NFC Interrogators and Tags

By Robert R. Sabella

When considering how two devices establish a connection, you can split Near Field Communication (NFC) into interrogators and tags. It doesn’t matter what the devices actually are; the terms reflect the roles that the devices play in the initial communication. This figure shows graphically how the connection works between the two.

interrogator-and-tag
The roles of interrogator and tag differ.

As you can see from the figure, one device is treated as powered, while the other isn’t. The sequence of steps that interrogator and tag rely on is as follows:

  1. The interrogator sends a signal to the tag. When the two devices are close enough, the signal powers the tag.

    The loosely coupled coils in the two devices actually create a high-frequency magnetic field between the two devices. After the field becomes established, the two devices can communicate.

  2. The interrogator sends the first message to the tag to determine what sort of communication the tag uses (such as Type A or Type B).
  3. The tag responds with the required information.
  4. (Optional) The interrogator establishes a secure communication channel with the tag when required.
  5. The interrogator sends out a command using the appropriate specification.
  6. The tag receives the command and determines its validity.
    If the command is invalid, the tag doesn’t do anything.
    If the command is valid, the tag responds with the appropriate data.
  7. Steps 5 and 6 repeat as needed to complete the communication.

The interrogator modulates the RF signal at 13.56 MHz. Be aware that the data transfers at a different rate. Depending on the tag type, transmission can occur at

  • 106 Kbps
  • 212 Kbps
  • 424 Kbps

This is also the software view of the process. The initiator sends commands to the tag that interrogate the tag in various ways. The commands sent depend on the phase of the process and the application used to perform the communication. Depending on the command, a tag could respond with information such as a serial number or data contained within its memory.