More questions on car remote-opening devices

[Colophon]

This thread linked to a How Stuff Works article that explains these gizmos pretty well.

However, I could do with a bit of clarification on this point:

Both the transmitter and the receiver use the same pseudo-random number generator. When the transmitter sends a 40-bit code, it uses the pseudo-random number generator to pick a new code, which it stores in memory. On the other end, when the receiver receives a valid code, it uses the same pseudo-random number generator to pick a new one. In this way, the transmitter and the receiver are synchronized. The receiver only opens the door if it receives the code it expects.

If you are a mile away from your car and accidentally push the button on the transmitter, the transmitter and receiver are no longer synchronized. The receiver solves this problem by accepting any of the next 256 possible valid codes in the pseudo-random number sequence. This way, you (or your three-year-old child) could “accidentally” push a button on the transmitter up to 256 times and it would be okay – the receiver would still accept the transmission and perform the requested function. However, if you accidentally push the button 257 times, the receiver will totally ignore your transmitter. It won’t work anymore.

First question: say you’ve fiddled with the remote and pressed the button, say, a dozen times while you are out of range, as I have been known to do. Will it “resynchronise” next time you open the car successfully, are you now forever 12 steps out of synch? That is to say, if you press the button while out of range another 12 times the following week, and so on and so on, will you eventually find yourself locked out as you get outside the 256-code window?

Second question: how does this system deal with the fact that you have more than one remote? When I bought my car I was given three keys, two of which had a remote on the fob. I have only ever used one of them, keeping one as a spare. How does the unlocking system stay synchronised with the other, unused, remote? Are there two separate circuits in the car, one for each remote?
Amazing how the SDMB gets you thinking about things that otherwise would never have crossed your mind…

[Joey_P]

I believe it will be forever 12 steps out of sync unless you resync it. Check in your manual for what to do if you replace the battery. I belive to re sync mine you have to hold down the lock and unlock button for a few seconds while in range of the car.

[aerodave]

In response to the second question: It’s not a matter of separate circuits, but of separate “counters.” It keeps track of the rolling codes for each transmitter separately. For my car, it’s limited to 4 remotes, because that’s how many it can keep track of.

And in response to your first question: Every time there’s a successful button-recognition event, they are again “in sync” (although it’s not really the right way to look at it…you’re giving the alarm brain too much credit for being able to keep track of this kind of thing). When the alarm gets a code from the transmitter that’s farther ahead than the next one it expected (say 12 numbers down the random sequence), it just picks up from there when it decides what to expect next. It doesn’t really worry anymore about the 10 or 12 it had to skip over previously. In other words, the alarm brain doesn’t have a memory of past events, it only has anticipation of future ones.

[Colophon]

Thanks, that makes sense. And there I was thinking I was just getting in my car, when in fact I’d just had my own “successful button-recognition event”