Example Applications - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 347

HF-enable

Reader 1

Entrance

Release signal

R eader 2

Reader 3

Turnstile

Entrance

Reader 4

MUX

Control unit

Busy signal

Figure 13.15 To achieve mutual decoupling the readers are switched alternately in time-
division multiplex operation

systems activate only one reader at a time, while the HF field of all other readers is
switched off altogether. To achieve this, a multiplexer sends a start signal to one reader
after the other in a cyclical sequence, which causes the addressed reader to switch on
its HF field and check for the presence of a transponder (Figure 13.15). If the reader
discovers a transponder, then it activates a busy signal . The busy signal tells the control
unit to suppress the cyclical start signal for the duration of the busy signal. The active
reader is now free to perform the data exchange it has begun with the transponder.
After the end of this transaction, the active reader stops transmitting the busy signal,
whereupon the multiplexer can continue its cyclical interrogation.

13.4 Access Control

Electronic access control systems using data carriers are used to automatically check
the access authorisation of individuals to buildings, (commercial or event) premises,
or individual rooms. When designing such systems we must first differentiate between
two fundamentally different systems with corresponding properties: online and offline
systems.

13.4.1 Online systems

Online systems tend to be used where the access authorization of a large number
of people has to be checked at just a few entrances. This is the case, for example,
at the main entrances to office buildings and commercial premises. In this type of
system, all terminals are connected to a central computer by means of a network.