Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 207

7.2.4.3 Binary searchalgorithm

The implementation of a binary search algorithm requires that the precise bit position
of a data collision is recognised in the reader. In addition, a suitable bit coding is
required, so we will first compare the collision behaviour of NRZ (non-return-to-zero)
and Manchester coding (Figure 7.19). The selected system is an inductively coupled
transponder system with load modulation by an ASK modulated subcarrier. A 1 level
in the baseband coding switches the subcarrier on, and a 0 level switches it off.
NRZCode The value of a bit is defined by the static level of the transmission channel
within a bit window (t BIT ) . In this example a logic 1 is coded by a static 'high' level;
a logic 0 is coded by a static 'low' level.
If at least one of the two transponders sends a subcarrier signal, then this is inter-
preted by the reader as a 'high' level and in our example is assigned the logic value
1. The reader cannot detect whether the sequence of bits it is receiving can be traced
back to the superposition of transmissions from several transponders or the signal from
a single transponder. The use of a block checksum (parity, CRC) can only detect a
transmission error 'somewhere' in the data block (see Figure 7.20).
Manchester code The value of a bit is defined by the change in level (negative or
positive transition) within a bit window ( t BIT ). A logic 0 in this example is coded by a
positive transition; a logic 1 is coded by a negative transition. The 'no transition' state
is not permissible during data transmission and is recognised as an error.
If two (or more) transponders simultaneously transmit bits of different values then
the positive and negative transitions of the received bits cancel each other out, so
that a subcarrier signal is received for the duration of an entire bit. This state is not
permissible in the Manchester coding system and therefore leads to an error. It is thus
possible to trace a collision to an individual bit (see Figure 7.20).
We will use Manchester coding for our binary search algorithm. Let us now turn
our attention to the algorithm itself.
A binary search algorithm consists of a predefined sequence (specification) of inter-
actions (command and response) between a reader and several transponders with the
objective of being able to select any desired transponder from a large group.
For the practical realisation of the algorithm we require a set of commands that
can be processed by the transponder (Table 7.3). In addition, each transponder has a
unique serial number . In our example we are using an 8-bit serial number, so if we

0 1

1 0

t BIT t BIT

t BIT t BIT

Manchester code NRZ code

Figure 7.19 Bit coding using Manchester and NRZ code