4. Disk-in Sensor
When a disk is inserted in the FDD, this sensor outputs a LOW level signal (HIGH
when no disk).
The output of the sensor is connected to the controller and sent as a "DISK
CHANGE" signal that shows whether a disk is changed; when a disk is changed, the
"DISKCHANGE" signal becomes LOW.
Once the "DISK CHANGE" signal becomes LOW, it remains low until the "STEP"
signal is received from the FDC #5.