ST506 disk interface

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The ST506 disk interface (other common names: ST412, ST506/ST412, or often the misleading MFM disk) was introduced in 1980 by Seagate with the 5MB hard disk of the same name. It quickly became a de facto interface standard in the industry, although it was never formalized, or even given a formal name (hence the plethora of names for it used by people).

It provides the analog signal from the head, not a digital bit stream (as later disk interface standards did). Most early device controllers used MFM encoding (hence the name, but that is misleading, for several reasons) on that to produce a bit stream, at a data rate of 5 Mbits/sec. Towards the end of its lifetime, RLL encoding was used instead.

(In addition to the replacement of MFM by RLL, 'MFM disk' is misleading as a name for this interface standard since MFM was widely used by other magnetic storage devices at the time; see here for some.)

The use of RLL increased storage capacity by 50%, by squeezing 26 sectors into a track instead of 17. Because of the identical hardware interface, RLL hard disks can be easily reformatted to use MFM, with suitable controllers. On the other hand, the opposite direction is not recommended, since the higher sector density of RLL in MFM disks leads to serious problems with the reliability of data storage.

The way to success was paved by the follow-up 10MB ST412 disk model, which was used by IBM for the IBM Personal Computer XT Model 5160. Soon many other hard disk manufacturers followed suit with their own models, and thereby helped the ST506 interface to become very successful. The largest ST506 hard drive believed to have ever been made is the MAXTOR 2190, which gains about 159 MB from 1224 cylinders, 15 heads, and 17 512-byte sectors per track.


The ST506 interface uses two flat cables, one a 34-pin control cable, and the other a 20-pin data cable. The control cable is shared by up to four disks whereas each disk has got its own dedicated data cable. (Many PC ST506 controllers support only two disk drives.)

The data cables transmit the read and write signals, the control cable carries the control data such as:

  • Ready
  • Track 0
  • Direction
  • Step
  • Index
  • Seek Complete
  • Head Select
  • Drive Select
  • Write Gate

External links