offers 433 MHz RF modules that each contain an antenna and a Linx transmitter or receiver. A header provides connections for +5V, ground, data, and a power-down input, plus a signal-strength indicator on the receiver. MaxStream’s 1.9XTend OEM RF module has built-in error detecting and the ability to transmit over longer distances. The module can interface directly to a UART. The range is up to 40 miles using high-gain antennas and outdoor, line-of-sight transmissions at 9600 bps or less. The transmitter uses spread-spectrum technology and encryption. The modules can transmit and receive raw data or implement either of two built-in methods of error detecting with retries. A command can configure a module to look for an acknowledgement after transmitting a packet of data. If the acknowledgement doesn’t arrive, the module automatically retries the transmission up to a specified number of times. Another command can configure the module to repeat all transmitted data a specified number of times. These modules are just a sampling of what’s available. The vendors mentioned above and others offer products to suit just about any application’s needs. Using Other RF Standards Several standards define interfaces and protocols for transmitting RF data in formats other than asynchronous serial data. Modules are available that contain intelligent controllers that convert between asynchronous serial data and these interfaces. Table 8-1 compares three options. Zigbee The Zigbee standard zigbee.org defines a hardware interface and software protocols suitable for monitoring and control functions and other applications that transmit data in networks at up to 250 kbps. The components are inexpensive and consume little power. The protocol supports encryption. 152 Going Wireless Table 8-1: Wireless systems can use standard protocols to carry data. Interface Data Throughput bps Maximum Distance meters Bluetooth 3M 100 Zigbee 250k 50, extendable with routers Wi-Fi 54M 40 or more The p prom-electric.ru