TRC1300, TRC1315
MARCSTAR
I E/D
REMOTE CONTROL ENCODER/DECODERS
SLWS011D – AUGUST 1996 – REVISED JANUARY 1997
10
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
hopping code (continued)
In some cases, the encoder is activated and sends security and function data code without the decoder
receiving and decoding the signal (if the receiver is out of range, for example). This would normally cause the
encoder and decoder to fall out of sync with each other. MARCSTAR I E/D devices circumvent this by allowing
the decoder to activate the function outputs when any one of the next 256 expected security codes is received
from a learned encoder. The 256 expected security codes are based on the currently-stored 40-bit security
code. In rare cases, the encoder might be activated more than 256 times without being near the decoder,
requiring the encoder and decoder pair to be manually resynchronized. In this case, the decoder can simply
learn the current encoder security code, using the procedure detailed in the decoder programming section of
this document, resynchronizing the pair.
Hopping code provides extremely high security for the encoder/decoder pair and prevents unauthorized access
to the receiver and decoder by means of signal interception and retransmission of the intercepted signal.
Transmitter-Lock
Since the MARCSTAR I E/D devices have a pin-selectable encoder/decoder mode, a safeguard
(Transmitter-Lock) has been designed into the devices. Transmitter-Lock prevents unauthorized parties from
defeating the MARCSTAR security by using a MARCSTAR I E/D device to intercept a transmitted security code
and then transmit the next expected security code to the decoder. The received security code would then be
recognized as coming from the original encoder and, therefore, valid causing the decoder function outputs to
be activated.
The safeguard works by setting an internal flag, stored in EEPROM, whenever the device, in the decoder mode,
learns a code from an encoder. This flag then causes a new 40-bit security code to be generated and stored
in the EEPROM if the device is later placed in the encoder mode and a transmission is ever attempted. So, once
a decoder learn cycle has occurred in a particular MARCSTAR I E/D device, the learned security code will be
overwritten by a new 40-bit security code before output in the encoder mode is permitted. This feature allows
the MARCSTAR I E/D devices to be used as either an encoder or a decoder without sacrificing the security
provided by separate dedicated encoder and decoder devices.
device/system security
Statistically, the probability that a random code would activate the MARCSTAR I E/D devices operating in the
decoder mode is calculated using the formula shown in equation 1.
Probability
+ valid
possible
where
valid = the number of security codes that activate the device
possible = the total number of possible security codes
(1)
A MARCSTAR I E/D device operating in the decoder mode responds to a total of 28 (256) security codes
(including the 256-code look-ahead feature) for
each of the four encoders it can learn (256
× 4 valid security
codes).
The total number of possible 40-bit security codes is 240 (1.0995 trillion).
Inserting this into the formula gives equation 2.
Probability
+ 2
8
4
240
+ 1
2
30
+
1
1.074
10
9
(2)
Therefore, the security of
the entire system is one in 1.074 billion — there is one chance in 1.074 billion that
a random security code would be recognized as valid by a MARCSTAR I E/D device operating in the decoder
mode.