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T55xx 125kHz RFID write protocol
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This is a set of example captures from portable RFID copiers.

Details:
http://www.ebay.com/itm/RFID-125KHz-EM4100-Card-Copier-Duplicator-5-Writable-Cards-5-Keyfobs-Tags-/151237092163?hash=item23366ee343:g:lNkAAOxyYSdTBrVN
http://www.ebay.com/itm/Handheld-Portable-125Khz-RFID-Duplicator-Copier-Writer-for-EM4100-T5577-Card-/191630611446?hash=item2c9e1313f6:g:D7UAAOSwyQtVo4pM
http://priority1design.com.au/t5557_rfid_transponder.html
http://www.atmel.com/Images/Atmel-9187-RFID-ATA5577C_Datasheet.pdf


Logic analyzer setup
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The logic analyzer used was a Saleae Logic 16 clone (at 2MHz):

  Probe       T5557 write
  -----------------------
  1           RFID signal


Data
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To transmit data to the tag you turn on and of the coil for x amount of
field clocks. This type of modulation is called OOK.

The purpose of these RFID copiers are to read a generic EM4100 tag and
copy the tag data to another tag. To do that you need to configure the
destination tag and write specially formatted data in the tag banks.
Thus the signal contains 4-5 write packets.