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SRT512

Anticollision

Doc ID 13277 Rev 4

23/46

Anticollision

The SRT512 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 SRT512 individually, and access its memory. The anticollision sequence is managed by

the reader through a set of commands described in Section 5: SRT512 operation:

●

Initiate()

●

Pcall16()

●

Slot_marker().

The reader is the master of the communication with one or more SRT512 device(s). It

initiates the tag communication activity by issuing an Initiate(), Pcall16() or Slot_marker()

command to prompt the SRT512 to answer. During the anticollision sequence, it might

happen that two or more SRT512 devices respond simultaneously, so causing a collision.

The command set allows the reader to handle the sequence, to separate SRT512

transmissions into different time slots. Once the anticollision sequence has completed,

SRT512 communication is fully under the control of the reader, allowing only one SRT512 to

transmit at a time.

The Anticollision scheme is based on the definition of time slots during which the SRT512

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 SRT512 answers after the command is issued. SRT512 devices are allowed to

answer only once during the anticollision sequence. Consequently, even if there are several

SRT512 devices present in the reader field, there will probably be a slot in which only one

SRT512 answers, allowing the reader to capture its Chip_ID. Using the Chip_ID, the reader

can then establish a communication channel with the identified SRT512. The purpose of the

anticollision sequence is to allow the reader to select one SRT512 at a time.

The SRT512 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 b3) 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 19.

SRT512 Chip_ID description

Each time the SRT512 receives a Pcall16() command, the Chip_slot_number is given a new

4-bit random value. If the new value is 0000b, the SRT512 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 SRT512 receives the Slot_marker(SN)

command, it compares its Chip_slot_number with the Slot_number parameter (SN). If they

match, the SRT512 returns its Chip_ID as a response to the command. If they do not, the

SRT512 does not answer. The Slot_marker(SN) command is used to define all the

anticollision slot numbers from 1 to 15.

ai07668b

8-bit Chip_ID

b0 to b3: Chip_slot_number

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