How to Read and Write RFID Tags: It’s Way More Than Copy-Paste
Alright, so you’ve got a pile of blank RFID tags and a reader. You want to read and write them—to put your own serial numbers on them. Sounds simple: read the blank, write the number, verify. In reality, it’s a dance with a stubborn partner. The “write” part is where most hobbyist projects die and where professional systems earn their keep. Let’s walk through what actually works, and what will make you pull your hair out.
First, let’s kill a myth. You can’t just write anything, anywhere on a tag. Think of a tag’s memory like a book with locked chapters:
- Chapter 1 (TID): Glued shut. Written at the chip factory. You can only read it. It’s the tag’s DNA.
- Chapter 2 (EPC): This is your main notepad. This is where you write your unique ID—the one your software will look for. Most of your work is here.
- Chapter 3 (User Memory): An extra notepad in some fancier tags. Want to store a manufacture date? It goes here. Many cheap tags don’t have this.
If you’re just slapping IDs on assets, you’re living in Chapter 2: the EPC bank.
The Gear: Not All Readers Can Write
This is the first trap. That $20 RC522 module for Arduino? It can write, but only to very specific, old types of 13.56 MHz tags (MIFARE Classic). It’s useless for modern UHF tags.
For UHF—the kind on warehouse boxes—you need a proper UHF reader/writer. A basic inventory handheld might only read. You need one that explicitly says it supports EPC Gen2 Write commands. Most of CYKEO’s handhelds and all their fixed readers can do this. The hardware difference is internal; it needs to be able to hold a stable, powerful signal long enough to pump data into the tag, which is harder than just listening.
The Actual Process: Where Things Get Annoying
Let’s say you have a CYKEO CR23 handheld and a blank UHF sticker tag.
Step 1: The Read (The Reality Check)
You point the handheld at the tag and pull the trigger to read. If it’s truly blank, the EPC might be all zeros or a generic factory code like 0000_0000. This step isn’t about getting useful data; it’s about confirming the tag is alive and in range. If you get nothing here, writing will fail 100% of the time. Check your reader’s power setting. For writes, I crank it to max (30 dBm) to be safe.
Step 2: The Write (The Tense Moment)
This isn’t typing text. You’re writing hexadecimal numbers. Your serial number “1001” becomes 0x03E9 in hex, but you have to pad it to the tag’s length. The software (like CYKEO’s Tag Manager) has a field where you type the hex string.
You hit “Write.” Here’s what actually happens inside that second:
- The reader shouts a special “I’m about to write” signal at the tag.
- The tag locks on, using more power than usual.
- The reader pumps the new hex code into the tag’s memory cells.
- The tag sends back a quick “got it” signal (or, more often, silence if it failed).
Step 3: The Verify (The Non-Negotiable)
You immediately read the tag again. The handheld should now show your hex code in the EPC field. If it shows the old blank code, the write failed. Do not assume success. Always verify.
Why Your Write Failed (The List From My Notebook)
I have a checklist on my wall for this. In order of likelihood:
- Tag is Moving or Too Far: Even a slight wobble during the write cycle corrupts it. Hold the reader rock-steady, an inch away.
- Weak Signal: The reader’s RF power was too low. Writes need more juice than reads. Crank it up.
- Bad Hex Data: You tried to write “1001” as text, not as the hex equivalent
0x03E9. The software will yell at you with a format error.
- Wrong Tag Type: You’re trying to write a UHF command to an HF tag. Or you bought cheap “pre-encoded” tags that are actually locked. Always test with a sample first.
- Interference: Your phone, a Wi-Fi router, or another RFID reader nearby is blasting noise on the same frequency. Try a different location.
A Real-World Example: Encoding 100 Tools
Here’s my process for batch encoding:
- Test One: Take one tag from the batch. Write a test code like
AAAA1111. Verify. If this fails, the whole batch might be bad.
- Setup the Job: In the software, I set the start number (e.g.,
TOOL_0001 converted to hex) and enable auto-increment.
- The Rhythm: Tap a tag (write), hear the good beep, flip tag aside. Tap next tag. The reader automatically changes the number. It’s mindless but tense.
- Spot Verify: Every 10th tag, I do a separate read scan with a second reader to double-check. Once found a whole sequence was off by one digit because my hand blocked the antenna on tag #23.
Learning how to read and write RFID tags is less about coding and more about process discipline and RF hygiene. The read is easy. The write is where you battle physics. Start by successfully writing and verifying one tag ten times in a row. Once you have that repeatability, you can scale. If you can’t get that first tag to work, check your hex, check your power, and hold the damn reader still.

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