Access to electronic data tags on items handled in bulk - reader sends selection code shorter than identification code and awaits response from one or more items to a partial identification

摘要

The identification tag reader sends a selection code word (11) of known length to the items in a container. The selection word corresponds to part of the identification word of the tags. Each identification tag responds by comparing the received selection word with its own identification word, and if the selection word matches part of the identification word emits a signal, with a delay determined by the unused part of the identification word. If there is no response a longer selection word is used, while if there is collision of responses the length of the selection word is reduced. Collisions are detected using a third part of the identification word which contains a redundant code.