I'm using the "Motorola EMDK for .NET" for a personal project and I see TagData.TagID and TagData.MemoryBankData differ (when the MEMORY_BANK.TID is selected).
The .NET documentation isn't informative, it simply states:
TagData.TagID Property
Gets the Tag ID
I dug into the "Motorola EMDK for C" API, which is the low-level interface that the .NET SDK wraps around, and it provided more details:
UNIT8* pTagID
Tag ID, for C1G2 this field refers EPC Data.
...that clears things up, except that the value for MEMORY_BANK.EPC doesn't match TagData.TagID either!
I looked at the Class 1 Gen 2 specification ( http://www.gs1.org/sites/default/files/docs/uhfc1g2/uhfc1g2_2_0_0_standard_20131101.pdf ) and none of the fields described in the tag's data structures seem to correspond to this TagID property.
I did wonder if TagData.TagID is some arbitrary, opaque value generated by the SDK as a means of uniquely identifying each tag, but I noticed that the same values are used in different sessions, so it's definitely deterministic, but what does it mean?
I wrote a program that dumps all of the memory banks of the tags and the TagID property, this is what I got:
TagData.TagID: "0000000000000000000C7D16" Bank: MEMORY_BANK_EPC , BankData: "743E30000000000000000000000C7D16"
TagData.TagID: "0000000000000000000C7D16" Bank: MEMORY_BANK_RESERVED, BankData: "0000000000000000"
TagData.TagID: "0000000000000000000C7D16" Bank: MEMORY_BANK_TID , BankData: "E20060031A8A034E"
TagData.TagID: "0000000000000000000C7D16" Bank: MEMORY_BANK_USER , BankData: "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
TagData.TagID: "3330323135385F303031" Bank: MEMORY_BANK_EPC , BankData: "EB1728003330323135385F303031"
TagData.TagID: "3330323135385F303031" Bank: MEMORY_BANK_RESERVED, BankData: "0000000000000000030004180CFA"
TagData.TagID: "3330323135385F303031" Bank: MEMORY_BANK_TID , BankData: "E2801130200029431C9701C2"
TagData.TagID: "3330323135385F303031" Bank: MEMORY_BANK_USER , BankData: "00000000"
TagData.TagID: "54573630354C5F303037" Bank: MEMORY_BANK_EPC , BankData: "B63E280054573630354C5F303037"
TagData.TagID: "54573630354C5F303037" Bank: MEMORY_BANK_RESERVED, BankData: "0000000000000000030004190CFA"
TagData.TagID: "54573630354C5F303037" Bank: MEMORY_BANK_TID , BankData: "E2801130200021A11D4501C2"
TagData.TagID: "54573630354C5F303037" Bank: MEMORY_BANK_USER , BankData: "00000000"
TagData.TagID: "54573630354C5F303132" Bank: MEMORY_BANK_EPC , BankData: "D5AA280054573630354C5F303132"
TagData.TagID: "54573630354C5F303132" Bank: MEMORY_BANK_RESERVED, BankData: "0000000000000000030004190CFA"
TagData.TagID: "54573630354C5F303132" Bank: MEMORY_BANK_TID , BankData: "E2801130200021F11D4501C2"
TagData.TagID: "54573630354C5F303132" Bank: MEMORY_BANK_USER , BankData: "00000000"
It appears I didn't look closely enough!
The TagData.TagID is contained within the EPC, for example:
TagData.TagID: "3330323135385F303031" Bank: MEMORY_BANK_EPC , BankData: "EB1728003330323135385F303031"
EPC: "EB1728003330323135385F303031"
TagID: "3330323135385F303031"
According to the C1G2 specification, the beginning of the EPC is reserved as such:
0x0000 to 0x000F - StoredCRC - 0xEB17
0x0010 to 0x001F - StoredPC - 0x2800
0x0020 to 0x020F - Actual EPC value - 0x3330323135385F303031
0x0210 to 0x021F - XPC_W1 (Optional) - null
0x0220 to 0x022F - XPC_W2 (Optional) - null
That clears that up then!
But my question now is: can TagID be trusted to be globally unique like TID is? My understanding is EPC does not need to be unique, it's just an arbitrary string and is not a serial number. If this is the case, then what happens in case of an EPC collision?
There is no way to guaranty that any of these fields are unique. All of them can be updated by printer or reader. So you should trust to your RFID tags supplier or print them by your own. EPC collision is not a problem for reader, only for your software.