TransCore Amtech IT2200, Инструкция по установке, обслуживанию и ремонту

Страница: 31 / 204

Theory of Operation
2-7
Tags
Tags are small, self-contained RFID devices that are capable of bidirectional
communications with a reader. Tag circuitry contains an internal antenna through
which the tag modulates RF signals. Tags are available for interior and exterior
installation, and for system diagnostics (check tags).
Electronic circuitry in the tag can send a message to a reader either continuously or
upon command from a reader, depending on the tag type and application.
Tags and readers must be compatible. The RF generated by the reader system must be
compatible to the tag. For example a 915-MHz RF reader system will not be able to
read from or write to 2450-MHz tags.
Tags also have different memory sizes, varying according to application. Some
applications require only a small amount of memory, for example, 128 bits; while
others may require more memory, e.g., 256 bytes (2048 bits), to contain more data.
Read-Only and Read/Write Tags
Tags can be read-only or read/write. Electronic tags may simply return a fixed or
programmed message such as an ID code. This is referred to as a tag read. With a read/
write tag, data can be transmitted to the tag for archiving. This is referred to as a tag
write.
A read-only tag has an ID number preprogrammed either at the factory or the end
user's facility. This ID number cannot be changed; it can only be read. Read-only
applications have been used successfully for many applications, including high-
performance electronic toll systems. In these applications, the tag’s only function is to
signal its presence to the reader and provide the reader with its ID number.
Read/write tags have two general types. One type has memory that can be written into
only once, but can be read many times (called WORM memory). A second, more
flexible type has nonvolatile (EEPROM) memory that can be read and written to
repeatedly. Thus, information on the tag can be changed repeatedly by commands
issued by the host computer. With this advanced technology, the user can store
information on, or exchange information with, the RFID tag.
Both read-only and read/write tags can be programmable by the end user or
programmed by the manufacturer.
Beam-Powered and Battery-Powered Tags
In systems designed for short-range reading, beam-powered tags are energized by a
carrier signal sent by the reader. The tag returns a signal to the reader that is derived
from the energizing signal and that carries tag information as a sequentially coded
binary message. This type of beam-powered tag system is less expensive than a
battery-powered system.
Battery-powered tags use power from an internal battery to power circuitry that is
used to decode modulated data sent from a reader and to encode signals sent back
using modulated backscatter. Battery-powered tags are more expensive than beam-
powered tags, but they offer greater reading range and faster response time. Battery-
powered tags may offer increased capabilities and range, but they are limited by the
life of the internal tag battery. An additional benefit of battery-powered tag systems is