This signal differs between floppy drives model:
*For 3inch floppy drives (pin26), it is a "Ready" signal. The /RDY signal turns on when a disk is inserted and spinning in the drive, usually after detecting at least 2 or 3 index pulses, depending on the drive model. It turns off immediately if the disk is removed or the motor stops.
*For PC 3.5inch floppy drives (pin34), it is a "Disk Changed" signal. The /DSKCHG signal determines whether the same disk loaded during the previous disk access is still in the drive.