TAG FUNDAMENTALS CHAPTER 3
READER INTERFACE GUIDE
DOC. CONTROL # 8101938-000 REV 05
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mechanism allows commands to be sent to one specific tag, a selective group of
tags, or the whole field of tags.
To understand the use of masks, a basic understanding of tag memory structure
is first required.
Class 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.
Checksum EPC Code (or User ID Code)
Lock
PC
yte
0 1 0 1 2 3 4 5 6 7 0 0
it
0-7 8-15 0-7 8-15 16-23 24-31 32-39 40-47 48-55 56-63 0-7 0-7
Class I Tag Memory Structure
The 64-bit ID Code (either a fully qualified EPC code or user-defined ID Code) is
address from left to right, where the leftmost bit (the Most Significant Bit) is bit 0,
and the rightmost bit (the Least Significant Bit) is bit 63. There is no restriction on
the data that resides in this portion of the tag.
The checksum is calculated over the 64 bits of the ID Code 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 PassCode (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 PassCode, 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 chapter.
Addressing a Subset of Tags
One of the more useful applications of the mask command is to address a subset
of tags in the field. This is achieved using partial masks.
For example, the following mask command can be issued to address only tag IDs
that start with the numbers "8000 0040":
set Mask = 32, 0, 80 00 00 40
The first value is the length of the mask - 32 bits (4 bytes x 8 bits/byte). The
second value is the starting position in the ID - bit 0. Finally, a sequence of hex
bytes is given specifying the mask – 80 00 00 40.
Subsequent commands that use a mask will now only be recognized by tags that
start with this tag ID. This can be useful if, for example, the reader is scanning
food items but is only interested in finding a certain brand of breakfast cereal. By
setting the mask to identify only the breakfast cereal tag IDs, any acquire
command on the food items will only return the items of interest. This
methodology works particularly well when combined with the EPC code strategy,
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