Linx Technologies LICAL-ENC-HS001 - Manual (Page 9)

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Encoder Operation
Upon power-up, the encoder sets the baud rate based on the state of the
SEL_BAUD line, pulls the TX_CNTL line low, and goes into a low-power
sleep mode. It remains asleep until either the KEY_IN, SEND, or CREATE_
PIN line goes high. These lines place the encoder in either Get Key Mode,
Send Mode, or Create PIN Mode as described in the following sections.
Encoder Get Key Mode
When the encoder registers activity on the KEY_IN line, it enters Get Key
Mode. In this mode, the encoder looks for an encryption key and user ID
from a decoder. When it receives this information, it sends a confirmation
on the DATA_OUT line to the decoder. It then looks for a final confirmation
from the decoder on the KEY_IN line. Once this confirmation is received,
the encoder takes the MODE_IND line high for one second to indicate that
the key has been successfully transferred and that the units may now work
together.
Encoder Send Mode
When the SEND line goes high, the encoder enters Send Mode. It pulls
the TX_CNTL line high to activate the transmitter and record the state of
the data lines. The encoder then encrypts the data using the saved key
and sends it through the DATA_OUT line. It continues doing this for as long
as the SEND line is high, updating the state of the data lines with each
transmission. Once SEND is pulled low, the encoder finishes the current
transmission, pulls TX_CNTL low to deactivate the transmitter, and goes to
sleep.
For simple applications that require only a single input, SEND can be tied
directly to the data input line, allowing a single connection. If additional lines
are used in this manner, diodes or dual contact switches are necessary to
prevent voltage on one data line from activating all of the data lines. The
Typical Applications section of this data guide demonstrates the use of
diodes for this purpose.
encoder that must be entered before the encoder activates. Furthermore,
since each encoder has its own key and the Control Permissions are stored
in the decoder, all the attackers would be able to do is duplicate the device
that they have already taken. They will not be able to grant themselves
greater authority, create a new controller, or replicate another encoder.
Before the encoder sends a packet, it calculates the Hamming Weight (the
number of ‘1’s in the string) of the packet to determine the duty cycle. If the
duty cycle is greater than 50% (more ‘1’s than ‘0’s), the encoder logically
inverts all of the bits. This ensures that every packet always contains 50%
or less ‘1’s. Since the FCC allows transmitter output power to be averaged
over 100ms, this allows a legal improvement in link range and performance
for many devices using an ASK / OOK transmitter. A 50% duty cycle is
generally the best compromise between data volume and output power.
Some other manufacturers may use a Pulse Width Modulation (PWM)
scheme or Manchester Encoding scheme to maintain a 50% duty cycle.
Both of these methods work, but are inefficient and do not make use of
the full link budget. The HS Series uses true serial data while maintaining a
50% duty cycle. Application Note AN-00310 covers these issues in detail.