ST-506

The ST-506 was the first 5.25 inch hard disk drive, introduced in 1980 by Seagate Technology (then Shugart Technology). It stored up to 5 megabytes after formatting and cost US $1,500 ($4,308 in today's dollars). The similar, 10-megabyte ST-412 was introduced in late 1981. The ST-225 was introduced shortly thereafter with 20 megabytes and half the height. All three used MFM encoding, a widely used coding scheme. A subsequent extension of the ST-412 interface used RLL encoding for a 50% increase in capacity and bit rate.

The ST-506 connected to a computer system through a disk controller. The ST-506 interface between the controller and drive was derived from the Shugart Associates SA1000 interface, which was in turn based upon the floppy disk drive interface, thereby making disk controller design relatively easy. The ST-506 interface and its variants (ST-412, ST-412RLL) were de facto industry standards for disk drives well into the 1990s.

Interface to controller

In the ST-506 interface, the drive was connected to a controller card with two cables; a third cable provided power. The control card translated requests for a particular track and sector from the host system into a sequence of head positioning commands, then read the signal from the drive head and recovered the data from it, similar to a floppy disk drive. (Some years later, this was sometimes called "dumb" drives, as higher-level interfaces such as SCSI and IDE had been introduced and disk drives using these interfaces are more automated—the host system requests a particular block of data and the drive carries out all the steps required to retrieve it.)