64628f99_nical_Details_and_Instructions__Description
[Signal Name] HD IN [Description] 
This is a command from the host machine to the floppy drive, to increase or decrease the amount of power to the read/write heads in case that the high density and double density modes require a different amount of magnetic power, to avoid data corruption between consecutive bits. Also used for differentiating behavior for the inner and outer tracks on the magnetic disk; The inner tracks, just like on a record, are much more compressed in space per bit than the outer tracks are and so magnetic power must be lowered to prevent bleeding to nearby areas.