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EPC BRI and EPC Memory on


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									                  BRI and EPC Memory on Gen2 Tags

A Gen2 tag’s EPC memory bank is composed of three parts: CRC-16, Protocol Control
word (PC) and the EPC. ISO 18000-6C is equivalent to EPCglobal Gen2 with a some
small exceptions, one being that EPC is renamed to UII (unique identification ID). The
Gen2 tag memory banks are illustrated in the following diagram (using the ISO term
“UII” in place of EPC):

Using the BRI you can read any part of EPC (UII) memory. You can write any part of
EPC memory except for the CRC. You’ll get a BRI “privilege error” (PVERR) if you
attempt to write the CRC.

Gen2 protocol provides tag memory access on word boundaries (16-bits or even-byte
address and length). With BRI you can read tag memory on odd-byte addresses or an
odd number of bytes. Internally, the BRI reads whole words and truncates the odd
byte(s) you didn’t request. However, BRI requires that you write whole words (i.e., you
must write tag memory on even-byte addresses and with an even number of bytes).

Use the BRI keyword “HEX” to read and write tag memory. The syntax is:

<R | W> HEX(<bank>:<offset> , <length>)

R HEX(1:2,2)                  Read the PC in EPC Memory Bank
W HEX(1:4,2)=H3008            Write the 1st word of the EPC

It is important to understand two things about the PC:

   -   the 5 MSB’s (bits 10h-14h of the EPC memory bank) represent the length (# of
       words) of the EPC component.
   -   Bit 7 (bit 17h of the EPC memory bank) is set one for ISO 18000-6C encoded
       tags (zero for EPCglobal encoded tags).

EPC’s are often 96-bits (length=6 words). This corresponds to PC=3000h. If the EPC
length is 5 words, the PC=2800h. Length of 4 words and PC=2000h, etc.

The BRI keyword “EPCID” is an important feature to understand. When reading tags,
the keyword EPCID can be substituted for HEX(1:4, <length>) where length is defined
by the 5 MSB’s of the PC. Example:

R HEX(1:2,2)
H0800                         PC length field = 1 word
R HEX(1:4,4)
H3008FFFF                     EPC component + 1 word beyond EPC
H3008                         Only EPC component is returned

When writing tags, the keyword EPCID can be substituted for HEX(1:4, <length>) where
length is the number of words in the value you specify. Writing using the BRI keyword
“EPCID” accomplishes 3 things:

   -   writes the EPC component you specify
   -   writes the length you specify into the PC (5 MSB’s)
   -   sets bit 17h (PC bit 7) to zero (EPC encoding, not ISO)


R HEX(1:2,2)
H0900                         PC length field = 1 word. ISO tag encoding bit set.
H3008                         EPC component = 1 word.
W EPCID=H300833B2             Write 2 word EPC component
R HEX(1:2,2)
H1000                         PC length field = 2 words. ISO tag encoding bit cleared.
BRI session examples on a live tag. Note: in these examples the BRI attribute
IDREPORT is set ON. Therefore, a read command “R” defaults to R EPCID (i.e., the ID
is automatically reported).

                                         1) Typical EPCID
                                         2) Read PC bits
                                         3) Length = 6 words (12 bytes, 96 bits)
                                         4) Change Length  5

                                         5) EPCID now 1 word shorter

                                         6) Change Length  4

                                         7) EPCID now another word shorter
                                         8) Reset Length = 6
                                         9) Distance tag too weak to write

                                         10) EPCID now back to 6 words long
                                         11) Change 1st 2 bytes of EPCID

                                         12) Length remains 6 words
                                         13) Change EPCID as 1 word ID

                                         14) Length is now 1 word
                                         15) Restore length to 6

                                         16) Confirm EPCID length = 6

                                         17) Change Length  5

                                         18) EPCID Length is now 5 words

                                         19) Write EPCID as 2 word ID

                                         20) Length = 2

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