The ccu is like the us gov. The mcu is like the state. The mcu caint do nothin without the ccu all up in their face. If the ccu aint kosher then the mcu wont function. Houston we has a problem. Happily most of the time it is a broken wire or a (screwy louie door lock giving good news to the ccu and giving bad news to the mcu) between these two amigos due to all that cha cha cha-ing a goin on up in sir mix alot land.