Transponder identification system

The system seeks to speed up the identification of transponders 12, 14, 16 by identification or interrogating devices 2, 4 in situations in which the devices 2, 4 may have overlapping detection fields 6, 8. The devices 2, 4 may initially be operated simultaneously (in non-exclusive mode) for each device to determine the identities of any transponder in its field. A transponder 14 in an overlap region 10 of the fields 6, 8 may not be identified during this process because the conflicting signals received by transponder 14 from devices 2, 4 may interfere. The devices 2, 4 are therefore next operated sequentially (in exclusive mode). For each transponder which it has identified itself, a device, such as device 2, then establishes an identification code which when transmitted in whole or in part will cause the transponder to provide a distinguishing response. This code may be the unique ID code of the transponder, but is preferably an abbreviated code. As device 2 transmits an abbreviated code to each of its respective transponders in the exclusive mode, the or each other device 4 obtains the full identity of each such transponder, either over a dedicated communications channel between the devices 2, 4 or by monitoring the transmissions made by device 2 to the transponders. Identification device 2 then tests whether any transponder already identified, either by itself or by another identification device 4, lies within its respective detection field. Once this has been done by device 2, another identification device 4 begins its own independent operation. The entire process, commencing with the simultaneous operation of devices 2, 4, is then repeated periodically.