Oklahoma has been using what I believe to be the TI tags in
their PikePass system which has been up and running for about 5 or 6 years.
I don't know if the heart of the system is like a PIC, but the tags which
are yellow plastic rectangular boxes about half the size of an audio cassette
contain a lithium battery which dies after about five years.  The strange
thing is that the processor in the tag is always active in that I can detect
a very faint amount of RF coming from it if I hold it near the ferrite
rod antenna on an AM radio.  I am guessing that when the tag is in view of
the microwave dish at the toll booth, the reflected signal carries the ID
number.  The processor, if it is that sophisticated, must gate a conductive
path that changes the way in which the tag reflects the signal.  I am
surprised that the processor doesn't go to sleep when no signal is there to
aluminate it.

        As for the potential for fraud, I think it is pretty low-tech and
consists of the same sort of stuff that dishonest people do with charge cards,
keys, or calling card numbers.  The account balance information is centrally
stored and the presence of a tag at a toll booth either starts or stops
the calculation of a toll.

        My guess is that the circuitry is probably more like a serial ROM
rather than any kind of processor.

Martin McCormick

In message <3.0.32.19970314064358.006afad4@huey>, Robert Lunn writes:
>        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.