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Difference between EPC and TID

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amal View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 8:31pm
The first thing you'll want to do is convert your thinking to bits and bytes, not characters. The ID values above are HEX representations of 24bit IDs. A word is 4 bytes or 32 bits. As you can see above, those "48 characters" are really only representing 24 bits. That's most likely because the EPC Gen2 standard requires that the TID memory be comprised of a 12-bit Tag Mask-Designer Identifier (Tag MDID) and a 12-bit Tag Model Number. While the ISO/IEC 15963 standard requires that the TID memory be comprised of an 8-bit tag manufacturer ID and a 48-bit tag serial number.

Also, there are some special tags out there that contain a randomizer that changes some or all of the TID bits on every read. This is sometimes used as an encryption salt, as well as a few other purposes. However, I seriously doubt you've got one of these special tags.

Can you clarify that the three separate TIDs you obtained for the same tag were from the same reader (3 separate reads producing 3 different TIDs), or were these different TIDs obtained from 3 separate readers?


Edited by amal - 29 January 2012 at 8:31pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aliweb Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 8:32pm
One more interesting thing. I used another Invengo XC2900 handheld reader to check that tag and found out it is also showing only first 16 characters for TID even though it has the option of changing data length just like DL6940. I changed data length to 12 but it had no effect.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aliweb Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 8:33pm
Regarding your question, I got those TIDs from the same reader which is DL6940. And these tags actually came with this reader.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 8:35pm
Hmm, did you get each of those different TIDs using the same reader settings, or were you changing settings between each read?

Either way, I think your best bet at this point is to see if you can contact your supplier to get a spec document for the tags you're using, complete with a memory map that details which memory blocks contain the TID and what the nature of it is.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 8:36pm
PS, the bottom line here is that a tag should not have 3 different TIDs. Unless it is a special tag with a randomizer component built in, the TID should not change.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aliweb Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 8:41pm
You said " A word is 4 bytes or 32 bits" but in the documentation they have mentioned:

In G2 tag operation, the word unit is used frequently. 1 word equals 2 bytes.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 8:47pm
Hmm, some times a "word" can represent different bit values, depending on the system you're talking about. Apparently in "G2 tag operation", it means 2 bytes or 16 bits. Odd then that a 4 or 8 word IDs could mean 64bit or 128bit IDs.

I'm sticking to the idea that to speed things along, you should try to get a spec doc from your supplier for the tags you're reading.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aliweb Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 8:50pm
Yeah I am going to ask for detailed tag spec from supplier. Thanks for your help because I learned a lot and got to know things I didn't know before.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aliweb Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 10:05pm
I have another question. What's the advantage of using single frequency over frequency rane?
I mean the readers I have got the option to change working frequency from 902MHz to 928MHz. I can either select a range for e.g. 910MHz to 915MHz or single working frequency for e.g. 915MHz for a reader so I wanted to know the difference between these two.
Will using single frequency increase signal strength and reading range of reader?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 08 May 2012 at 6:52pm
Sorry for the late reply... I lost track of this thread. Some UHF readers can adjust their frequency (but still remain firmly in UHF territory) to try to avoid various sources of interference from different kinds of equipment. Being able to adjust the frequency the reader operates at is a way to mitigate the effects of this interference, as well as avoid interfering with other devices that are operating in the 900MHz spectrum. Some readers can intelligently scan the spectrum from time to time and automatically adjust it's frequency in response to these kinds of interference issues, which is a preferred mode of operation.

However, there are readers that use a "scan range" approach. They don't employ any intelligence, but instead continuously adjust their own operating frequency essentially scanning through the entire operational range looking for tags. This is the "shotgun" approach to RFID, in which the entire frequency range is dominated by a single device. By hitting every single possible frequency within it's configurable range, the reader doesn't care if there is another device operating at 912.68MHz, it will bulldoze right through that frequency range every few seconds (or less). While the reader is using 912.68MHz to scan for tags, it will be interfering with the other device using that frequency, and it will not likely be able to talk to tags very well at that frequency. No matter though, the reader will just hop to the next frequency and try again. It's a very "dirty" way to operate a reader.

There is one positive reason for a frequency scanning UHF reader, and that is in situations where there are no other devices operating in the 900MHz spectrum, but there are other interference issues like metal or liquids in the area, either from equipment or walls or whatever else there might be in the way. A frequency range approach helps mitigate these interference issues by trying many different frequencies, which all behave slightly differently when propagating through the same space and bouncing off the same walls. One wall might contain materials that reflect 913.456MHz but pass 910MHz just fine. A metal shelving system might interfere with 912MHz but be fine with 915MHz. It's hard to tell what kinds of interference issues might cause reliability problems in warehouse or manufacturing environments, so being able to simply scan an entire range of frequencies to get the maximum range and reliability out of your reader can be a good thing when used in controlled situations.
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