定义 NDEF 消息

本文介绍了定义 NDEF 消息的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我正在使用 Raspberry Pi 和 RFID-RC522 板开展 Python 项目.作为 NFC 标签，我使用 NXP NTAG213.我现在的计划是在标签上存储链接.我可以毫无问题地读/写它们.但我不明白如何为标签上存储的数据定义 NDEF 标头.

当我用智能手机在标签上写一个链接并用我的程序读取它时，标签上存储的数据如下所示:

网址http://www.gmx.at"的 NDEF 标头是

[3, 11, 209, 1, 7, 85, 1, ... (数据)]

当我编写另一个链接时，我意识到其中一些参数会发生变化，但有些参数仍然保持不变.

我发现了这个教程它描述了 NDEF 标头的不同字段，但我仍然不知道如何设置它们以存储指向网站的链接.

如果有人能描述我需要如何正确计算/定义链接的参数，我会很高兴.

解决方案

为了了解 NDEF 格式以及 NDEF 格式的数据如何存储在 NFC Forum Type 2 标签(这是由NTAG213)，建议您阅读以下NFC论坛规范:

第 2 类标签操作规格
NFC 数据交换格式 (NDEF)
URI 记录类型定义

您从标签中读取的数据是一个 NDEF 消息 TLV 对象，其中包含一个 NDEF 消息，该消息由一个 URI 记录组成.

NDEF 消息 TLV:
0x03 TLV 标签 = NDEF 消息 TLV0x0B TLV 长度 = 11 字节0xD1 ... 0x74 TLV 值 = NDEF 消息
这意味着该标签包含一个长度为 11 个字节的 NDEF 消息.NDEF 消息是 0xD1 ... 0x74.
NDEF 消息:
0xD1 记录头位 7 = MB = 1:NDEF 消息的第一条记录位 6 = ME = 1:NDEF 消息的最后一条记录位 5 = CF = 0:链的最后或唯一记录位 4 = SR = 1:短记录长度字段位 3 = IL = 0:无 ID/ID 长度字段位 2..0 = TNF = 0x1:类型字段代表 NFC 论坛众所周知的类型名称0x01 类型长度 = 1 字节0x07 有效载荷长度 = 7 字节0x55 类型字段 = "U"(在 US-ASCII 中) = 类型名称 urn:nfc:wkt:U 的二进制形式0x01 ... 0x74 有效载荷字段 = URI 记录有效载荷
这意味着 NDEF 消息包含一个 URI 记录(类型名称 urn:nfc:wkt:U)，跟在 URI 记录类型定义之后.
URI 记录负载:
0x01 标识符字节 = URI 前缀http://www."0x67 ... 0x74 URI 字段(UTF-8 编码)=gmx.at"
这意味着 URI 记录指向 URI http://www.gmx.at".

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.

解决方案

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".

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