Specifications

April – June 2002 ExtroNews 13.2 13
describes how heat moves through a
solid, convection describes how heat
moves through a liquid or gas. Of the
three methods, radiation is the most
powerful. Radiation moves at the speed of
light and is the phenomenon we perceive
in the dull red glow of the coals in a fire or
the element of an electric oven. Unlike
visible light which can be refracted,
filtered, or reflected, IR propagation may
be modeled by some properties of visible
light, but also propagates via radiation,
convection, and conduction.
Not the Lone Radiator
I think of IR control as “the invisible
frontier.” Why? IR control is a land of
unsettled territory. The trail is strewn with
various digital control protocols and
methods. Early makers of IR remote
controls developed unique control
protocols and chip sets for their products.
Latecomers to IR control must decide
whether to use an available protocol or
develop yet another of their own design.
Some system protocols are skeletons of
others which, over time and out of the
necessity to avoid control interference
with other brands, branch out in different
directions like the old desert mesquite.
Meanwhile, most peaceful folk are
amassing a vociferous collection of these
ray guns; not to mention that each
modern household now contains a new
“junk drawer”, a veritable mass grave,
housing the “dead” IR remote controls…
a kind of contemporary Boot Hill.
At every turn of the system installation
trail there can be outside interference in
the form of IR noise, sunlight, fluorescent
lighting fixtures, and heat sources. There
are some organizations that, like the
marshals of the old Wild West, are
attempting to bring law and order in the
form of standards to this necessary sector
of systems control and integration. Who are
these masked men?
One of them is IrDA-Infrared Data
Association. This organization has
developed standard intercommunications
for two key application areas: data and
control.
IrDA DATA
This is the popular two-way protocol for
short range, high speed data exchange
between enabled appliances, both portable
and fixed installation. They claim
application in over 300 million electronic
devices currently. Data communication
speed ranges from 9600 baud upwards in
steps to 4 Mbps. This is a low power
interface intended to auto-magically
operate when two appliances are within
range of one another…that range being at
least one meter and upward of two meters.
IrDA DATA is structured through a
mandatory set of three protocols and a set
of optional protocols. The optional set
includes seven other functions providing
specific exchange services and data
handling facilities. Important attributes of
this protocol set are automatic service
discovery, device-to-device connection,
data packet protection, and continuous bi-
directional operation.
IrDA Control
This variation allows cordless devices
such as mice, keyboards, game pads, etc. to
interact with host devices. Important
aspects of this protocol are minimum five
meter range, bi-directional communication,
up to 75 Kbps transmission rate, protected
data packets, and utilization of a 1.5 MHz
subcarrier. Similar in design to IrDA DATA,
IrDA CONTROL uses a mandatory set of
three protocols: PHY (Physical layer), MAC
(Media Access Control), and LLC (Logical
Link Control).
TECHNICALLY SPEAKING...
continued on next page
Even with new protocols and design
improvements, every installation design
using IR control must take into account the
environment. IR receive sensors must be
kept away from unwanted sources of
ambient IR radiation, like sunlight exposure,
incandescent lamps, and switched
fluorescent light fixtures.
Infra-Shades
Ever wonder why infrared equipment
typically has a dark red plastic window in
front of the receiver? The receiver consists
of a photodiode coupled with a bias circuit,
a small amplifier, and perhaps, a
demodulator. The photodiode’s sensitivity
curve, or optical bandwidth, includes the
infrared region plus most, or all, of the
visible spectrum, and possibly the
ultraviolet portion as well. Allowing visible
light and ultraviolet light energy to strike
the photodiode will begin to make it
conduct and, depending on the strength of
these competing energy bands, will
decrease the photodiode’s sensitivity to
infrared. If the diode is already in a
conducting state due to other light energy,
the incremental amount of response
afforded by infrared reception may be
incidental to the ambient current condition
in the diode. The dark red filter blocks all
ultraviolet and most all visible spectrum
energy from reaching the photodiode. This
situation returns the diode to a state of
mild, or no, conduction until infrared
energy passes through the red filter.
Therefore, most all the diode’s sensitivity is
dedicated to infrared reception.