ST-412

ST-506 can refer to either the hard disk drive (HDD) or its interface

The ST-506 hard disk drive was the first 5.25 inch hard disk drive, introduced in 1980 by Shugart Technology (now Seagate Technology). It stored up to 5 megabytes after formatting and cost US $1,500 ($4,360 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 adopted by numerous HDD manufacturers such that the interface became a de facto industry standards for disk drives well into the 1990s.

The limitations of the ST-506 interface is 5.00 Mbit/s transfer rate, 16 heads, 4 drive units and a 20-foot (6.1 m) cable length. Number of tracks only increase the access time.

In the ST-506 interface, the drive connects to a controller card with two data cables, while a third provides power. 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. Four of the pins, "HD SLCT 0" through "HD SLCT 3", allow the selection among up to 16 heads, although only four are available on the two-platter ST-506. Once the heads are properly positioned, data is read or written serially through a set of pins in the 20-pin data cable. The limited bandwidth of the data cable was not an issue at the time and is not the factor that limited the performance of the system. However, the unshielded cable can sometimes be susceptible to high levels of noise.

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Wikipedia