What Is an RFID Tag?
Ray Floyd posted on February 21, 2014 |

RFID systems are relatively simple: there are readers and there are tags. Of the two major components, it is the tag that offers the greatest variation of choices that must be made for the particular application of the technology. There are many types of tags, but they can all be found in one of three general classes: passive, semi-passive or active. The final decision of which type to use will be driven by the requirements of the application. The application requirements must address such parameters as data storage, read distance, read speed, off tag sensors, physical constraints, multiple tag environments and other application specifics. What then are the options available to the system designer?


Passive Tags
Passive tags are the most prominent tag in use within RFID today. In general they are the most simple in design. The tag does not contain a battery, and depends on the strength of the reader RF signal to cause the tag to generate a response. In general, the passive tag contains a serial number, typically 96 to 128 bits in length. The serial number will most often be just a serial number with no connection to a particular product or application. The serial number can be read and then used to establish a relationship to a product within an application data base.

Courtesy - Wikipedia

Since the passive tag does not contain a battery and depends on the reader signal to generate a response, the read range is typically short, ranging from a few inches to no more than 10 feet. The read speed is slow with reads taking 25 to 50 milliseconds to complete. From these two parameters, applications that come to mind would be personnel access, gated community access control, parking lot access and billing, and similar applications. In each application the tag moves at a relatively slow speed, allowing time to read the tag information and make the correlation to information within the database.

The tag may take many forms, from identification badges to license plate encapsulated tags to flexible tags for attaching to curved surfaces. Tag prices range from a few pennies to $10.

In some documentation, the passive tag is also referred to as a WORM device (write once, read many), as once the serial number is written it cannot be changed, but can be read millions of times.


Courtesy - Wikipedia
Semi-Passive Tags
The semi-passive tag operates similarly to the passive tag, using the reader signal to cause a response from the tag. The primary difference is that the semi-passive tag does have a battery, not for generating a response, but to power electronics that are used in conjunction with off-board sensors such as a thermal sensor. The sensor reading is incorporated into the tag return signal along with the tag serial number. Unsurprisingly, the semi-passive tag has most of the limitations noted for the passive tag in terms of slow read speeds and short read distances.

The price point for semi-passive tags is higher than the passive tag, with prices ranging from $10 to $50.


Courtesy - PrePass, a service provided by HELP Inc.
Active Tags
Active tags contain a battery and do not depend on the reader signal to generate a response. As a result, the active tag can be read at much greater distances, with read distances up to 100 yards seeming quite common. Active tags may be either read only or read/write, thus allowing data modification by the reader. Data storage on active tags has a range up to 8K bytes, a common value. Data rates are also faster in the active tag, thus making electronic toll collection and weigh station bypass highly successful applications of RFID. There are also active tags that provide off-tag communication via RS232/RS485 protocols, or JBUS to provide such information as fuel levels and odometer readings for transportation gate control applications. As one might suspect, the active tag is more expensive with prices ranging from $20 to over $100 per tag.

The variety of tags is almost endless. In any given application the designer must decide the application requirements and then look at the offerings. The choice may be driven by performance, cost, both, or even something else – as there are many choices driving the final solution.

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