The Free On-line Dictionary of Computing (30 December 2018):

ST-506

    The first full-height 5.25 inch hard disk drive
   for personal computers, introduced in 1980 by Shugart
   Technology (now Seagate Technology).  The ST-506 stored up
   to 5 megabtyes after formatting using MFM encoding.  It
   transferred data at 625 kilobytes per second.

   The ST-506 (like the ST-412) was interfaced to a computer
   via a disk controller.  The interface was a faster version
   of the Shugart Associates SA1000 interface, which was in
   turn based upon the floppy disk drive interface.  Two cables
   connected the controller to the disk.  The 34-pin control
   cable controlled mechanical motion and data was read or
   written serially using two pins of the 20-pin data cable.

   Other companies copied the interface, creating a universal de
   facto standard that was further strengthened by its revision
   to support Seagate's 10 MB ST-412 drive that was adopted for
   the IBM PC XT.

   Around 1990, SCSI and ATA superseded ST-506.  These
   eliminated the problems of matching controllers to drives by
   physically integrating a controller with the drive, allowing
   interleave ratios and other disk parameters to be optimised
   by the manufacturer rather than the system integrator.

   Connector pin-out (http://www.gamesx.com/hwb/co_ST506.html).

   (2007-03-06)