My garage door opener is compatible with either the 828LM & 888LM combination OR the new 821LM.
What are pros and cons of each? I only have one door. It seems the 821LM has a sensor on the door itself to confirm its open/close state - is this beneficial ? is it possible to get the older 828LM & 888LM combo out of sync with reality?
The choice between the 828LM Gateway and the 821LM Wifi Hub would be determined by the type of overhead controller you have.
If you have an Overhead Controller that is directly MYQ compatible, you can use the 828LM and 888LM door button.
If not, you can check the compatibility guide found in this forum thread to determine if your Overhead is compatible with the 821LM (most relatively newer ones will be if they have a learn button on them)
If your garage Overhead controller is compatible with both, I would recommend using the 828LM Gateway, as it does not rely on wifi. If you can position it (directly wired to router) within range of the 888LM, I typically hear of fewer incidents with that model.
Most homes will have no trouble with communication between the 828LM and 888LM.
What about the presence of a door sensor on the 821LM model? Is it possible to confuse the 828LM by something tripping the safety eye as the door is closing? Just wondering if it can get into a state where it thinks its closed (because it told the opener to close) but its actually still open.
I’ve not heard of that being possible. I do not recall specifically an issue with an 828LM ever reporting incorrect status. I have heard of the 821LM doing so (mostly having to do with the tilt sensor mounting/getting knocked off/battery dying.)
I got some more info on the liftmaster forum on how it all works in case anyone else is interested:
https://myqcommunity.liftmaster.com/liftmastermyq/topics/how-does-888lm-know-the-door-is-open-closed
The 888LM replacement control panel seems to be a transceiver; it talks to opener via wires from existing control panel, and in turn it has an RF receiver for a new protocol called “Security + 2.0 technology”. The 828LM Internet Gateway communicates with 888LM via “Security + 2.0 technology” - there are also additional remotes that speak this new protocol that you can program to the 888LM to control your opener. You can keep existing remotes programmed to the opener directly.
Liftmaster rep reports that the door state is reported to the 888LM via the wires, why you don’t need a tilt sensor like the 821LM employs.