User's Manual

CHAPTER 4 TAG READING FUNDAMENTALS

NANOSCANNER READER USER GUIDE • DOC # 8101029-000 REV A 32

Masks and Tag Memory Structure

Many commands aimed at Alien RFID tags require the setting of a mask, which

directs the commands only at the tags who’s ID matches the mask. This

mechanism allows commands to be sent to one specific tag, a selective group of

tags or the whole field of tags.

However to understand the use of masks, a basic understanding of tag memory

structure is first required.

LASS I TAG MEMORY

Class I tags from Alien contain 96 bits of programmable memory, of which 64 bits

are user-programmable. The remaining 32 bits are controlled by the reader to

record state and checksum information inside the tag.

Class I Tag Memory Structure

Tag memory is address from left to right, where the leftmost byte is byte zero,

and the leftmost bit is bit zero.

The 64 bit ID code (either a fully qualified EPC code or user-defined ID code)

resides from bytes 2 to 9, or bits 16 to 79.

Bytes 0 and 1 contain a 2 byte checksum calculated over the 64 bits of tag ID

only. The checksum is calculated and programmed into the tag automatically by

the reader. This checksum is calculated using the CCITT-16 standard.

The Lock and Pass Code (PC) bytes stored at the end of tag memory are used to

lock a tag and kill a locked tag. Each of these codes takes exactly one byte. The

user can control the value of the pass code, passing it in as a parameter to the

Lock command. The reader takes full control of the Lock byte, allowing it to flag

the tag as either locked or unlocked.

(For further details on programming tag IDs and tag memory, please see the Tag

Programming Guide).

CLASS BPT TAGS

Class BPT tags from Alien support 96 bits of programmable ID memory. Unlike

the Class I tag, all 96 bits are user-addressable and user-programmable. There

are no pre-requisites for the content of this memory space.

0 1 2 3 4 5 6 7 8 9 10 11

EPC Code (or User ID Code) Lock PC Checksum

Byte

Bit

0-7 8-15 16-23 24-31 32-39 40-47 48-55 56-63 64-71 72-79 80-87 88-95