Hard disks use a light-emitting diode or LED to indicate drive activity. The hard disk activity LED is a very useful indicator that generally tells the PC user at a glance when the system is
active. The first PC hard disks shipped with a faceplate (or bezel) on the front. The hard
disk was mounted into an external hard drive bay (in place of a floppy disk drive) and its
integral LED was visible from the front of the PC, because the drive's front was actually
protruding from the case, much as floppy disk drives still do.
It was quickly realized that having the disks mounted internally to the case made more
sense than using external drive bays, but the LED was still desirable. So a remote LED was mounted to the case and a wire run to a
two-pin connector on the hard disk itself. This system worked fine when there was just one
hard disk, but became a problem in systems that had two or three hard disks. Eventually,
the case LED was made to connect to the hard disk controller instead, to show
activity on any of the hard disks that were managed by the controller.
Modern PCs have integrated IDE/ATA controllers built into the chipset on the
motherboard, so the LED is usually connected to special pins on the motherboard itself.
For systems that use add-in controllers, the LED is connected to the controller, as it was
in the days before integrated controllers. Over time, as connecting the LED to the
controller has become the standard, most manufacturers have dropped entirely the LED
connector on the disk itself on IDE/ATA drives.
Since support for SCSI drives is not present in the vast majority of PC motherboards,
they often do still come with an external LED connector.
Next: Hard Disk
Logic Board
