Going Wirelesscan transmit at up to 38,400 bps over distances of up to 80 ft. A pair of these
modules can replace an RS-232 serial cable.
Some IR systems don’t use a carrier and instead encode each logic-0 bit as a sin-
gle pulse. To save power, the encoder can transmit pulses that are shorter than a
full bit’s period. For example, Microchip Technology’s MSC2120 infrared
encoder/decoder uses pulses that are less than 20% of the bit width.$
The Infrared Data Association (IrDA) defines a hardware interface and proto-
cols for IR communications over distances of up to 1 meter. The limited dis-
tance makes IrDA suitable mainly for communicating with nearby peripherals.
The IrDA specifications define several physical layers that each specify a format
for transmitted data. The Serial IrDA (SIR) physical layer supports transmitting
asynchronous data at up to 115,200 bps.1-
For longer distances or when a light of sight isn’t available or practical,
radio-frequency (RF) communications are an option.-
RF communications must comply with government regulations. In the U.S.,
the Federal Communications Commission (FCC) administers regulations that
divide the RF spectrum into bands and specify uses for each band. The regula-
tions are in the Code of Federal Regulations, Title 47 (http://wire-
less.fcc.gov/rules.html). Part 15 of the regulations covers unlicensed
transmissions.
Many small-scale applications can use bands that allow unlicensed transmis-
sions, which don’t require a licensed operator. Transmitters that use these bands
must comply with regulations that apply to the bands, however. The FCC spec-
ifies the allowed radiated power for the fundamental frequency and for radia-
tion outside the necessary bandwidth. Radiated power is specified as field
strength, which is a measure of the voltage of the radiated signal at a defined
distance from the transmitting antenna. For some bands, the regulations also
specify the allowed type of content and purpose of the transmissions.
Commercial products that intentionally radiate RF energy must obtain certifi-
cation by passing FCC compliance tests. The FCC authorizes private test labs