The ST-506 was the first 5.25 inch hard disk drive, introduced in 1980 by Seagate Technology (then Shugart Technology). It stored up to 5 megabytes after formatting and cost US $1,500 ($4,308 in today's dollars). The similar, 10-megabyte ST-412 was introduced in late 1981. The ST-225 was introduced shortly thereafter with 20 megabytes and half the height. All three used MFM encoding, a widely used coding scheme. A subsequent extension of the ST-412 interface used RLL encoding for a 50% increase 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, which was in turn based upon the floppy disk drive interface, thereby making disk controller design relatively easy. The ST-506 interface and its variants (ST-412, ST-412RLL) were de facto industry standards for disk drives well into the 1990s.
Interface to controller
In the ST-506 interface, the drive was connected to a controller card with two cables; a third cable provided power. 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, similar to a floppy disk drive. (Some years later, this was sometimes called "dumb" drives, as higher-level interfaces such as SCSI and IDE had been introduced and disk drives using these interfaces are more automated—the host system requests a particular block of data and the drive carries out all the steps required to retrieve it.)
A 34-pin control cable controlled 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. The limited bandwidth of the data cable was not an issue at the time and was not the factor that limited the performance of the system (although the unshielded cable could sometimes be susceptible to high levels of noise).
Seagate's second generation ST-412 disk drive, among 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 microprocessor would then move the mechanism to the desired track as fast as possible. The ST-506 disk drive without buffered seek, averaged 170 ms (similar to a floppy drive or modern optical drive) while the mechanically very similar ST412 disk drive with buffered seek averaged 85 ms. By the late 1980s, ST-412 drives were capable of average seek times between 15 and 30 milliseconds.
Compatible systems and developments
Many other companies quickly introduced drives using the same connectors and signals, creating a hard drive standard based on the ST-506. IBM chose to use it, acquiring adapter cards for the PC/XT from Xebec and for the PC/AT from Western Digital. As a consequence of IBM's endorsement, most of the drives in the 1980s were based on the ST-506. However, the complexity of the controller and cabling led to newer solutions like SCSI, and later, ATA (IDE). A few early SCSI drives were actually 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 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||13||14||~HD SLCT 0|
|Key (no pin)||15||16||Reserved|
|GROUND||17||18||~HD SLCT 1|
|GROUND||25||26||~DRV SLCT 0|
|GROUND||27||28||~DRV SLCT 1|
|GROUND||29||30||~DRV SLCT 2|
|GROUND||31||32||~DRV SLCT 3|
|No connection||7||8||Key (no pin)|
|No connection||9||10||No connection|
|+MFM WRITE||13||14||-MFM WRITE|
|+MFM READ||17||18||-MFM READ|
- Power Connector
- Pin 1 +12 V DC
- Pin 2 +12 V return
- Pin 3 +5 V return
- Pin 4 +5 V DC
- "Disc-storage innovations keep coming while manufacturers ponder user needs". EDN. May 20, 1980. p. 59.
- Seagate ships one billionth hard drive, Computerworld, April 22, 2008
- the principal difference was that the data rate was increased from 4.34 to 5.00 Mbit/s.
- "Simplify system design with a single controller for Winchester/floppy combo," Electronic Design, October 25, 1979, pg 76-80.
- ST-506 / ST-412 Interface
- Seagate ST506/412 OEM Manual
- "Xebec Lands Key IBM Controller Pact". Computer System News. November 29, 1982. pp. 1, 29.
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