showing the maintenance and/or repair history of the corresponding article.  
         [0040]    Other information related to the article may likewise be stored in the memory  40 . For example, the integrity of the information stored in the memory  40  can be assured by keeping a record of the modifications to the stored information and of the identity of the personnel making the modifications. As another example, records related to the production of the article may be stored in the memory of the tag.  
         [0041]    Accordingly, any information about the article may be stored with the article instead of in a remote computer system or on paper.  
         [0042]    Because the records are carried by the RFID tag  16  attached to a corresponding article, the RFID tag  16  eliminates the need to maintain paper or computer records of the life history of an article, the RFID tag  16  eliminates the problem of lost or misplaced records, and the RFID tag  16  improves operational efficiency by eliminating the requirement to retrieve records prior to accessing and/or operating on the article.  
         [0043]    The RFID tag  16  includes a battery  42  that is coupled so that it supplies power through a switch  44  to the frequency agile RF transmitter  32 , through a switch  46  to the direct sequence spread spectrum RF receiver  34 , directly to the microprocessor  36 , directly to the second transceiver  38  (if necessary), and directly to the memory  40 .  
         [0044]    Moreover, a plurality of sensors (not shown) may be coupled to the microprocessor  36 . These sensors may include, for example, a temperature sensor, a humidity sensor, and other sensors such as a pressure sensor, a proximity sensor, an electromagnetic sensor, an optical sensor, a mechanical sensor, a chemical sensor, and/or the like. The microprocessor  36  stores the information from the sensors in the memory  40 , and this information may be read from the memory  40  by the short range reader  14  or by the long range reader  12 .  
         [0045]    The microprocessor  36  may be arranged to further sense the voltage level of the battery  42 . Accordingly, the microprocessor  36  stores this voltage level in the memory  40 , and this stored voltage level may be read from the memory  40  by the short range reader  14  or by the long range reader  12 . Thus, if the voltage level of the battery  42  as read by either the short range reader  14  or the long range reader  12  indicates that the battery  42  needs charging or replacement, suitable remedial action may be taken.  
         [0046]    Because of the frequency agile RF transmitter  32 , the direct sequence spread spectrum RF receiver  34 , and/or the battery  42 , the RFID tag  16  is capable of relatively long range activation while providing a low power method for command-response activation by the long range reader  12 . This long range activation allows the RFID tag  16  to be placed at distances remote from the long range reader  12  for purposes of interrogating the RFID tag  16  for its unique tag number and possibly other information.  
         [0047]    The frequency agile RF transmitter  32  and the direct sequence spread spectrum RF receiver  34  allow the tagging system  10  to operate in the FCC defined Industrial Scientific and Medical (ISM) bands at maximum legal power. Both frequency hopping as used by the frequency agile RF transmitter  32  and direct sequence spread spectrum communications as used by the direct sequence spread spectrum RF receiver  34  circumvent jamming by narrow-band signals using different methods of spreading the signal over a large bandwidth. The direct sequence spread spectrum RF receiver  34  can receive signals from the long range reader  12  within milliseconds of activation. By contrast, a frequency agile receiver must search a long frequency hopping sequence in order to receive signals from the long range reader  12 . The time required to make this search is typically longer than the time required to detect a direct spread spectrum sequence because the direct spread spectrum signal is either on a fixed frequency or on one of only a few frequencies.  
         [0048]    Also, the RFID tag  16  may be duty cycled in order to conserve the energy stored in the battery  42 . Thus, each duty cycle has an ON time and an OFF time. During the ON time, the RFID tag  16  is fully powered permitting the direct sequence spread spectrum RF receiver  34  to receive communications directed to the RFID tag  16  and to permit the frequency agile RF transmitter  32  to transmit information to in response to the communications. During the OFF time, all non-essential power consuming devices of the RFID tag  16 , such as the frequency agile RF transmitter  32  and/or the direct sequence spread spectrum RF receiver  34 , are effectively disconnected from the battery  42  so that energy drainage of the battery  42  is reduced. The OFF time to ON time ratio of the duty cycle, for example, may be on the order of 1000:1, although other ratios could be used.  
         [0049]    An embodiment of the long range reader  12  is shown in additional detail in FIG. 3. The long range reader  12  includes a direct sequence spread spectrum RF transmitter  50  and a frequency agile RF receiver  52  coupled between the antenna  18  and a microprocessor  54 . The frequency agile RF receiver  52  of the long range reader  12  implements frequency hopping in receiving information from the frequency agile RF transmitter  32  of the RFID tag  16 . Moreover, the direct sequence spread spectrum transmitter  50  of the long range reader  12  implements direct sequence spread spectrum transmission in transmitting communications to the direct sequence spread spectrum RF receiver  34  of the RFID tag  16 .  
         [0050]    The long range reader  12  further comprises a memory  56  coupled to the microprocessor  36 . The memory  56  stores the information that the long range reader  12  receives from the RFID tag  16 . The memory  56  also stores the software that supports a communication protocol as described herein.  
         [0051]    This communication protocol governs the message format that is used between the long range reader  12  and the RFID tag  16 . According to this protocol, a message is comprised of a plurality of frames as shown in FIG. 4. Each frame is preferably no longer than the length of time the frequency agile RF transmitter  32  is allowed to dwell at any given frequency.  
         [0052]    Each of the frames shown in FIG. 4 has the construction shown in FIG. 5. Accordingly, each frame has a frame header and a number of time slots TS 0 -TSN. The frame header contains information about the long range reader  12  that is reading the RFID tag  16 . As shown in FIG. 6, the header contains (i) the state of the long range reader  12 , (ii) the hop sequence currently being used by the long range reader  12  to receive messages from the RFID tag  16 , and (iii) the current position (i.e., frequency) of the long range reader  12  in this hop sequence. The frame header can also contain such other information as may be useful in the tagging system  10 . For example, the frame header may also contain the number (N+1) of the time slots in the corresponding frame.  
         [0053]    The long range reader  12  may have several reader states including, for example, an active communication state and a beacon state. In the active communication state, the long range reader  12  commands responses from one or more selected tags such as the RFID tag  16 . In the beacon state, the tags, such as the RFID tag  16 , self-initiate the transmission of messages to the long range reader  12 .  
         [0054]    The hop sequence and/or the current position in the hop sequence as contained in the frame header are/is useful to tags that have limited signal processing capability. Such tags, for example, may have no capability themselves to determine the frequency (i.e., the current position in the hop sequence) onto which they should transmit their responses.  
         [0055]    Moreover, each time slot may also include a time slot header and data as shown in FIG. 7, and each time slot header, as shown in FIG. 8, may contain the hop sequence and the current position in the hop sequence of the long range reader  12 . The time slot header may also contain the relative position, such as a time slot number ( 0 ,  1 , . . . , or N), of the corresponding time slot in the frame. This relative position information may be used by the RFID tag  16  to establish a relative timing interval into which the RFID tag  16  can transmit data. By transmitting the hop sequence and the current position in the hop sequence at the beginning of each time slot, the RFID tag  16  is aided in its rapid acquisition of the current hop sequence and frequency. Because the RFID tag  16  can acquire, from the header in each time slot, sufficient information about the frequency and timing of the long range reader  12 , the RFID tag  16  may power down until such time that it expects the complete header information to be transmitted by the long range reader  12 . Therefore, the RFID tag  16  is able to substantially reduce the amount of power that it uses to determine the frequency and timing to be used by its frequency agile RF transmitter  32  in transmitting information in the data portion of the time slot.  
         [0056]    As indicated above, the long range reader  12  transmits all headers, whether frame headers or time slot headers. The RFID tag  16  transits only in the data portion of the time slots. The RFID tag  16  may implement a non-deterministic method of selecting a time slot for the transmission of data. By using a non-deterministic method of selecting a time slot, the possibility of a plurality of tags transmitting data into the same time slot is minimized. For purposes of illustration, such a non-deterministic method of selecting a time slot could be embodied by a pseudo-random number generator that pseudo-randomly generates a number of a time slot into which its corresponding tag transmits its data. This implementation results in a communications protocol similar to, but not identical to, the Aloha protocol, a standard communications protocol.  
         [0057]    The long range reader  12  can communicate directly with a specific tag or a group of specific tags. When the long range reader  12  is communicating directly with a specific tag or a group of specific tags, the long range reader  12  may suspend the transmission of time slot headers. This suspension indicates to all other tags that their communications are to be suspended. Also, all data may be transmitted between the long range reader  12  and the RFID tag  16  in packets having packet numbers so that both the long range reader  12  and the RFID tag  16  can detect missing or duplicate data. Moreover, acknowledgements can be used to signify a successful transmission between the long range reader  12  and the RFID tag  16 . A failure to receive an acknowledgement can cause re-transmission of the information. Once a transaction between the long range reader  12  and a specific tag or group of tags is complete, the long range reader  12  resumes transmitting the headers.  
         [0058]    As discussed above, the RFID tag  16  may be duty cycled such that, during ON times, the RFID tag  16  can receive interrogations from the long range reader  12  and such that, during OFF times, the RFID tag  16  cannot receive interrogations from the long range reader  12 . Alternatively, the long range reader  12  may infrequently interrogate the RFID tag  16 . Thus, during the OFF portions of the duty cycles, and/or during the long periods between interrogations by the long range reader  12 , an article to which the RFID tag  16  is attached can be removed without authorization from a secured area, and the long range reader  12  may not detect this unauthorized removal of the article in sufficient time to stop it.  
         [0059]    However, the presence of the second transceiver  38  enables the RFID tag  16  to be immediately activated when the RFID tag  16  is brought into the short range of the short range reader  14 . The short range reader  14 , for example, may be located at an appropriate activation portal (typically installed at an entryway). In this case, the short range reader  14  can be arranged to emit an interrogation signal that is read by the RFID tag  16  through the second transceiver  38 . The RFID tag  16  sends a response to the interrogation signal through the second transceiver  38  in order to indicate that the RFID tag  16  is within the range of the short range reader  14 . Therefore, an appropriate alarm can be given in sufficient time to prevent the unauthorized removal of the article.  
         [0060]    Alternatively, a portal may be arranged to interrogate the RFID tag  16  and the RFID tag may be arranged to transmit a response through the second transceiver  38  to the short range reader  14  that is located elsewhere than at the portal. As a further alternative, a portal may be arranged to interrogate the RFID tag  16  and the RFID tag  16  may be arranged to wake up the first transceiver  30  causing the first transceiver  30  to transmit a response through to the long range reader  12  that is located remotely from the portal. As a still further alternative, the portal and/or the short range reader  14  may passively receive a signal from the second transceiver  38  of the RFID tag  16  in order to indicate that the RFID tag  16  is within the range of the portal and/or the short range reader  14 .  
         [0061]    As shown in FIG. 9, when it is time for the RFID tag  16  to receive messages from the long range reader  12 , as indicated by a block  100 , the RFID tag  16  powers up, as indicated by a block  102 . Accordingly, power is supplied to the direct sequence spread spectrum RF receiver  34  so that it can then listen for a message from the long range reader  12 . To power up the direct sequence spread spectrum RF receiver  34 , the microprocessor  36  may simply close the switch  46  to couple the battery  42  to the direct sequence spread spectrum RF receiver  34 . The microprocessor  36  controls the duty cycle of the switch  46  so that it has an appropriate OFF to ON ratio, such as the 1000:1 duty cycle ratio described above.  
         [0062]    When the RFID tag  16  detects a received message at the direct sequence spread spectrum RF receiver  34  as indicated by a block  104 , the RFID tag  16  parses the header information as indicated by a block  106  and, as indicated by a block  108 , stores the reader state, the hop sequence, the current position in the hop sequence, and any other information that is contained in the header.  
         [0063]    As shown in FIG. 10, when it is time for the RFID tag  16  to transmit data to the long range reader  12  as indicated by a block  110 , the RFID tag  16  powers up, as indicated by a block  112 . Accordingly, power is supplied to the frequency agile RF transmitter  32  so that it can then transmit data to the long range reader  12 . To power up the frequency agile RF transmitter  32 , the microprocessor  36  may close the switch  44  to couple the battery  42  to the frequency agile RF transmitter  32 . The microprocessor  36  controls the duty cycle of the switch  44  so that it has an appropriate OFF to ON ratio such as the 1000:1 ratio described above. Thereafter, the RFID tag  16  selects the time slot in which it is to transmit the data as indicated by a block  114 , the RFID tag  16  selects the frequency at in which it is to transmit the data as indicated by a block  116 , and the RFID tag  16  causes the frequency agile RF transmitter  32  to transmit the data in the selected time slot using the selected frequency as indicated by a block  118 .  
         [0064]    The time at which the RFID tag  16  is to transmit data (the block  106 ) depends on the state of the long range reader  12 . If the reader state as contained in the header and stored by the RFID tag  16  (block  104 ) indicates that the long range reader  12  is in the beacon mode, the RFID tag  16  self-originates the transmission of data. In this state, the RFID tag  16 , for example, may be arranged to transmit data periodically based on a timer. When the timer indicates that it is time to transmit, the RFID tag  16  starts at the block  100  so that the direct sequence spread spectrum RF receiver  34  can determine the frequency at which the frequency agile RF transmitter  32  is to transmit and processing continues through the remainder of the blocks shown in FIGS. 9 and 10. In the case where the RFID tag  16  is in the beacon mode, the block  110  may be a simple pass through to the block  112 .  
         [0065]    If the long range reader  12  is an active communication state, the RFID tag  16  at the block  110  determines whether an interrogation signal for the RFID tag  16  has been received.  
         [0066]    Other reader states for the long range reader  12  are also possible.  
         [0067]    As indicated above, the time slot in which the RFID tag  16  transmits data may be selected based on the pseudo-randomly generated number. The frequency at which the RFID tag  16  transmits data is selected based on the current position in the hop sequence as parsed (block  106 ) and stored (block  108 ) by the RFID tag  16 .  
         [0068]    The protocol as described above facilitates a long duty cycle and/or infrequent interrogations by the long range reader  12  because the RFID tag  16  can quickly receive the necessary information (such as hopping frequency) to permit it to transmit data.  
         [0069]    As shown in FIG. 11, when it is time for the RFID tag  16  to transmit data to the short range reader  14  (or portal) as indicated by a block  130 , the second transceiver  38  of the RFID tag  16  transmits the appropriate information as indicated by a block  132 .  
         [0070]    The time at which the RFID tag  16  transmits data to the short range reader  14  (or portal) and the data to be transmitted may be in accordance with any of the embodiments and/or alternatives described above. For example, if the RFID tag  16  receives a signal from the short range reader  14  requesting information stored in the memory  40 , it is time to transmit, and the second transceiver  38  transmits the requested information to the short range reader  14 . As another example, when the RFID tag  16  is being used as a security device, the RFID tag  16  responds to an interrogation signal from the short range reader  14  or a portal (time to transmit) by sending a signal to the short range reader  14 , portal, or long range reader  12  indicating that the RFID tag  16  is within the range of the short range reader  14  or portal.  
         [0071]    Certain modifications of the present invention have been disclosed above. Other modifications will occur to those practicing in the art of the present invention. For example, the functions of the long range reader  12  as described above have been confined to reading information from the RFID tag  16 . However, the long range reader  12  can also be arranged to write information to the RFID tag  16 .  
         [0072]    Also, as described above, the long range reader  12  is arranged to read the tag ID of the RFID tag  16 , and the short range reader  14  is arranged to read other information from the RFID tag  16 . However, the long range reader  12  may be arranged instead to read any combination of tag ID and other information from the RFID tag  16 , and the short range reader  14  may be similarly arranged to read any combination of the tag ID and other information from the RFID tag  16 .  
         [0073]    Moreover, although the RFID tag  16  is shown as a microprocessor based tag in FIG. 2, the RFID tag  16  may instead comprise one or more digital circuit elements, and/or a programmable logic array, and/or a dedicated integrated circuit, etc.  
         [0074]    Furthermore, the long range reader  12  as described above has a range of several hundred feet and could have an expected range of approximately 500 feet. However, this range could be longer or shorter depending on the application and/or other factors. Similarly, the range given above for the short range reader  14  could be other than as described above.  
         [0075]    Additionally, the transmitter of the first transceiver  30  of the RFID tag  16  is described above as the frequency agile RF transmitter  32 , and the receiver of the first transceiver  30  of the RFID tag  16  is described above as the direct sequence spread spectrum RF receiver  34 . However, the RFID tag  16  may instead advantageously use other types of transmitters and receivers.  
         [0076]    Also, the frequency agile RF transmitter  32  and the direct sequence spread spectrum RF receiver  34  are duty cycled as described to conserve the energy of the battery  42 . Additionally, the microprocessor  36  may also power itself down during the OFF portion of the duty cycle of the RFID tag  16  requiring only the power necessary to determine the ON portion of the duty cycle.  
         [0077]    Moreover, as described above, one or more elements of the RFID tag  16 , such as the direct sequence spread spectrum RF receiver  34 , may be duty cycled. The duty cycles can have equal or unequal ON times and/or equal or unequal OFF times.  
         [0078]    Furthermore, the switches  44  and  46  described above may be electromechanical switches, semiconductor switches, logic elements, etc.  
         [0079]    Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.