Standardisation - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 224

for the response from a full/half duplex transponder — a sequential transponder in the
field requires the activation field to charge up its charging capacitor.
If a full/half duplex transponder is present within the range of the activation field,
then this transponder sends its data during the operating interval of the field (2 in
Figure 9.1). While data is being received the operating interval can be extended to
100ms if the data transfer is not completed within the first 50ms.
A sequential transponder in the range of the activation field (3 in Figure 9.1) begins
to transmit data within the 3ms pause. The duration of the pause is extended to a
maximum of 20ms to permit the complete transmission of a data record.
If portable or stationary readers are operated in the vicinity of one another, then
there is a high probability that a reader will emit its activation field during the 3ms
pause of the other reader. This would result in neither of the readers being able to
receive the data signal of a sequential transponder. Due to the relatively strong acti-
vation field in comparison to the field strength of a sequential transponder this effect
occurs in a multiple of the reader's normal read radius. Appendix C of the standard
therefore describes procedures for the synchronisation of several readers to circumvent
this problem.
Portable and stationary readers can be tested for the presence of a second reader
(B in Figure 9.2) in the vicinity by extending the pause duration to 30ms. If the
activation field of a second reader (B) is received within the 30ms pause, then the
standard stipulates that the activation field of the reader (A) should be switched on for a
maximum of 50ms as soon as the previously detected reader (B) switches its activation
field on again after the next 3ms pause. In this manner, a degree of synchronisation
can be achieved between two neighbouring readers. Because data is only transmitted
from the transponder to the reader (and the activation field thus always represents an
unmodulated HF field), an individual transponder can be read by two portable readers
simultaneously. To maintain the stability of the synchronisation, every tenth pause
cycle is extended from 3ms to 30ms to detect any other readers that have recently
entered the area.
Stationary readers also use a synchronisation cable connected to all readers in the
system. The synchronisation signal at this cable is a simple logic signal with low and
high levels. The resting state of the cable is a logic low level.

Duration (ms): 50 ms 3 50 ms 3 50 ms 3 50 ms

Activation field B:

Pause B:

Activation field A:

Pause A:
Switch reader on: 30 ms pause

Interference Synchronisation

Figure 9.2 Automatic synchronisation sequence between readers A and B. Reader A inserts
an extended pause of a maximum of 30ms after the first transmission pulse following activation
so that it can listen for other readers. In the diagram, the signal of reader B is detected during
this pause. The reactivation of the activation field of reader B after the next 3ms pause triggers
the simultaneous start of the pulse pause cycle of reader A