UHF Tag Memory: EPC, TID, User Memory, Lock & Reuse

The four memory banks of a Gen2 tag

Every tag built to the EPC Gen2 standard (ISO/IEC 18000-63) organises its memory in four banks. Readers address each bank by number.

The GS1 EPC Tag Data Standard describes the same layout and calls the TID bank "information about the tag itself, as opposed to the physical object to which the tag is affixed". Locking a bank stops it from being rewritten; it does not hide the EPC, which readers still see in every inventory.

Memory by chip, from the data sheets

Two details are easy to miss. Impinj's spec table lists the access and kill password as "shared" on every M700 chip, so check the chip data sheet before you plan different access and kill passwords. And the UCODE 9 data sheet lists its access password as hardwired to zero, so that chip cannot hold a password of your own: on a UCODE 9, protecting the EPC from rewriting means locking it permanently. The life figures in the last column are explained in our guide how long RFID tags last.

Our product pages state the memory of each tag. A few examples: the UFLT-5610 laundry tag has a 128-bit rewritable EPC, a 96-bit read-only TID, 32-bit access and kill passwords and no user memory; the UHF crate tag has an EPC of up to 496 bits and 512 bits of user memory; the IDTS-7545 on-metal tag has a 128-bit EPC and a 96-bit TID.

EPC vs TID: which number to use

The EPC is your number. You choose what goes in it: an item or asset number in a closed system, or a GS1 identity such as an SGTIN when tagged goods leave your hands (our SGTIN-96 encoding guide covers retail numbering). Readers return the EPC every time they see the tag, so it is the number your software works with.

The TID is the chip's number. The GS1 EPC Tag Data Standard, Release 2.3 (October 2025), sets out what it holds. Its first 32 bits carry the class identifier E2h, a 9-bit mask-designer identifier (MDID, the chip maker) and a 12-bit tag model number, and they must be permalocked at manufacture. An extended TID (XTID) can follow, with a serial number and a description of the chip's optional features, and it too is permalocked at manufacture. The standard adds that, as of Gen2v3, tags with class E2h shall support a serialised TID with a unique serial number. The UCODE 8 and UCODE 9 data sheets show this in practice: a 96-bit, factory-locked TID with a 48-bit unique serial number.

Use them together. Because the TID is written and locked at the factory, recording it next to the EPC when you encode lets your software spot a tag whose EPC matches your records but whose TID does not, and tell a dead tag from one that was badly encoded. For cloning and protection, see our RFID data security guide.

Write, lock, permalock and kill, in plain words

• Write. A reader sends new data to the EPC or user bank. Each write uses one of the chip's rated write cycles; reading uses none.
• Access password. A 32-bit password in the reserved bank. The default is zero. Once a non-zero password is written, a reader has to present it before it can change locked memory.
• Lock. Makes a bank writable only by a reader that presents the access password. It can be undone by whoever holds that password.
• Permalock. Makes the lock state permanent. A permalocked EPC can never be changed again, by anyone, including us, so permalock only data that is final.
• Kill. A reader that presents the tag's kill password can kill it, and a killed tag never responds again. The kill password defaults to zero, and a tag whose kill password is zero cannot be killed this way, so one has to be written first. The UCODE 9 data sheet describes its 32-bit kill password as there "to permanently disable the tag".

Can you reuse a UHF tag? Three real cases

Reuse is mostly about the housing, because the chips in this table allow at least 10,000 writes. Three cases where reuse works well:

• Returnable crates, kegs and bins. A hard tag such as our UHF crate tag (ABS/PC, IP68, 100,000 write cycles; ₹30 + GST) stays on the crate for its life. Most systems keep its EPC fixed as the crate number and record each trip in software; re-encoding is there if a crate changes owner or pool. See proving reuse targets on returnable packaging.
• Jewellery trays and hard tags. Jewellery tags are re-encoded as stock changes; our JT3515 jewellery hard tag lists 100,000 write cycles and 20 years of data retention. See the jewellery inventory guide.
• Event wristbands. A reusable C03 silicone wristband (₹65 + GST) can be re-issued for the next event, either re-encoded or re-mapped to a new attendee in the event system; a Tyvek band (₹22 + GST) is made for one event. See RFID for events.

Four things stop reuse: an adhesive label that has already been stuck down, an EPC that a previous user permalocked, a tag that was killed, and an antenna damaged by use. A tag bought from another supplier may also arrive locked with a password you do not have; ask before you buy second-hand stock.

How to put your own number on a tag

• At one PC: the UHF desktop reader 103 is a USB reader-writer that needs no driver or separate power supply, with a reading radius adjustable up to 30 cm and multi-tag reading and writing: ₹15,000 + GST.
• Batches from Excel: the 120L desktop reader comes with free utility software that imports data from an Excel sheet and writes it into tags in sequence: ₹28,000 + GST.
• Print and encode labels: the IT4RL desktop RFID printer and encoder prints a label and writes its chip in one pass, and its free Labelsoft software can print the tag's EPC or TID on the label for cross-checking: ₹1,35,000 + GST.
• Check in the field: the CX1500N handheld comes with CX Flow Pro on every unit, and CX Flow Pro is provided free with the CX1000. It covers inventory, finding an item, and importing a list to tally what is present and what is missing, which is how you confirm that a batch was encoded as planned.

Whichever you use, encode in a fixed order: write the EPC, read it back, record the EPC and TID together, and lock only once the batch checks out.

Related guides

• Impinj vs NXP chips: M700 vs UCODE 9
• NXP UCODE X vs Impinj M800
• RFID scanner to Excel in keyboard mode
• Glossary: TID, EPC, user memory and passwords