The ST-506 was the first 5.25 inch hard disk drive. Introduced in 1980[1] by Seagate Technology (then Shugart Technology), it stored up to 5 megabytes after formatting and cost $1500.[2] The similar (but more expensive) 10 MB ST-412 was introduced in late 1981. Both used MFM encoding (already widely used in disk drives). A subsequent extension of the ST-412 used RLL for a 50% boost 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[3] which was in turn based upon the floppy disk drive interface[4] thereby making disk controller design relatively easy.[1] The ST-506 Interface and its variants (ST-412, ST-412RLL) were defacto industry standards for disk drives[5] well into the 1990s.
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In the ST-506 interface, the drive was connected to a controller card with two cables; a third cable provided power. The drives were "dumb", so-called because the control card translated requests for a particular track and sector from the host system into a sequence of head positioning commands, then read the signal from the drive head and recovered the data from it. A 34-pin control cable would control the mechanical motions of the drive with pins such as "HD SLCT 0" through "HD SLCT 3" used to select one of up to 16 heads (only four were available on the two-platter ST-506 itself) and "STEP" / "DIRECTION IN" used to move the heads to the appropriate track. Data then could be read or written serially using the appropriate two pins of the 20 pin data cable. This led to slow potential performance due to the limited bandwidth of the data cable, although this was not an issue at the time. Modern disk drive systems have considerable processing power on-board, so that the host system only needs request a particular block of data and the drive internally carries out all the steps required to retrieve it.
Seagate's second generation ST-412 disk drive amongst other things added buffered seek capability to the interface. In buffered seek mode, the controller could send STEP pulses to the drive as fast as it could receive them, without having to wait for the mechanism to settle. An onboard microcontroller would then move the mechanism to the desired track as fast as possible. The ST-506 disk drive without buffered seek, averaged 170 msec (similar to a floppy drive or modern optical drive) while the mechanically very similar ST412 disk drive with buffered seek averaged 85 msec.[6] By the late 1980s ST412 drives were capable of average seek times between 15-30 milliseconds
A number of other companies quickly introduced drives using the same connectors and signals, creating an ST-506-based hard drive standard. IBM chose to use it, acquiring adapter cards for the PC/XT from Xebec[7] and for the PC/AT from Western Digital. As a consequence of IBM's endorsement, most of the drives in the 1980s were ST-506-based.
However the complexity of the controller and cabling led to newer solutions like SCSI, and later, ATA (IDE). A few early SCSI drives were in fact ST-506 drives with a SCSI to ST-506 controller on the bottom of the drive; however, most SCSI and all ATA drives had built the controller into the drive, thereby eliminating the ST-506 interface in such models.
From ST506/ST412 OEM Manual[6] In the following table, "~" denotes a negated (active low) signal.
| GROUND | 1 | 2 | ~HD SLCT 3 (or ~Reduced Write Current) |
| GROUND | 3 | 4 | ~HD SLCT 2 |
| GROUND | 5 | 6 | ~WRITE GATE |
| GROUND | 7 | 8 | ~SEEK CMPLT |
| GROUND | 9 | 10 | ~TRACK 0 |
| GROUND | 11 | 12 | ~WRITE FAULT |
| GROUND | 13 | 14 | ~HD SLCT 0 |
| Key (no pin) | 15 | 16 | Reserved |
| GROUND | 17 | 18 | ~HD SLCT 1 |
| GROUND | 19 | 20 | ~INDEX |
| GROUND | 21 | 22 | ~READY |
| GROUND | 23 | 24 | ~STEP |
| GROUND | 25 | 26 | ~DRV SLCT 0 |
| GROUND | 27 | 28 | ~DRV SLCT 1 |
| GROUND | 29 | 30 | ~DRV SLCT 2 |
| GROUND | 31 | 32 | ~DRV SLCT 3 |
| GROUND | 33 | 34 | ~DIRECTION IN |
| ~DRV SLCTD | 1 | 2 | GROUND |
| No connection | 3 | 4 | GROUND |
| No connection | 5 | 6 | GROUND |
| No connection | 7 | 8 | Key (no pin) |
| No connection | 9 | 10 | No connection |
| GROUND | 11 | 12 | GROUND |
| +MFM WRITE | 13 | 14 | -MFM WRITE |
| GROUND | 15 | 16 | GROUND |
| +MFM READ | 17 | 18 | -MFM READ |
| GROUND | 19 | 20 | GROUND |
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