Linx Technologies LICAL-TRC-MT - Manual (Page 6)

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Overview
Many products and applications call for the transfer of button presses
or switch closures across a wireless link. Traditionally, a remote control
link has operated in only one direction, from a transmitter to a receiver.
The cost associated with transceivers has been too high to practically
implement in low-cost products. With the increasing availability of low-cost
transceiver solutions, bidirectional links are now practical and open a new
world of opportunity.
In a wireless environment, maintaining the reliability and uniqueness of
a transmitted signal is generally of great importance. In a unidirectional
system, IC devices called encoders and decoders are often utilized to
simplify this process. The encoder side turns the status of a number
of input lines into an encoded serial bit-stream output intended for
transmission via an RF or infrared link. Once received, the decoder
decodes, error checks and analyzes the transmission. If the transmission is
authenticated, the decoder’s output lines are set to replicate the states of
the encoder’s input lines.
To accommodate bidirectional links, a new type of device has been
developed. Called a transcoder, this device combines a remote control
encoder and decoder into a single device, and is capable of sending
commands as well as receiving them. It is also able to receive an automatic
confirmation from the remote side indicating that its command was
received.
The Linx MT Series is a revolutionary transcoder product designed for
wireless remote control applications. The same device can be used as an
encoder, decoder, or transcoder and is ideal for both uni and bidirectional
applications and even mixtures of the two. The MT Series is easily
implemented, making it ideal for even the most basic applications, but
its rich feature set also allows it to meet the needs of far more complex
applications. These features include the ability to identify the originating
transmitter, establish user permissions, select output latch modes on a
“per pin” basis, and a powerful serial interface that allows control and
information exchange with external microcontrollers or a PC.
Consider a brief example of how just one of the MT’s innovative features
could be used to transform a relatively simple application, the common
garage door opener. In competitive devices, encoded transmissions
are generally either recognized or denied based on the address. If the
addresses match, the state of all data lines are recognized and output.
The MT Series allows a user or manufacturer to establish a user identity
and profile that determines which inputs are acknowledged. Let’s apply
this capability practically to an example: a three door garage houses
Dad’s Corvette, Mom’s Mercedes, and Son’s Yugo. With most competitive
products, any user’s keyfob could open any garage door as long as the
addresses match. In a Linx MT-based system, each individual keyfob could
easily be configured to open only certain doors (guess which one Son gets
to open!)
While reviewing this data guide keep in mind that it seeks to cover the full
scope of the MT’s capabilities. The implementation for a simple one button
remote is different than a powerful targeted control, command, or status
network. While it is unlikely that all of the features of this part will be utilized
at any one time, their availability provides great design flexibility and opens
up many new opportunities for product innovation.
Transceiver Power Control
The transcoder has the option to control power to an external transceiver
through the TR_PDN line. This line can be connected to a power down or
supply line of a Linx transceiver or a similar input on another transceiver.
This allows the transcoder to power down the transceiver when it is not
required, thereby reducing current consumption and prolonging battery life.
The transcoder pulls the TR_SEL line low to place the transceiver into
receive mode and looks for valid data for 16mS or 32mS, depending on
the baud rate. If data is present on the TR_DATA line, then the transcoder
enters Receive Mode. If no data is present, then the transcoder pulls the
TR_PDN line low to power down the transceiver and goes to sleep for
150ms or 295ms. The “off” time is approximately nine times the “on” time,
resulting in a 10% duty cycle, greatly reducing the transceiver’s current
consumption. However, there may be a lag time from when the transmitting
transcoder activates to when the receiving transcoder responds. The
transcoder enters Receive Mode when it sees a valid packet, so there
would only be a lag for the first packet.
This cycle continues until data is received placing the transcoder into
Receive Mode, until a status input line is taken high placing it into Transmit
Mode, or the CRT/LRN line is taken high placing it into Serial, Learn, or
Create Modes. If a faster response time is desired, then the TR_PDN line
can be left disconnected.