A vehicular rearview mirror system suitable for use with a vehicle control, the vehicle control generating an output having a particular protocol representing a state of the output. The vehicle rearview mirror system includes a variable reflective element which assumes a partial reflectance level in response to a drive signal. The rearview mirror system further includes a mirror control that is responsive to the output of the vehicle control for generating at least one mirror control output. The at least one mirror control output includes a value for the drive signal. The mirror control determines the protocol of the output of the vehicle control including monitoring the output of the vehicle control in attempting to conclude a state of the output of the vehicle control.