SRIX4K Anticollision
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7 Anticollision
The SRIX4K provides an anticollision mechanism that searches for the Chip_ID of each
device that is present in the reader field range. When known, the Chip_ID is used to select
an SRIX4K individually, and access its memory. The anticollision sequence is managed by
the reader through a set of commands described in Section 5: SRIX4K operation:
● Initiate()
● Pcall16()
● Slot_marker().
The reader is the master of the communication with one or more SRIX4K device(s). It
initiates the tag communication activity by issuing an Initiate(), Pcall16() or Slot_marker()
command to prompt the SRIX4K to answer. During the anticollision sequence, it might
happen that two or more SRIX4K devices respond simultaneously, so causing a collision.
The command set allows the reader to handle the sequence, to separate SRIX4K
transmissions into different time slots. Once the anticollision sequence has completed,
SRIX4K communication is fully under the control of the reader, allowing only one SRIX4K to
transmit at a time.
The Anticollision scheme is based on the definition of time slots during which the SRIX4K
devices are invited to answer with minimum identification data: the Chip_ID. The number of
slots is fixed at 16 for the Pcall16() command. For the Initiate() command, there is no slot
and the SRIX4K answers after the command is issued. SRIX4K devices are allowed to
answer only once during the anticollision sequence. Consequently, even if there are several
SRIX4K devices present in the reader field, there will probably be a slot in which only one
SRIX4K answers, allowing the reader to capture its Chip_ID. Using the Chip_ID, the reader
can then establish a communication channel with the identified SRIX4K. The purpose of the
anticollision sequence is to allow the reader to select one SRIX4K at a time.
The SRIX4K is given an 8-bit Chip_ID value used by the reader to select only one among up
to 256 tags present within its field range. The Chip_ID is initialized with a random value
during the Ready state, or after an Initiate() command in the Inventory state.
The four least significant bits (
b
0
to b
3
) of the Chip_ID are also known as the
Chip_slot_number. This 4-bit value is used by the Pcall16() and Slot_marker() commands
during the anticollision sequence in the Inventory state.
Figure 21. SRIX4K Chip_ID description
Each time the SRIX4K receives a Pcall16() command, the Chip_slot_number is given a new
4-bit random value. If the new value is 0000
b
, the SRIX4K returns its whole 8-bit Chip_ID in
its answer to the Pcall16() command. The Pcall16() command is also used to define the slot
number 0 of the anticollision sequence. When the SRIX4K receives the Slot_marker(SN)
command, it compares its Chip_slot_number with the Slot_number parameter (SN). If they
match, the SRIX4K returns its Chip_ID as a response to the command. If they do not, the
SRIX4K does not answer. The Slot_marker(SN) command is used to define all the
anticollision slot numbers from 1 to 15.
ai07668b
b7 b6 b5 b4 b3 b2 b1 b0
8-bit Chip_ID
b0 to b3: Chip_slot_number
