The output of the exclusive 'or' gate UD3 pin 6 will be "low" until C46 has
charged through R25. Once the voltage across C46 reaches 2 volts, the output of
UD3 pin 6 will go "high". This occurs when the disk is powered on, or a reset
pulse is generated by a device connected to the serial bus. The reset pulse on
the serial bus interface is input on, pin 6 of P2 or P3. This "low" to "high"
going pulse on pin 6 of UD3 is input to the microprocessors reset interrupt
input. This causes a restart on reset routine to be executed giving control of
the disk drive operation to the Disk Operating System (DOS).