ST-506/ST-412

[1] Beginning with its selection as the hard drive subsystem for the original IBM XT[1] disk drive controllers supporting the ST-412 interface grew to become ubiquitous in the personal computer industry,[2] The ST-412 interface and its variants were the de facto industry standard for personal computer hard disks until the advent and wider adoption of the IDE or ATA interface in the early 1990s.

The limitations of the ST-412 interface are 5 million transitions per second maximum on data lines, 16 heads, 4 drive units and a 20-foot (6.1 m) cable length.

The control card translates requests for a particular track and sector from the host system into a sequence of head positioning commands, including setting the direction of head movement, in or out, and sending individual "STEP" commands to move.

The ST-412 disk drive, among other improvements, added buffered seek capability to the interface.

The controller sends the required STEP pulses to the drive as fast as it can receive them.

The process of moving portions of the command interpretation off the controller card and onto the drive itself in order to improve performance is a common feature of later hard drive connection schemes, notably SCSI, with its rich command set, and the storage-focused IDE systems.

While integrated controllers have many benefits, they also have a disadvantage: the mechanical drive (called the "head-disk assembly", or HDA) and the controller are effectively fused into a monolithic black box, so that if something goes wrong with the drive, it is nearly impossible to do anything about it—the data is usually irretrievably lost.

Such data recovery techniques are much more difficult to execute on integrated drives, because the needed analog signals from the disk are not available at a standard interface and the internal data recording method, sector format, and disk organization of nearly every integrated drive model is different and secret.

However, the complexity of the controller and cabling led to newer solutions like SCSI, and later, ATA (IDE).

[13] Atari also used Adaptec ACB-4000A SCSI to ST-506 converter inside its own line of SH204/SH205 external ACSI drives.