User's Manual

MPR series User’s Manual: Draft version 0.95 11/4/04 page 34
The return link uses a simple form of subcarrier modulation, F2F. Each bit time begins with a transition in
the tag state. To transmit a binary 0, the tag adds one transition in the middle of the bit. To transmit a
binary 1, 3 additional transitions are employed. (Thus, a string of binary 0’s has a fundamental frequency
of (1/Tbit), whereas the fundamental frequency of a string of binary 1’s is (2/Tbit), hence the name of this
coding scheme.) Each tag bit occupies _ of the time used for a reader bit, so that the nominal data rates are
about 140 Kbps in the US and 30 Kbps in Europe.
Instead of employing a single long RESET and synchronization for a sequence of exchanges, as is done in
class 0, class 1 provides packet-by-packet tag synchronization. Each packets starts with a 64 ms CW period
to power up any tags in listening range, followed by data. During binary tree traversal, the reader then
sends a binary 1 to mark the edge of a response time slot or ‘bin’, so the tags have no need to maintain an
accurate clock to time the edges of the 8 possible response bins. A tag that response begins its packet with
a fixed 8-bit preamble, followed by the next few bits of its EPC, or in the case of a full scroll the remainder
of the EPC.
Tags have six possible states: Power Up, Awake, Asleep, Reply, Program , and Dead. Responses to
commands depend solely on the current state and not on how the tag arrived there. The basic commands
are:
• ScrollID: a tag whose EPC bits match the filter bits responds with its complete EPC
• Quiet: a tag whose EPC bits match the filter bits goes to sleep
• Kill: Permanently disable the tag if a valid argument (passcode) is provided.
• PingID: a tag whose EPC bits match the filter responds with the next 8 bits of its EPC
• Talk: a tag whose EPC bits match the filter bits wakes up
• ScrollallID: all tags hearing this command respond with their full EPC
• Pincscroll: Optional command allowing quick scroll of full ID from any tag that is the sole
responder in a given bin.