Prior to firmware 1.8, it was possible to use 2nd generation Buttons (R9861006D01) with the CS/CSE series Base Units and to use 3rd generation Buttons (R9861500D01 and R9861500D01C) with the CSM and CSE Base Units.
Since the firmware 1.8 release, this is no longer possible as the 2nd generation Button has reached the end of its lifetime and has not been found to be secure enough to deliver the CS and CSE standards. This means they will no longer receive updates, meaning new functionalities will not be available and issues will not be solved and compatibility between these two generations has been disabled, meaning they can no longer be paired or used between generations.
The 2nd generation Buttons were shipped and sold with CSC-1 and CSM-1 Base Units and can be recognized by looking at the layout & info on the sticker at the back. More info on how to identify your Button and Base Unit as well as an overview of which Button works with which Base Unit can be found in KB 3998.
