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ST-506 / ST-412 Interface
The original hard disk interface used in the PC world was developed in 1980 by Seagate Technologies, to work with that company's 5 MB ST-506 hard disk. It was later revised to support the 10 MB ST-412, which was the first hard disk drive model used in the IBM PC/XT. You can obviously see where the name of the interface comes from, given this bit of history! In common parlance, this older interface is sometimes called "MFM" or "RLL", so you may hear someone refer to a drive using this interface as an "RLL drive" for example. These actually refer to the encoding method used for storing data on the disk. Both encoding methods were used on these early drives. (In fact, RLL is used on some IDE and SCSI drives also, making it a poor name for referring to drives of one particular interface.)
The ST-506/ST-412 interface differs from the IDE/ATA and SCSI standards in one very important respect: the hard disks were "dumb", meaning there was no built in logic board as modern drives have. All of the smarts resided in the controller card that plugged into the PC. This caused a host of problems relating to compatibility, data integrity and speed, because the raw data from the read/write heads was traveling over a cable between the controller and the drive. This interface also required a lot more work on the part of the user, because while a newer drive ships with an integrated controller card built into the drive that is optimized for that drive, these older ones didn't have this, and therefore the person setting up the drive had to program the interleave ratios and other factors into the drive to achieve maximum performance.
By today's standards, this interface and the drives that use it are microscopic in capacity (although enormous in physical size), slow, cumbersome, error-prone and completely obsolete. You will never see ST-506/ST-412 used in a new system, and in fact, it's hard to find them in any systems still being used, unless you look around or know a very frugal person. :^) You can recognize this interface in older systems by the use of two ribbon cables (instead of the single cable used by IDE/ATA and SCSI). One of the cables is 20 pins wide and carries data, and the other is 34 pins and carries control signals.