ST-506
Encyclopedia
Seagate Technology
Seagate Technology is one of the world's largest manufacturers of hard disk drives. Incorporated in 1978 as Shugart Technology, Seagate is currently incorporated in Dublin, Ireland and has its principal executive offices in Scotts Valley, California, United States.-1970s:On November 1, 1979...
(then Shugart Technology), it stored up to 5 megabytes after formatting and cost $1500. The similar (but more expensive) 10 MB ST-412 was introduced in late 1981. Both used MFM
Modified Frequency Modulation
Modified Frequency Modulation, commonly MFM, is a line coding scheme used to encode the actual data-bits on most floppy disk formats, hardware examples include Amiga, most CP/M machines as well as IBM PC compatibles. Early hard disk drives also used this coding.MFM is a modification to the original...
encoding (already widely used in disk drives). A subsequent extension of the ST-412 used RLL
Run Length Limited
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. This is used in both telecommunication and storage systems which move a medium past a fixed head. Specifically, RLL bounds the length of stretches ...
for a 50% boost in capacity and bit rate
Bit rate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
.
The ST-506 connected to a computer system through a disk controller
Disk controller
The disk controller is the circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive.Early disk controllers were identified by their storage methods and data encoding. They were typically implemented on a separate controller card...
. The ST-506 interface between the controller and drive was derived from the Shugart Associates
Shugart Associates
Shugart Associates was a computer peripheral manufacturer that dominated the floppy disk drive market in the late 1970s and is famous for introducing the 5¼-inch minifloppy disk drive....
SA1000 interface which was in turn based upon the floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
drive interface thereby making disk controller design relatively easy. The ST-506 Interface and its variants (ST-412, ST-412RLL) were defacto 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 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 bandwidthBandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
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
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
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. By the late 1980s ST412 drives were capable of average seek times between 15-30 milliseconds
Compatible systems and developments
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/XTPC/XT
PC/XT has multiple possible meanings:* "PC/XT" may simply be shorthand for the IBM Personal Computer XT.* "PC/XT" may also be intended to mean "the IBM Personal Computer and/or the IBM Personal Computer XT"....
from Xebec and for the PC/AT from Western Digital
Western Digital
Western Digital Corporation is one of the largest computer hard disk drive manufacturers in the world. It has a long history in the electronics industry as an integrated circuit maker and a storage products company. Western Digital was founded on April 23, 1970 by Alvin B...
. 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
SCSI
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, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
, 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.
Connector pinouts
From ST506/ST412 OEM ManualIn 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 |
Power Connector
- Pin 1 +12v DC
- Pin 2 +12v return
- Pin 3 +5v return
- Pin 4 +5v DC