>I have a system here that weighs trucks, but the operator have to
>enter the plate number of each truck and sometimes it weigh about
>1000 trucks a day, so it would be nice if I could put one transponder
>in each truck so when it arrives the transponder send this informations
>and some other.

        You should investigate the TIRIS system from Texas Instruments.
        This is a passive RF transponder, and is widely used around the
        world for the sort of application you describe.

        TI also, of course, sell the interrogator for reading the tags.
        You can get either the basic RF unit (which essentially outputs
        the demodulated RF data stream, from which you must extract the
        tag data) or an intelligent unit (which outputs a formatted RS-
        232 data stream).

        The tags themselves are available in large number of different
        packages, ranging from glass pellets for injection into cattle
        up to solid blocks for attachment to railway cars.

        I believe the also now sell a user programmable tag.

        Other well-known names for similar products are Destron, and
        Trovan.

___Bob

PS:     A quick alta-vista search on 'passive transponder' certainly
        gives you a useful links, including 'www.trovan.com'.