WO1999012122A1

WO1999012122A1 - Method for resolving signal collisions between multiple rfid transponders in a field

Info

Publication number: WO1999012122A1
Application number: PCT/US1998/017413
Authority: WO
Inventors: Donald L. Black; Dale Yones
Original assignee: Micron Communications, Inc.
Priority date: 1997-09-03
Filing date: 1998-08-20
Publication date: 1999-03-11
Also published as: US6265962B1; US6091319A; US5986570A; AU9029998A

Abstract

Communications between an RFID interrogator and an RFID transponder require that no more than one transponder be present in the reading range of the interrogator and transmitting into motion at any given time. If multiple transponders are in the field, then a collision between the return signals of the transponders occurs, rendering the signals unreadable. A method to resolve the collisions and allow for accurate transmission of each transponder's data is given. This method is especially effective over other methods when the transponder is a read-only type of device, whereby there is no communications interrogator on board the read-only transponder.

Description

METHOD FOR RESOLVING SIGNAL COLLISIONS BETWEEN MULTIPLE RFID TRANSPONDERS

IN A FIELD

FIELD OF THF, INVENTION

Tlie present invention relates to electronic identification devices and systems, and

in particular, to devices where their application in a system will allow more than one device to

be present in the interrogator field at any moment in time. Furthermore, the system requires

that all the devices in the field are properly identified by the interrogator.

RAΓKOROTTNT) OF TIIF TNVFNTTON

There are methods currently in use that provide varying degrees of success in

resolving the collision of the tags, and especially in tags with read/write capability, where

communications to the tags is facilitated by an on-board interrogator in the RFID tag. In such

a tag, the interrogator can send signals to allow tags to respond with a random number that is

manipulated by the interrogator and transmitted to all tags in the field. Only the tag that

matches the computed number generated by the interrogator will transmit its data. This process

continues until all tags have transmitted their data. This method significantly increases the complexity of the transponder by requiring the transponder to include an interrogator and being

only partially effective for regulation of collisions.

Another method of collision resolution is to cause tags to transmit at different

frequencies, thereby avoiding a collision of signals. This method significantly increases the

complexity of the interrogator while being only partially effective for resolution of the collision.

Yet another method involves the use of spread spectnim techniques. The

technique can be either direct sequence spread spectnim (DSSS) or frequency hopping. Either

method requires correlation of the signal in the interrogator and requires a very complex

interrogator system.

-I- Another method of collision resolution uses part of the unique identification code of the transponder to provide a specific time whereby the transponder transmits its data, and

all other times, the transponder remains inactive. This feature is limited by the vast number

of unique transmission slots available, and the time required to read all the possible tags in the

field. Moreover, a single transponder in the field could take an inordinate amount of time to

be read. If the unique identification code is truncated to allow for faster performance, then the

probability of an un-resolvable collision occurs due to the duplication of codes.

One other method for resolution is the utilization of a random oscillator and a

binary counter on each transponder which enables transmission from the transponder when the

counter is in a specific state. All other times the transponder is inactive. This method has the

disadvantage of requiring alignment of all the transponders asynchronously before coherent data

can be received by the interrogator.

All the above methods require either the use of expensive and complicated

interrogator systems, a read-write tag, or they are excessively slow to resolve a useful number

of tags.

SUMMARY OF THE INVENTION

The present invention addresses the limitations of the above solutions to the

collision problem by providing a simple system for collision resolution that does not require the

transponder to have read-write capability nor does the solution require additional complexity

to the interrogator to perform the anti-collision function.

The invention comprises a method for sending data from a transponder having

at least one memory, a transmission criteria, a transmit state and a full cycle flag. The method comprises the steps wherein the transponder: detects the interrogator, such as by receiving a

carrier signal from the interrogator; determines that it is time to transmit the data by verifying

that it is the transmit-armed state and that the carrier signal has been modified in a

predetermined manner; transmits its data in groups of one or more data bits; determines

whether its complete data has been read by the interrogator during the transmission of its data;

and sets a full cycle flag after sending its complete data transmission. The transponder

determines that the interrogator has read the complete data transmission by verifying that the

carrier signal has not been modified until the full cycle flag is set. If the transponder

determines that the interrogator did not read the complete data transmission, then the

transponder stores a number in memory, iteratively changes the number until the number stored

in memory satisfies the transmission criteria, and then transmits its data. This process of

transmitting and determining whether the complete data has been read by the interrogator

during data transmission is repeated until the transponder determines that the interrogator has

read the complete data transmission.

The invention also comprises a method for an interrogator to read one or more

RFID transponders in a field by: providing a carrier signal; detecting the presence of at least

one transponder; modifying the carrier signal in a predetermined manner, such as by sending

out a continuous carrier signal; receiving data from all active transponders in the field;

determining whether it has received a valid data transmission by checking the validity of each

group of data as it is received; and upon determining an invalid data transmission, modifying

the carrier signal-such as by suppressing the signal for a predetermined number of clock

cycles— to inform all active transponders in the field that there was an incomplete read. The step of modifying the carrier signal is performed prior to the transponder sending its complete

data transmission and it is performed substantially simultaneously upon the determination that

invalid data transmission has been received. The interrogator iteratively repeats the steps of

receiving data and determining whether it has received valid data, until the interrogator

determines that it has read the complete data for each transponder in the field. The step of

determining that the interrogator has received an invalid data transmission comprises detecting

the interrogator's inability to compute a proper synchronization word, a proper CRC, or a

proper word length. After determining that the interrogator has received complete data

transmission by determining that the CRC is valid, the carrier signal is modified in a

predetermined manner, such as by suppressing the signal for a predetermined number of clock

cycles. The complete data for each transponder is transmitted from the interrogator to a

computer system for processing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in detail with reference to an example of the

embodiment with the aid of the drawings as follows:

FIG. 1 is a block diagram of a passive RFID system.

FIG. 2 is a block diagram of an RFID interrogator.

FIG. 3 is a block diagram of a passive RFID transponder.

FIG. 4 is a flow chart of the sequence of events that occur within the transponder

during collision resolution.

FIG. 5 is a flow chart of the sequence of events that occur within the interrogator

during collision resolution. FIG. 6 is a timing diagram showing clock waveforms from the interrogator.

DETAILED DESCRDTTION OF THE INVENTION

Passive, read-only RFID systems are well known in the art and only a brief

overview of an RFID system will be given here for reference purposes. Referring to FIG. 1 ,

a passive, read-only RFID system consists of an interrogator 10 and a transponder 20. The

interrogator provides a carrier signal 12 necessary for powering and synchronizing the

transponder 20, and receives modulated data 22 transmitted from the transponder 20. The

interrogator 10 decodes the data from the transponder 20, verifies that the modulated

transponder data 22 is valid, and sends this data to a computer system, not shown, for

processing.

The interrogator 10 may be connected to a series circuit 11 that is resonant at the

desired carrier frequency, or it may be connected to an antenna, depending upon frequency of

operation and the desired communications method between the interrogator 10 and the

transponder 20.

The transponder 20, in one embodiment, consists of a parallel resonant LC

circuit 21. In general, this circuit is resonant at the carrier frequency of the interrogator 10;

however, it could be configured such the resonant frequency changes as a function of the

transmitted data; or multiple resonant circuits could be utilized to optimize both power

reception (resonance at the carrier frequency) and optimize signal strength from the transponder

(resonance at the return frequency from the transponder).

Likewise, multiple antennas could be used for the same purpose. Referring to FIG. 2, a block diagram of a typical RFID interrogator is given.

The main components of the interrogator are the microcontroller 18, amplifier and filter 16,

demodulator and drivers 14 and I/O circuitry 19.

The microcontroller 18 contains firmware and the necessary I/O lines to provide

proper operation of the interrogator. The microcontroller 18 generates a frequency that

corresponds to the carrier frequency of the system. For low frequency transponders, this

frequency is from the range of 100 kHz to 13.56 MHz. This frequency is filtered to generate

a relatively harmonic free sine wave. This sine wave signal is buffered by the driver and

demodulator block 14 to provide increased current drive capability. The output of the drivers

circuit, not shown, is connected to the series LC circuit 11. The nature of a series resonant

circuit driven at the resonant frequency is such that the impedance of the circuit approaches the

series resistance of the inductance in the circuit, and the voltage amplitude across the inductor

and the capacitor increases to a large magnitude, far in excess of the applied voltage across the

series LC circuit.

This large amplitude voltage across the inductor is the carrier referred to in this

invention and is necessary to provide sufficient power to the transponder at substantial distances

from the interrogator.

The carrier signal 12 is radiated without any underlying information being

transmitted, and when a transponder 20 is placed within the field of the interrogator 10, the

carrier signal 12 is magnetically coupled to the parallel resonant circuit 21 within the

transponder 20. When the transponder 20 is moved to a certain distance from the reader, sufficient power will be applied to the transponder 20 to allow it to begin transmitting its data back to the interrogator 10 by reflecting a load across the magnetically coupled medium.

This reflected load will cause the amplitude of the carrier frequency to vary

slightly, and these variations in the carrier frequency are detected by the demodulator. 14 This

method of communications is well known and some of the earlier work is described in U.S.

Patent #1 ,744,036 and U.S. Patent #4,333,072, which are hereby incorporated by reference.

The output of the demodulator 14 will consist of the raw encoded data from the

transponder 20 as well as a very large component of the carrier frequency and its haπnonics.

Consequently, it is necessary to provide filtering of this signal. The amp & filter block 16

perform this function. This amp & filter block 16 provides a bandpass filter which allows the

signal components to pass but the carrier frequency and its harmonics are sufficiently

suppressed. The resultant signal is amplified to provide proper signal levels for the

microcontroller 18. In one embodiment, the output of the .amp & filter 16 is connected directly

to the microcontroller 18.

The microcontroller 18 executes a routine whereby the data from the amp &

filter 16 is decoded into the proper logical states, and an error detection routine is executed,

whereby, the data received and decoded is determined to be valid or invalid. If the data is

valid, the data is sent to a host computer through the I/O block 19. If the data is determined

to be invalid, the interrogator 10 ignores the received data and continues processing data from

the tag. In other embodiments, and specifically for anti-collision features, the reader

determines that invalid data should be construed as a collision of data from multiple

transponders 20, and the reader suppresses a number of carrier clocks in addition to ignoring the received data from the transponder(s) 20. The need for clock suppression will be explained

in detail when the anti-collision invention is discussed.

Referring to FIG. 3, a passive RFID transponder block diagram is shown. The

main function blocks are given as: rectifier 22, regulator 23, clock generator 25, timing 26,

modulator 28, anti-collision arbitrator 29, control logic and state machine 27, and memory 30.

The memory 30 includes: row address 34, column address and sense amplifiers 32, and

memory array 36.

The rectifier 22 is typically a full-wave rectifier which converts the AC carrier

signal to a DC voltage. The rectifier 22 may also contain clamping diodes or an automatic gain

control circuit to prevent an over-voltage condition when the interrogator is placed in close

proximity to the transponder coil.

The regulator 23 controls the amplitude of the DC voltage from the rectifier 22

and provides a stable operating voltage for the transponder 20. The regulator 23 may also

contains a power-on-reset circuit which prevents the transponder from beginning operation or

transmitting data before there is sufficient voltage available for proper operation.

The clock generator 25 performs signal conditioning on the carrier signal to

provide a square wave clock signal for timing purposes. Additionally, "glitch" suppression and

clock overlaps are eliminated by this circuitry. Another function performed by this circuitry

is the detection of missing clocks from the carrier signal. This function will be explained in

detail in the section describing the anti-collision function.

The modulator block 28 provides the proper encoding of data and generation of

the signals to be transmitted to the interrogator. The anti-collision arbitrator 29 controls the performance of the transponder 20

when collisions between tags is detected. The anti-collision arbitrator 29 also assigns a random

address to the transponders 20 and controls the timing of the data to be transmitted to the

interrogator 10. This section will be discussed in detail in subsequent paragraphs.

The row address block 34 is used to address the rows within the memory array

36.

The column address and Sense Amplifier block 32 contains logic addresses each

column within the memory array 36 and also contains the sense amplifier to deteπnine the logic

state of the specific bit in the array that is being addressed.

The control logic and state machine block 27 executes a specific routine which

ensures that proper synchronization data, stored data in the array and Cyclic Redundancy Check

(CRC) codes are sent in the proper sequence to the interrogator. This block also inhibits

further transmission of data from the transponder 20 once it has been determined that

transponder 20 has successfully transmitted its data.

The timing 26 block provides the proper timing for the desired data rate from

the transponder. Additionally, the timing 26 block may provide the necessary signals for

synchronization with the interrogator.

The memory array 36 stores the unique identification code in a read-only

transponder 20, and stores identification and user programmable data in a read/write

transponder 20.

Referring to FIG. 4 and FIG. 5, flow charts for the operation of the interrogator

10 and the transponder 20, respectively, are shown. During normal operation of the system, the interrogator 10 is sending out a carrier signal 12 for example a 125 kHz sine wave, for a

first predetermined number of clock cycles. The interrogator 10 then suppresses the carrier

signal 12 for next a second predetermined number of clock cycles. This sequence continues

until one or more transponders 20 are detected. The detection of transponders 20 is

accomplished by detecting data from the demodulator in the interrogator 10. Once the

interrogator 10 detects the presence of transponder(s) 20, the interrogator 10 begins sending out

the carrier signal 12 continuously until a collision is detected. Collisions can be detected by

the Microcontroller's 18 inability to resolve whether the received data from the AMP &

FILTER 16 is a logic ' 1 ' or a logic '0' .

If a collision is detected, the interrogator 10 will suppress the carrier signal 12

for a second predetermined period of clocks and then begin transmitting a first predetennined

period of clocks. This cycle continues until data from a transponder 20 is detected.

Again, when data from a transponder 20 is detected, the interrogator 10 transmits

a continuous carrier signal 12 and processes the data received from the transponder 20.

Other conditions that denote the presence of multiple transponders 20 is the

inability to compute a proper synchronization word, inability to compute a proper CRC or

improper word length. For example, a person with ordinary skill in the art would recognize

that a possible word length is 96 bits, but the invention is not limited by the length of the data

word. It is therefore advantageous for the interrogator to examine the received data to

determine that it has a proper synchronization word, a proper CRC, and a proper word length.

Upon the failure of any of these conditions, may be treated as a collision, as discussed above. Once the interrogator 10 has determined that it has received the full data word

of the proper length, and the CRC is computed correctly, the interrogator will suppress the

carrier signal 12 for the second predetermined number of clock periods. It is now necessary

to refer to the transponder flow chart, FIG 4. to understand the difference in behavior of the

transponder 20 due to when carrier signal 12 is suppressed for the second predetermined

number of clock periods.

When a transponder 20 is sufficiently powered by the interrogator 10, it resets

the circuitry within the transponder to a known state. Specifically, it sets the anti-collision

arbitrator 29 to the 'transmit armed' state. The transponder does not transmit data, it merely

waits until it determines that the carrier signal 12 has been suppressed for a predetennined

number of clock periods. When the transponder 20 determines that the interrogator 10 is again

sending a carrier signal 12 after the suppression for a predetermined number of clock periods,

and the transponder 20 determines that it is in the 'transmit armed' state, it begins to execute

a sequence of commands that will cause data from the memory array 26 to be modulated and

transmitted to the interrogator 10.

If, due to the conditions previously discussed when describing the interrogator

flow chart a collision is detected and the intenogator suppress the carrier signal 12 for a second

period, the transponder 20 will determine that a suppression of clocks has occurred and, if the

full-cycle flag has not been set, the transponder 20 will cease transmitting its data.

Concurrently, the transponder 20 will load a new number into its address register. Preferably,

this will be a random number. As previously described, the interrogator 10 will continuously cycle through

generation of clocks for a first predetermined amount of time and suppression of clocks for a

second predetermined amount of time.

Each time the interrogator 10 suppresses the carrier signal 12 for a second

predetermined number of clock cycles, the address is decremented by one count. Alternatively,

the count may be increased or decreased by a different number of cycles, each address in each

transponder 20 is altered. When a transponder 20 has its address reach a preset critical value,

the anti-collision to 'transmit armed' state and the transponder 20 begins to transmit its data.

Preferably, the critical value is when the address is zero.

If no collisions are detected, the transponder 20 will transmit its full cycle and

the interrogator 10 will send out a continuous carrier signal 12 until it verifies that all data has

been properly received.

Likewise, for the other transponders 20 in the field, the detection of missing

clocks causes their addresses to be altered, and preferably decremented. This cycle continues

until .another transponder 20 has a address of the critical value and the anti-collision arbitrator

changes 'transmit armed' state, at which time the transponder 20 begins to transmit its data.

If more than one transponder 20 has their address as the critical value, the reader

detects the collision and stops the clocks for a second predetermined number of cycles. At that

time, only the transponders 20 with an address of the critical will recompute their addresses,

preferably with a random number.

The operation of a specific embodiment is described in Fig. 4 which is a flow

chart outlining the process steps for the transponder 20. The transponder 20 enters the field generated by the interrogator 10 and resets 100 by setting itself to the transmit-armed state and

by removing the full-cycle flag. The number in memory 30 is set to zero and modulation is

turned off 105. The transponder 20 detects the presence of an interrogator by checking 110

whether the transponder 20 sees modification of the carrier signal in a predetermined manner,

which for this embodiment is carrier signal 12 loss. The transponder 20 iteratively keeps

checking until the transponder 20 sees a loss and then determines whether the number in

memory 30 satisfies the transmission criteria. In the embodiment described in Fig. 4, the

transponder 20 determines whether the number in memory 30 is equal to zero 115 to satisfy the

transmission criteria. If the number in memory 30 does not satisfy the transmission criteria

which for this embodiment the number would not equal zero, then the number is modified.

For this embodiment, modification is achieved by decrementing the number 120 and then

returning to the iterative process of checking to determine whether there is any carrier signal

loss 110. If the number in memory 30 equals zero, then the random number generator is turned

on until the next loss of carrier 125. The transponder 20 modulates the data to the interrogator

10 after waiting four bit periods 130. The transponder 20 iteratively determines whether it has

waited four bit periods 135 and if so, modulation occurs on the first data bit.

The transponder 20 transmits the data one bit at a time 140, starting with the first

data bit. The transponder 20 then iteratively determines whether the carrier signal 12 has been

modified in a predetermined manner, which in this embodiment is the determination 145 of

whether the transponder 20 sees any carrier signal 12 loss. If the transponder 20 determines

that there is a loss in the carrier signal 12, then the transponder 20 detects a collision and the random number generator generates a new number, the random number generator is turned off

180, and the transponder 20 iteratively determines whether there is any carrier signal loss 110.

If the transponder 20 determines that there is no carrier loss after transmitting

the data bit 145, then the transponder 20 determines whether the last data bit has been sent

(complete data transmission), which in this embodiment would require the determination of

whether the 96th data bit had been sent 150. If the last data bit has not been sent, then the next

data bit is transmitted 140 to the interrogator 10. If the transponder determines that the last

data bit has been sent, then the transponder 20 sets the full-cycle flag 155.

After setting the full cycle flag, the transponder 20 transmits the first data bit 160

to the interrogator 10. The transponder 20 then iteratively determines whether the carrier signal

12 has been modified in a predetermined manner, which in this embodiment is the

determination 165 of whether the transponder 20 sees any carrier signal 12 loss. If the

transponder 20 determines that there is no carrier loss, then the transponder 20 iteratively

transmits 160 the next bit of data. If the transponder 20 determines that there is a loss in the carrier signal 12, then

the transponder 20 determines 175 whether the first bit was just transmitted and whether the

full-cycle flag is set. If the data is not the first data bit then the transponder iteratively

transmits 160 the next data bit. If the full-cycle flag has not been set and the data is the first

bit, then the transponder 20 detects an error and the random number generator generates a new

number, the random number generator is turned off 180, and the transponder 20 iteratively

determines whether there is any carrier signal loss 110. If the transponder 20 determines 175 that the first data bit has been sent and the

full-cycle flag is set, then the transponder 20 determines that the interrogator 10 has read the

complete data set and the transponder 20 goes dormant 190.

An embodiment is described in Fig. 5, which shows the process steps for the

interrogator 10. The interrogator 10 is powered up 200 and immediately begins transmitting

205 the carrier signal 12. The interrogator 10 looks 215 for transponders 20 during the first

predetermined period 210 and if the interrogator 10 does not detect 215 a transponder 20, the

interrogator 10 suppresses 270 the carrier signal 12 for a second predetermined period to signal

a new tag or transponder 20 to transmit or to sync colliding tags. The interrogator 10 next

determines 275 whether the period is complete and continues to suppress 280 the carrier signal

12 until the interrogator determines 275 that the period done, in which case, the interrogator

10 turns on 285 the carrier signal 12. The interrogator 10 then repeats the process and looks

210 for transponders 20 during the first predetermined period. If the interrogator 10 detects

215 the presence of a transponder 20, the interrogator 10 leaves the carrier signal 12 on until

it receives 220 invalid data due to a collision or error.

The interrogator 10 reads 225 the first bit of data, looking for a sync word. The

interrogator 10 iteratively determines 230 whether each bit of data is good and if the data bit

is bad, the interrogator 10 detects a collision and suppresses 270 the carrier signal 12 for a

second predetermined period to signal a new tag or transponder 20 to transmit or to sync

colliding tags. The interrogator 10 next determines 275 whether the period is complete and

continues to suppress 280 the carrier signal 12 until the interrogator determines 275 that the

period done, in which case, the interrogator 10 turns on 285 the carrier signal 12. The interrogator 10 then repeats the process and looks 210 for transponders 20 during the first

predetermined period. If the interrogator 10 determines 230 that each data bit is good, at the

eleventh bit, the interrogator 10 determines whether the sync word valid and if the sync word

is invalid, the interrogator 10 detects an error and suppresses 270 the carrier signal 12 for a

continues to suppress 280 the carrier signal 12 until the interrogator determines 275 that the period done, in which case, the interrogator 10 turns on 285 the carrier signal 12. The

interrogator 10 then repeats the process and looks 210 for transponders 20 during the first

predetermined period.

If the interrogator 10 determines 240 that the sync word is valid, the interrogator

10 continues to read 245 each data bit to obtain data and the CRC. The interrogator 10

determines 250 whether each data bit is valid and if the data is bad, the interrogator 10 detects

a collision and the interrogator 10 suppresses 270 the carrier signal 12 for a second

predetermined period to signal a new tag or transponder 20 to transmit or to sync colliding tags.

The interrogator 10 next determines 275 whether the period is complete and continues to

suppress 280 the carrier signal 12 until the interrogator determines 275 that the period done,

in which case, the interrogator 10 turns on 285 the carrier signal 12. The intereogator 10 then

repeats the process and looks 210 for transponders 20 during the first predetermined period.

If the interrogator 10 determines 250 that the data bit is good, the interrogator

10 iteratively continues to read the next data bit until the interrogator 10 determines 255 that

it has read the ninety-sixth data bit, wherein the interrogator 10 computes 260 the CRC and tests. If the interrogator 10 determines 265 that the CRC is not good, then the interrogator 10

detects a collision and the interrogator 10 suppresses 270 the carrier signal 12 for a second

in which case, the interrogator 10 turns on 285 the carrier signal 12. The interrogator 10 then

If the interrogator determines 265 that the CRC is good, then the interrogator

10 suppresses 290 the carrier signal 12 for a second predetermined period when the interrogator

10 reads the first data bit in order to signal to the transponder 20 that it was read. The

interrogator 10 next determines 295 whether the period is complete and continues to suppress

300 the carrier signal 12 until the interrogator determines 295 that the period done, in which

case, the interrogator 10 turns on 305 the carrier signal 12 and process 310 the transponder

code. The interrogator 10 then repeats the process and looks 210 for transponders 20 during

the first predetermined period.

An example of the anti-collision timing is shown in Fig. 6. The carrier signal

12 sent by the interrogator is represented pictorially by 420, wherein the signal interrupts are

shown as square wave pulses in 425. The interrogator 10 or reader sends 400 interrupts and

bursts to look for tags or transponders 20. The first tag is within the reader's field upon the

end of the suppression of the carrier signal 12 at position 400, the transponder 20 and the reader

detects 400 the presence of the first tag and transmits the carrier signal continuously. The first

tag transmits its data 430 to the reader. The reader reads the first tag, waits till the first bit is repeated for the first tag

and sends an interrupt at time position 405 which starts the second 440 and third 450 tags

transmitting. The reader detects the collision and sends an interrupt at time position 410. The

reader continues to send interrupts and bursts at time position 415 looking for tags. When the

reader detects the presence of the second tag, it sends a continuous carrier signal while reading

the tag. Because the second tag generated a random number of three, the tag transmits its data

on the third interrupt 445. Similarly, the third tag generated a random number of four and it

sends its data after waiting one more time slot on the fourth interrupt 455.

The present invention has been described above with reference to a preferred

embodiment. However, those skilled in the art will recognize that changes and modifications may

be made in this embodiment without departing from the scope of the present invention. Those

skilled in the art will recognize that the various specific tasks and devices described herein in

connection with this embodiment may be altered significantly without departing from the scope

of the present invention. These and other changes and modifications which are obvious to those

skilled in the art are intended to be within the scope of the present invention.

Claims

WHAT TS CT AIMED TS:

1. A method for sending data from a transponder having at least one memory

and a transmission criteria comprising the steps of:

a. detecting the presence of an interrogator;

b. transmitting the data;

c. determining whether the interrogator read the complete data

transmission while the transponder is transmitting the data and upon determining that the

interrogator did not read the complete data transmission;

i. storing a number in the memory;

ii. changing the number;

iii. iteratively repeating the step of changing of the number

until the number stored in memory salsifies the transmission

criteria;

iv. transmitting the data;.

2. The method as recited in claim 1, further comprising iteratively repeating

step (c) until the transponder determines that the interrogator has read the complete data

transmission.

3. The method as recited in claim 2, wherein the step of detecting the presence

of an interrogator comprises receiving a carrier signal from the interrogator.

4. The method as recited in claim 3, further providing the step of powering the

transponder.

5 The method as recited in claim 4, wherein the transponder has a transmit

state which can be either transmit-armed or transmit-unarmed and wherein the powering step

comprises turning the transponder on, setting the transmit state to transmit-armed, and setting the

memory to a number which satisfies the transmission criteria

6 The method as recited in claim 5, wherein the step of turning the

transponder on comprises generating power from the carrier signal

7. The method as recited in claim 5, wherein the transponder has a full cycle flag and wherein the step of transmitting the data comprises determining that the transponder is

in the transmit-armed state and that the carrier signal has been modified in a predetermined manner,

transmitting the data in groups of one or more bits, performing the determining step after each

group of data bits is transmitted, iteratively repeating the steps of transmitting the data in groups

and performing the determining step, and setting the full cycle flag after verifying that all the data

bits have been transmitted

8. The method as recited in claim 7, wherein the step of the transponder

determining that the interrogator read the complete data transmission comprises verifying that the

carrier signal has not been modified until the full cycle flag is set

9. The method as recited in claim 1, wherein the step of storing a number in

memory comprises generating a random number and assigning the number to memory

10 The method as recited in claim 1, wherein the changing the number step

comprises determining that the carrier signal has been modified in a predetermined manner and

altering the number in memory each time the carrier signal is modified in a predetermined manner

1 1 The method as recited in claim 9, wherein the number is altered by

decrementing by one

12 The method as recited in claim 11, wherein the transmission criteria is

satisfied when the number equals zero

13 A method for an interrogator reading one or more RFID transponders in

a field comprising the steps of

a providing a carrier signal,

b detecting the presence of at least one transponder,

c receiving data from all active transponders in the field,

d determining whether the interrogator has received a valid data

transmission, and

e upon determining an invalid data transmission, modifying the carrier

signal to inform all active transponders in the field that there was an incomplete read

14 The method recited in claim 13, wherein steps (c) and (d) are iteratively

repeated until the interrogator determines that is has read the complete data for each transponder

in the field

15 The method recited in claim 14, further comprising the step of transmitting

the complete data for each transponder from the interrogator to a computer system for processing

16 The method recited in claim 14, wherein the interrogator includes a

demodulator and the step of detecting the presence of one or more transponders comprises

receiving data from the demodulator and modifying the carrier signal in a predetermined manner

17. The method recited in claim 16, wherein the step of modifying the carrier

signal in a predetermined manner comprises sending out the carrier signal continuously.

18. The method as recited in claim 14, wherein the step of determining whether

the interrogator has received a invalid data transmission comprises detecting the interrogator 's

inability to compute a proper synchronization word, a proper CRC, or an proper word length.

19. The method as recited in claim 14, wherein the step of modifying the carrier signal in a predetermined manner comprises suppressing the carrier signal for a predetermined number of clock cycles.

20. The method as recited in claim 14, wherein the step of modifying the carrier signal is performed prior to the transponder sending its complete data transmission.

21. The method as recited in claim 20 wherein the step of modifying the carrier

signal is performed substantially simultaneously upon the determination that invalid data

transmission has been received.

22. The method as recited in claim 14, wherein the receiving step comprises

receiving the data in groups of one or more bits and checking the validity of each group of data

as it is received.

23. The method as recited in claim 14, wherein the step of determining complete

data transmission comprises determining that the CRC is valid and modifying the carrier signal in

a predetermined manner.

24. The method as recited in claim 21, wherein step of modifying the carrier

signal in a predetermined manner comprises suppressing the carrier signal for a predetermined

number of clock cycles.