ST506 is the designation for two related items: the ST506 hard disk drive (HDD) and its associated ST506 interface. They are closely related to the more ubiquitous ST412 HDD and the ST412 interface. While the hard disk drives quickly competed with other vendors' models, the interface variants were the de-facto industry standard for personal computer hard disks until the advent and wider adoption of the IDE or ATA interface in the early 1990s.
The ST506 HDD was the first 5.25 inch hard disk drive, introduced in 1980 by Shugart Technology (now Seagate Technology). It stored up to 5 megabytes after formatting and cost US $1,500 (equivalent to $4,561 in 2018). The similar, 10-megabyte ST412 HDD was introduced in late 1981. The ST225 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 ST412 interface, the ST412HP interface, used RLL encoding for a 50% increase in capacity and bit rate.
The ST506 drive connected to a computer system through a disk controller. The ST506 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 limitations of the ST412 interface are 5 million transitions per second maximum on data lines, 16 heads, 4 drive units and a 20-foot (6.1 m) cable length. The standard channel code for the ST412 (and ST506) is MFM with one data bit per transition for a data rate of 5 Mb/sec. The ST412HP RLL variant averages 1.5 data bits per transition for a data rate of 7.5 Mb/sec
Interface to controller
In the ST506 interface, the drive connects to a controller card with two data cables, while a third provides power. The control card translates requests for a particular track and sector from the host system into a sequence of head positioning commands, including setting the direction of head movement, in or out, and sending individual "STEP" commands to move. Four of the pins, "HD SLCT 0" through "HD SLCT 3", allow the selection among up to 16 heads, although only four are available on the two-platter ST506. Once the heads are properly positioned, data is read or written serially through a set of pins in the 20-pin data cable. The limited bandwidth of the data cable was not an issue at the time and is not the factor that limited the performance of the system. However, the unshielded cable can sometimes be susceptible to high levels of noise.
The ST412 disk drive, among other improvements, added buffered seek capability to the interface. In this mode, the controller can send STEP pulses to the drive as fast as it can receive them, without having to wait for the mechanism to settle. An onboard microprocessor then moves the mechanism to the desired track as fast as possible. The ST506 disk drive without buffered seek, averages 170 ms (similar to a floppy drive or modern optical drive) while the mechanically very similar ST412 disk drive with buffered seek averages 85 ms. By the late 1980s, drives with an ST412 interface were capable of average seek times between 15 and 30 milliseconds.
The process of moving portions of the command interpretation off the controller card and onto the drive itself in order to improve performance is a common feature of later hard drive connection schemes, notably the rich set of commands in SCSI, and the storage-focused IDE systems. IDE, in effect, is a system for extending the computer bus so the interface controller can be built into the drive enclosure rather than having to be plugged into the computer's backplane. This allows a single controller card to talk to multiple drives, as well as reducing latency and noise between the controller and drive hardware. In these systems, the operational details of the drive, like head selection and seeking, is entirely hidden from the host and handled within the drive's microprocessor. These became known as "smart" drives, while ST506-like devices retroactively became known as "dumb".
Compatible systems and developments
Many other companies quickly introduced drives using the same connectors and signals, creating a hard drive standard based on the ST506. 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 ST506. 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 ST506 drives with a SCSI to ST506 controller on the bottom of the drive. Likewise a few early IDE drives were just drives with an ST412 interface attached to a controller board or chip. Ultimately all SCSI and ATA drives had built the controller into the drive, thereby eliminating the ST506/412 interface in such models.
From ST506/ST412 OEM Manual. In the following tables, "~" 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|
|Pin 1||+12 V DC|
|Pin 2||+12 V return|
|Pin 3||+5 V return|
|Pin 4||+5 V DC|
- "Changes to the ST-506/ST-412 Interface". Retrieved 15 January 2019.
- "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.
- ST506 / ST412 Interface
- Seagate ST506/412 OEM Manual
- "System Architecture: a look at hard drives". Archived from the original on 2010-11-18. Retrieved 2008-07-25.
IDE drives on-board controllers are configured to appear to the computer like standard ST506 drives
- "Xebec Lands Key IBM Controller Pact". Computer System News. November 29, 1982. pp. 1, 29.
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