Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 208

NRZ coding

Transponder 1

101 10 010

Transponder 2

10101010

Combined signal
at the reader

Decoded
data stream

10111010

Manchester coding

Transponder 1

10110010

Transponder 2

10101010

Combined signal
at the reader

Decoded
data stream

1 0 1 ?? ?? 0 1 0

Figure 7.20 Collision behaviour for NRZ and Manchester code. The Manchester code makes
it possible to trace a collision to an individual bit

are to guarantee the uniqueness of the addresses (serial numbers) a maximum of 256
transponders can be issued.
The use of the commands defined in Table 7.3 in a binary search algorithm will now
be demonstrated based upon a procedure with four transponders in the interrogation
zone of the reader. The transponders in our example possess unique serial numbers in
the range 00-FFh ( 11111111 bin.) (Table 7.4).

The first iteration of the algorithm begins with the transmission of the command
REQUEST ( ≤ 11111111) by the reader. The serial number 11111111b is the highest
possible in our example system using 8-bit serial numbers. The serial numbers of all
transponders in the interrogation zone of the reader must therefore be less than or equal
to 11111111b, so this command is answered by all transponders in the interrogation
zone of the reader (see Figure 7.21).
The precise synchronisation of all transponders, so that they begin to transmit their
serial numbers at exactly the same time, is decisive for the reliable function of the
binary tree search algorithm . Only in this manner is the determination of the precise
bit position of a collision possible.

= 0 − 255 dec. or 00000000 −