Defining a NDEF Message

问题

I am working on a Python project with my Raspberry Pi and the RFID-RC522 board. As NFC tags I use NXP NTAG213. My plan now is to store links on the tags. I can read/write on them without a problem. But I don't understand how to define the NDEF header for the stored data on the tags.

When I write a link on the tags with my smartphone and read it with my program the stored data on the tag looks like this:

The NDEF header for the URL "http://www.gmx.at" is

[3, 11, 209, 1, 7, 85, 1, ... (Data)]

I recognized that some of these parameters change when I write another link, but some still stay the same.

I found this tutorial which describes the different fields of the NDEF header but I still don't get how I need to set them to store a link to a website.

I would be very happy if someone can describe how I need to calculate/define the parameters for a link correctly.

回答1:

In order to understand the NDEF format and the way how NDEF formatted data is stored on an NFC Forum Type 2 tag (which is the tag platform that is implemented by the NTAG213), I suggest that you read the following NFC Forum specifications:

Type 2 Tag Operation Specification
NFC Data Exchange Format (NDEF)
URI Record Type Definition

The data that you read from the tag is an NDEF Message TLV object containing an NDEF message that consists of one URI record.

NDEF Message TLV:

0x03             TLV tag = NDEF Message TLV
  0x0B           TLV length = 11 bytes
  0xD1 ... 0x74  TLV value = NDEF message

This means that the tag contains an NDEF message with a length of 11 bytes. The NDEF message is 0xD1 ... 0x74.

NDEF Message:

0xD1             Record header
                   Bit 7 = MB = 1: first record of NDEF message
                   Bit 6 = ME = 1: last record of NDEF message
                   Bit 5 = CF = 0: last or only record of chain
                   Bit 4 = SR = 1: short record length field
                   Bit 3 = IL = 0: no ID/ID length fields
                   Bit 2..0 = TNF = 0x1: Type field represents an NFC Forum
                                         well-known type name
  0x01           Type length = 1 byte
  0x07           Payload length = 7 bytes
  0x55           Type field = "U" (in US-ASCII) = binary form of type name urn:nfc:wkt:U
  0x01 ... 0x74  Payload field = URI record payload

This means that the NDEF message consists of one URI record (type name urn:nfc:wkt:U) following the URI Record Type Definition.

URI record payload:

0x01             Identifier byte = URI prefix "http://www."
0x67 ... 0x74    URI field (UTF-8 encoded) = "gmx.at"

This means that the URI record points to the URI "http://www.gmx.at".

来源：https://stackoverflow.com/questions/35363563/defining-a-ndef-message

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