Standardisation - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 225

If one of the connected readers detects a transponder, then the synchronisation cable
switches to the high level while data is transmitted from the transponder to the reader.
All other readers extend their current phase (activation/pause).
If the detected data carrier is a full/half duplex transponder, then the synchronised
readers are in the 'activation field' phase. The activation period of the activation field
is now extended until the synchronisation cable is once again switched to low level
(but with a maximum of 100ms).
If the signal of a sequential transponder is received, the synchronised readers are in
the 'pause' phase. The synchronisation signal at the cable extends the pause duration
of all readers to 20ms (fixed value).

9.1.2.2 Full/half duplex system

Full/half duplex transponders, which receive their power supply through an activation
field, begin to transmit the stored identification data immediately. For this a load
modulation procedure without a subcarrier is used, whereby the data is represented
in a differential bi-phase code (DBP). The bit rate is derived by dividing the reader
frequency by 32. At 134.2 kHz the transmission speed (bit rate) is 4194 bit/s.
A full/half duplex data telegram comprises an 11-bit header, 64 bits (8 bytes) of
useful data, 16-bit (2-byte) CRC and 24-bit (3-byte) trailer (Figure 9.3). After every
eight transmitted bits a stuffing bit with a logic 1 level is inserted to avoid the chance
occurrence of the header 00000000001. The transmission of the total of 128 bits takes
around 30.5ms at the given transmission speed.

9.1.2.3 Sequential system

After every 50ms the activation field is switched off for 3ms. A sequential transponder
that has previously been charged with energy from the activation field begins to transmit
the stored identification data approximately 1 to 2ms after the activation field has been
switched off.
The modulation method used by the transponder is frequency shift keying (2 FSK).
The bit coding uses NRZ (comparable to RS232 on a PC). A logic 0 corresponds with
the basic frequency 134.2 kHz; a logic 1 corresponds to the frequency 124.2 kHz.
The bit rate is derived by dividing the transmission frequency by 16. The bit rate
varies between 8387 bit/s for a logic 0 and 7762 bit/s for a logic 1 depending upon the
frequency shift keying.

Stuffing bit

“1”

“0”

Header

Identification CRC Trailer

Figure 9.3 Structure of the load modulation data telegram comprising of starting sequence
(header), ID code, checksum and trailer