So I just installed a new garage door opener, and somehow taught it how
to recognize the two transmitters that came with it. The instructions
assured me the openers would somehow 'roll their code' each time the
door was opened as a security measure.
How does this work? For sure the transmitters don't cycle to a new code
each time the button is pressed, else they'd get out of sequence too
easily. Could it be there's some form of cross check, that the door
opener transmits a signal the opener has to respond to?
typically the way that they work is that the receiver will accept
the current code, and the next 10 or so codes in the sequence as valid.
It resets the current code based on the last valid received code. The
openers will sometimes recognize more than one transmitter as well,
keeping two or more sets of rolling codes. There is wide variation in
this among models and manufacturers.
The transmitter does all the work. The communication is one-way. Google for
"Keeloq" to see one of the technologies. It is very sophisticated . The
transmitter sends a serial number, a secure code that tells the receiver
what the next one will be, and information on up to four buttons plus low
battery. The receiver then calculates what the next code will be. If they
get out of sync beyond a certain number of presses, it only takes one more
button press to resync.