5.1. Physical Components of an RFID Reader
Since the reader communicates with tags using RF, any RFID reader must have one or more antennas . Because a reader must communicate with some other device or server, the reader must also have a network interface of some sort. Examples of common network interfaces are the serial Universal Asynchronous Receiver/Transmitters (UARTs) for RS 232 or RS 485 communications and the RJ45 jack for 10BaseT or 100BaseT Ethernet cables ; some readers even have Bluetooth or wireless Ethernet communications built in. Finally, to implement the communications protocols and control the transmitter, each reader must have either a microcontroller or a microcomputer. Figure 5-1 shows the physical components of an RFID reader.
Figure 5-1. Physical components of a reader
5.1.1. Antenna Subsystem
Although the antennas themselves are simple in concept, engineers work constantly to get better reception at lower power and to adapt the antennas to special circumstances. Some readers have only one or two antennas, packaged with the readers themselves; other readers may be able to manage many antennas at remote locations. The primary limitation on the number of antennas a reader can control is the signal loss on the cable connecting the transmitter and receiver in the reader to the antennas. Most installations keep the reader within about six feet (two meters) of the most distant antenna, but much longer runs are possible.
Some readers use one antenna to transmit and one to receive. In this sort of configuration, the tag's direction of motion through the reader's fields is particularly important. If the transmitting antenna is "ahead" of the receiving antenna, the receiving antenna will have a longer amount of time to receive signals from the tag. If the antennas are reversed, the tag will spend much less time energized and within range of the receiving antenna. Figure 5-2 shows a conveyor belt with tagged boxes passing first a transmitting (TX) antenna and then a receiving (RX) antenna .
The arrow indicates the conveyor's direction of motion. The tag on each box becomes energized as it passes the TX antenna and begins broadcasting a response. Because it is further down the conveyor, the RX antenna has a longer period in which to receive the response than it would if the two antennas were reversed, which means that there is a better chance that the tag will be read.
Figure 5-2. Preferred placement of receiving and transmitting antennas
The computing device that controls a reader can vary in complexity from a simple state machine on a chip, which might be used for a tiny embedded reader on a telephone or PDA, to a complete microcomputer system capable of running a server operating system as well as end user applications and accumulating a large amount of data on an internal hard disk. The controller is responsible for controlling the reader side of the tag protocol (described in Chapter 4) as well as determining when information read from a tag constitutes an event to send to the network. The reader controller is also responsible for managing the reader's end of the reader protocol (described in Chapter 6).
5.1.3. Network Interface
Reading tags and recognizing events wouldn't be much use if the reader never told anyone about those events. Readers communicate with the network and other devices through a variety of interfaces. Historically, most RFID readers have had serial interfaces using RS 232 or RS 422 (point to point, twisted pair) or RS 485 (addressable, twisted pair). In recent years, more and more readers have supported Ethernet, and in the last 18 months, some have begun to support built-in wireless Ethernet, Bluetooth, and even ZigBee (for more on ZigBee, see Chapter 11).
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