Chapter 8
typically don’t have dedicated channels for sending flow-control data. Buffers or
software flow control can help ensure that a transmitter doesn’t send data when
the receiver isn’t ready.
Radiated energy from sources other than the transmitter can appear as noise at
the receiver and cause errors in received data. Chapter 2 described methods for
detecting errors. These methods are also useful in wireless communications. As
mentioned above, modulation and spread-spectrum technologies can also help
a receiver reject noise.!
For short range, line-of-sight communications, beamed infrared energy is an
option. Infrared energy is electromagnetic radiation with wavelengths in the
range of 760–1000 microns, slightly longer than the wavelengths of visible red
light.1 8
One or more LEDs that emit IR energy can serve as a transmitter. Lenses can
direct the energy in a wide or narrow beam as needed. At the receiver, a photo-
diode can detect the transmitted energy.
Many IR applications use OOK modulation to reduce errors caused by stray IR
energy in the environment. Common carrier frequencies are 38 kHz and 455
kHz. To send a logic 0, the carrier transmits for the width of the bit. To send a
logic 1, the transmitter remains off. The receiver detects and decodes pulses at
the carrier frequency and ignores any other detected IR energy.
The transmitting computer can logically AND the data signal with the output
of an oscillator at the carrier frequency. For the receiver, components are avail-
able that combine a photodetector that responds to IR wavelengths, a demodu-
lator that detects pulsed IR energy at a specific frequency, and an amplifier with
a TTL-compatible output. An example is the TSOP7000 IR Receiver from
Vishay Semiconductors.
Reynolds Electronics (www.rentron.com) is a source for components and mod-
ules for IR data links. The Fyre-Fly module contains an IR transmitter and
receiver that use a 455 kHz carrier. The module has an RS-232 interface and