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The ST-506 interface was a standard hard disk controller (HDC) and ST-506 interface used to connect personal computers (PC) and hard disk drives (HDD). It connected to a controller card with two cables and a third power cable and was the first five and a quarter-inch hard disk drive by Seagate Technology, formerly Shugart Technology.
The ST-506 is also known as modified frequency modulation (MFM) - an encoding scheme for floppy drives and older HDDs.
The ST-506 interface was derived from Shugart's SA1000 interface, which in turn was derived from the five and a quarter-inch floppy drive interface - facilitating easy disk controller design. The ST-506, ST-412 and ST-412RLL interfaces were de facto HDD standards through the 1990s. Unlike modern hard disk drive systems, on-board processing power was not provided, which was a non-issue during that period.
The ST-506 disk drive lacked buffered seek capability and averaged a seek time of only 170 ms, versus ST-412 drives with buffered seek capability, which averaged 85 ms and 15-30 ms by the late 1980s.