Chapter 8
connect to an antenna. The companion RXM-433-LR receiver module has a
433-MHz receiver that can connect to an antenna and an asynchronous serial
output that can connect to a UART’s data input.
The modules use OOK modulation. They don’t implement any error-detecting
protocols so the computers that connect to the asynchronous serial interfaces
are responsible for detecting errors. A link can operate over distances of up to
3,000 ft at up to 10,000 bps. Parallax, Inc. 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 sig-
nal-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 trans-
mission 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..
!
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.9
The Zigbee standard (zigbee.org) defines a hardware interface and software pro-
tocols 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.