How fmu board send pwm info to ioboard throught serial port in code

I looked throught the file Fmu.cpp. I have knew the relative codes,but did not find out serial port info here. I am so confused. Any one can help me ?
PX4FMU::pwm_output_set(unsigned i, unsigned value)
if (_pwm_initialized) {
up_pwm_servo_set(i, value);