ST-506

Seagate ST 506 5¼-inch HDD with cover removed.

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. 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 was interfaced to a computer system using a disk controller. The ST-506 interface was derived from the Shugart Associates SA1000 interface^[2]. which was in turn based upon the floppy disk drive interface^[3] thereby making disk controller design relatively easy.^[1]

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 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.

The ST-412 update to the interface added buffered seek capability to the interface.^[4] 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 stepper motor to move. An onboard microcontroller would then send the pulses to the stepper motor at whatever rate it could handle, or reprogram the servo on a voice-coil actuator to move to the desired track. Buffered seek improved seek times greatly, and made drives using it capable of average seek times between 15-30 milliseconds by the late 1980s (older drives, like the ST-506, averaged 100-200 ms, the same as a floppy drive or modern optical drive).

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^[5] 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.

Connector Pinouts

From ST506/ST412 OEM Manual^[4] In the following table, "~" denotes a negated (active low) signal.

Power Connector
Pin 1 +12v DC
Pin 2 +12v return
Pin 3 +5v return
Pin 4 +5v DC

Notes

1. ^ ^{a b} "Disc-storage innovations keep coming while manufacturers ponder user needs," EDN, May 20, 1980, pg 59
2. ^ the principal difference was that the data rate was increased from 4.34 to 5.00 Mbit/s.
3. ^ "Simplify system design with a single controller for Winchester/floppy combo," Electronic Design, October 25, 1979, pg 76-80.
4. ^ ^{a b} http://www.bitsavers.org/pdf/seagate/ST412_OEMmanual_Apr82.pdf
5. ^ "Xebec Lands Key IBM Controller Pact", Computer System News, November 29, 1982, pg. 1, 29.