the storage controller 10 also referred to as being u201c added u201d or u201c included u201d in the storage controller 10 the annex controller 54 wields this control in response to its own independent monitoring of the health of the i o annex module 28 fig1 illustrates various signals provided to or generated by the annex controller 54 the annex controller 54 receives the signals tach_a and tach_b generated by the fans 52 during normal operation each of these signals includes regularly spaced pulses at a frequency proportional to the rotational speed of the corresponding fan 52 the controller uses the tach_a and tach_b signals 1 as feedback signals in a control loop that regulates fan speed and 2 to monitor whether the fans 52 are operating at all the fan_a_pwm and fan_b_pwm signals are the pwm control signals 56 for the fans 52 the annex controller 54 also receives the temp_sensor signal to detect any over temperature condition that may develop and generates the signals cntl_good pic_pwrgd board_reset and annex_flt in operation if both of the fans 52 fail then the annex controller 52 drives the pic_pwrgd signal low to cause the associated sp 16 to remove the i o annex module 28 from the system this is a u201c fail.