After transmitting for 1 second, a device must
wait 30 seconds before transmitting again.
%
A quick way to implement an RF link for asynchronous serial data is to use
transceivers with built-in asynchronous serial interfaces. Sources include Linx
Technologies, Inc. (www.linxtechnologies.com) and MaxStream, Inc.
(www.maxstream.net). Chips, modules, and complete units are available for
incorporating into designs.
The Linx TXM-433-LR transmitter module has an asynchronous serial input
that can connects to a UART??™s data output and a 433-MHz output that can
Chapter 8
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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 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.
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