EM4022
Free running example
Transponder 1
Transponder 2
Transponder 3
Data stream in collision
Data stream detected
Fig. 10
In the EM4022 the ACK signal is implemented as two
consecutive gaps with the appropriate timing and
received at a specific time after a code has been
transmitted.
Bi-directional protocols
Allowing bi-directional communication between reader
and transponders can speed up the basic free-running
protocol.
Communication from the reader to
The slow-down mode is a compromise between the free-
running mode and the switch-off mode. Etch time a
transponder is read, the reader send an ACK to double
the random repetition rate. This reduces the collisions
and in the time increases the saturation level.
transponders is achieved by turning the illuminating
energy field off for short periods. The transponders
detect these gaps in the energy transmission and
interpret them as required.
Figure 12 show a typical case of this mode of operation.
Switch-off and Slow-down Modes
Reducing the effective population of transmitting
transponders in the reader field can speed up the free-
running protocol. One method to achieve this is by either
switching transponders off or slowing them down once
they have been detected. To achieve this, the reader
sends an ACK signal to a transponder after its code has
been successfully received. The transponder then either
switches off completely or reduces its repeat rate until it
is powered down. This reduces the number of collisions
between transponder transmissions, thereby reducing
the time required to read a group of tags.
Fast Mode
A second method of speeding up the reading of tags, is
to inhibit other transponders from transmitting while one
transponder is transmitting. This is done by sending a
MUTE signal to all of the transponders when the start of
a transmission is detected. The transponders stay muted
long enough (128 bit of its own clock) to allow the
transmission of one code (see figure 13). This allows the
transponder that has started transmitting to complete its
transmission without any collisions.
The other
transponders continue with their own protocols
automatically after a time out, or continue immediately
upon detection of an ACK signal indicating that the
transmission that caused the MUTE has been
completed.
Figure 11 shows a typical situation where a collision
occurs between transponder 1 and 3. Then, as soon as
transponder 2 is read, the reader sends an ACK signal to
this one switching it off as long hi is powered up from the
field. This eliminates the collision between transponder 1
and 2 in the next step The Switch-off protocol’s main
advantage is that identical transponders can be counted.
In the EM4022 the MUTE signal is implemented as a
single gap received while the transponder is not
transmitting.
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