Last updated Seagate ST506 5¼-inch HDD with cover removed5.25 inch MFM hard disk drive
The ST-506 and ST-412 (sometimes written ST506 and ST412[1]) were early hard disk drive products introduced by Seagate in 1980 and 1981 respectively,[1] that later became construed as hard disk drive interfaces: the ST-506 disk interface and the ST-412 disk interface. Compared to the ST-506 precursor, the ST-412 implemented a refinement to the seek speed, and increased the drive capacity from 5 MB to 10 MB, but was otherwise highly similar.[1]
Beginning with its selection as the hard drive subsystem for the original IBM XT[1]disk drive controllers supporting the ST-412 interface grew to become ubiquitous in the personal computer industry,[2] The ST-412 interface and its 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.
Both interfaces used MFM encoding; the subsequent extension of the ST-412 interface, the ST-412HP interface, used RLL encoding for a 50% increase in capacity and bit rate.
History
The ST506 HDD was the first 5.25inch hard disk drive, introduced in 1980[3] by Shugart Technology (now Seagate Technology). It stored up to 5 megabytes after formatting (153 cylinders, 4 heads, 26 sectors/track, 256 bytes/sector) and cost US$1,500 (equivalent to $4,933in 2021).[4] The similar, 10-megabyte ST412 HDD was introduced in late 1981 (with 306 cylinders). 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,[5] which was in turn based upon the floppy disk drive interface,[6] thereby making disk controller design relatively easy.[3]
The ST412 interface was adopted by numerous HDD manufacturers such that the interface became a de facto industry standard for disk drives[7] well into the 1990s.
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 (6.1m) cable length. The standard channel code for the ST412 (and ST506) is MFM with one data bit per transition for a data rate of 5Mbit/s. The ST412HP RLL variant averages 1.5 data bits per transition for a data rate of 7.5Mbit/s.
Interface to controller
A 34-pin control cable and a 20-pin data cable for an ST412 drive connected to a controller card. A 4-pin Molex connector supplying power to the drive can not be seen in this image.
In the ST506 interface, the drive connects to a controller card with two ribbon cables carrying signals, while a third cable provides power. The two signal cables are the wide 34-pin control cable and the narrow 20-pin data cable. The control cable interface is very similar to the standard Shugart floppy disk interface; like that floppy disk interface, it can support four drives. The data cable carries a read signal and write signal, both as differential binary signals: the two signal states correspond to the two possible differential signal polarities. The data represented by these signals is MFM, so the absolute signal states are not significant: the data is represented in the timing of the state transitions, like in floppy disk systems. While up to four drives can share a control cable, each drive has its own dedicated data cable connecting it to the hard disk controller (HDC). Most HDCs supported only two drives.
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 control cable 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. The original ST506/ST412 interface defined only two HD SLCT lines, providing supporting for only four heads, but a third HD SLCT line was shortly added to the design; a fourth was added a not much later by redefining the reduced Write Current signal, needed only by very early drives, as HD SLCT 3. Once the heads are properly positioned and the appropriate head is selected, data is read or written serially through a set of pins in the 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 in the drive then moves the mechanism to the desired track as fast as possible. The ST506 disk drive without buffered seek averages 170ms (similar to a floppy drive or modern optical drive) while the mechanically very similar ST412 disk drive with buffered seek averages 85ms.[8] 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 SCSI, with its rich command set, 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 unit rather than being plugged into the computer's backplane. This allows a single "controller" card—really just an interface card—to communicate with multiple dissimilar drives, while it also reduces latency and noise between the controller and drive hardware.[9] Effectively, the roles are reversed: instead of the controller doing almost all of the complex processing and the drive just transferring encoded data between the magnetic disks and the controller, the drive does almost all of the complex processing and the "controller" just transfers decoded data between the drive and the host system. In these systems, the operational details of the drive, like head selection and seeking, are entirely hidden from the host and handled within the drive's dedicated controller. These became known as "smart" drives, while ST506-like devices retroactively became known as "dumb".
While integrated controllers have many benefits, they also have a disadvantage: the mechanical drive (called the "head-disk assembly", or HDA) and the controller are effectively fused into a monolithic black box, so that if something goes wrong with the drive, it is nearly impossible to do anything about it—the data is usually irretrievably lost. With a separated controller and disk system like that of the ST506 interface, sometimes the problem can be resolved by connecting the disk drive, containing the actual (perhaps very important) data, to another compatible controller. Furthermore, an ST506 style interface makes it possible and easy not only to replace the controller without throwing away the data, but to get access to the analog data signals from the disk drive and process them through a special data recovery system that may be able to reconstruct data that a normal controller cannot read. Such data recovery techniques are much more difficult to execute on integrated drives, because the needed analog signals from the disk are not available at a standard interface and the internal data recording method, sector format, and disk organization of nearly every integrated drive model is different and secret.
Compatible systems and developments
Western Digital WD1006
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[10] 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.[11] Atari also used Adaptec ACB-4000A SCSI to ST506 converter inside its own line of SH204/SH205 external ACSI drives.[12] 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.
Connector pinouts
From ST506/ST412 OEM manual.[8] In the following tables, "~" denotes a negated (active low) signal.
Control Connector
Control connector pinout
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
Data Connector
Data connector pinout
~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
Power Connector
Pin 1
+12V DC
Pin 2
+12V return
Pin 3
+5V return
Pin 4
+5V DC
Notes
1 2 3 4 "Beginnings: MFM drives of the 1980s". redhill.com. Red Hill Technology. n.d. Retrieved 16 September 2020. Note: 'ST-412' is correct. You will often see 'ST412' written in error.
Interfaces are listed by their speed in the (roughly) ascending order, so the interface at the end of each section should be the fastest. Category
Related Research Articles
Parallel ATA (PATA), originally AT Attachment, also known as ATA or IDE is standard interface designed for IBM PC-compatible computers. It was first developed by Western Digital and Compaq in 1986 for compatible hard drives and CD or DVD drives. The connection is used for storage devices such as hard disk drives, floppy disk drives, and optical disc drives in computers.
Industry Standard Architecture (ISA) is the 16-bit internal bus of IBM PC/AT and similar computers based on the Intel 80286 and its immediate successors during the 1980s. The bus was (largely) backward compatible with the 8-bit bus of the 8088-based IBM PC, including the IBM PC/XT as well as IBM PC compatibles.
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interfaces. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements. The initial Parallel SCSI was most commonly used for hard disk drives and tape drives, but it can connect a wide range of other devices, including scanners and CD drives, although not all controllers can handle all devices.
Shugart Associates was a computer peripheral manufacturer that dominated the floppy disk drive market in the late 1970s and is famous for introducing the 5+1⁄4-inch "Minifloppy" floppy disk drive. In 1979 it was one of the first companies to introduce a hard disk drive form factor compatible with a floppy disk drive, the SA1000 form factor compatible with the 8-inch floppy drive form factor.
The disk controller is the controller circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive. It also provides an interface between the disk drive and the bus connecting it to the rest of the system.
The Zip drive is a removable floppy disk storage system that was introduced by Iomega in late 1994. Considered medium-to-high-capacity at the time of its release, Zip disks were originally launched with capacities of 100 MB, then 250 MB, and finally 750 MB.
Modified frequency modulation (MFM) is a run-length limited (RLL) line code used to encode data on most floppy disks and some hard disk drives. It was first introduced on hard disks in 1970 with the IBM 3330 and then in floppy disk drives beginning with the IBM 53FD in 1976.
Run-length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. RLL codes are defined by four main parameters: m, n, d, k. The first two, m/n, refer to the rate of the code, while the remaining two specify the minimal d and maximal k number of zeroes between consecutive ones. This is used in both telecommunication and storage systems that move a medium past a fixed recording head.
SyQuest Technology, Inc. (Nasdaq: SYQT) was an early entrant into the hard disk drive market for personal computers. The company was founded on January 27, 1982, by Syed Iftikar who had been a founder of Seagate, along with Ben Alaimo, Bill Krajewski, Anil Nigam and George Toldi. Its earliest products were the SQ306R, a 5 MB 3.9" (100 mm) cartridge disk drive and associated Q-Pak cartridge for IBM XT compatibles. Subsequently a non-removable medium version was announced, the SQ306F.
Each generation of floppy disk drive (FDD) began with a variety of incompatible interfaces but soon evolved into one de facto standard interface for the generations of 8-inch FDDs, 5¼-inch FDDs and 3½-inch FDDs. For example, before adopting 3½-inch FDD standards for interface, media and form factor there were drives and media proposed by Hitachi, Tabor, Sony, Tandon, Shugart and Canon.
Cylinder-head-sector (CHS) is an early method for giving addresses to each physical block of data on a hard disk drive.
Micropolis Corporation was a disk drive company located in Chatsworth, California and founded in 1976. Micropolis initially manufactured high capacity hard-sectored 5.25-inch floppy drives and controllers, later manufacturing hard drives using SCSI and ESDI interfaces.
A floppy-disk controller (FDC) is a special-purpose integrated circuit and associated disk controller circuitry that directs and controls reading from and writing to a computer's floppy disk drive (FDD). The FDC is responsible for reading data presented from the host computer and converting it to the drive's on-disk format using one of a number of encoding schemes, like FM encoding or MFM encoding, and reading those formats and returning it to its original binary values.
Parallel SCSI is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the other. A SCSI device attaches to the bus but does not interrupt it. Both ends of the bus must be terminated.
Write precompensation is a technical aspect of the design of hard disks, floppy disks and other digital magnetic recording devices. It is the modification of the analog write signal, shifting transitions somewhat in time, in such a way as to ensure that the signal that will later be read back will be as close as possible to the unmodified write signal. It is required because of the non-linear properties of magnetic recording surfaces.
Lt. Kernal is a SCSI hard drive subsystem developed for the 8-bit Commodore 64 and Commodore 128 home computers. The Lt. Kernal is capable of a data transfer rate of more than 38 kilobytes per second and 65 kilobytes per second in Commodore 128 fast mode.
The Advanced Disc Filing System (ADFS) is a computing file system unique to the Acorn computer range and RISC OS-based successors. Initially based on the rare Acorn Winchester Filing System, it was renamed to the Advanced Disc Filing System when support for floppy discs was added and on later 32-bit systems a variant of a PC-style floppy controller.
Hard disk drives are accessed over one of a number of bus types, including parallel ATA, Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses with which they cannot communicate natively, such as IEEE 1394, USB, SCSI, NVMe and Thunderbolt.
The Commodore D9060/D9090 Hard Disks were the only family of hard drives that Commodore made for both the home and business market. The electronics are identical in the D9060 and the larger D9090 unit; the only difference is the size of the installed hard drive, with a jumper set to distinguish between 4 or 6 disk heads. Originally intended for the metal-cased PET/CBM series of computers, they are compatible with the VIC-20, Commodore 64 and later models with an adapter.
Finis Conner is an American entrepreneur and pioneer of the disk drive industry, founding industry leaders Shugart Associates, Seagate Technology and Conner Peripherals. Conner Peripherals, a major HDD manufacturer, was founded in 1987, and by 1990 had become the fastest-growing start-up in the history of U.S. commerce up until that point. Conner Peripherals was acquired by Seagate in 1996.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
