64628f99_nical_Details_and_Instructions__Description
[Signal Name] DRIVE A DRIVE B [Description] 
This is a command from the host machine to the floppy drive to activate the drive (not necessarily to spin the motor, but just let the electronics know to begin operation.). This is used in conjuction with the MOTORA ON, MOTORB ON signals. The floppy disk drives common in the US use this type of signal instead of the DS0, DS1, DS2, DS3 types of signal. Most drives are permanently set to DRIVE B, and you must use a ribbon cable with a twist in the middle of it to switch it to DRIVE A; however most computer PCs nowadays check both lines and assign whoever responds to drive A.
USB Floppy Disk Drive Converter/Emulator Technical Details and Instructions
Description