And what kind of information does the reader transmit to a microcontroller—and how is this accomplished?
Let me flip your question around.
A reader usually initiates communication with a tag. This is true for all passive RFID systems, as well as for some active systems. Real-time location systems that use active RFID tags set the tags to broadcast their serial number every few seconds or minutes. Typically, a reader will broadcast a signal; if more than one tag responds, the device will issue another command instructing the tags to respond in a specific sequence, so that they can be read individually.
In simple terms, the reader might say "If the first digit in your serial number is a one, please respond" (readers are always polite). If two or more tags have a serial number that starts with one, the interrogator might then ask all tags that have a serial number starting with one and a second digit of zero to respond, and so forth—this is known as singulation.
The tag will respond back to the commands until it has shared its full serial number with the reader, which might then issue this command: "Please transmit the information stored in memory block one." The tag might have a shipping location stored in block one, or some other information, which it would then transmit to the reader. This communication between the tag and reader would take place in accordance with the specific air-interface protocol they were designed to use.
There are air-interface protocol standards adopted by the International Organization for Standardization (ISO), and there are also proprietary protocols. These protocols determine whether the method of communication is frequency shift keying, phase shift keying or amplitude shift keying. Essentially, a standard wave form is used to communicate a zero, and then a one is communicated by shifting the wave's frequency, phase or amplitude.
—Mark Roberti, Founder and Editor, RFID Journal
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