ST-506

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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 data 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.^[4]

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 head 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.

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). In most cases SCSI drives were in fact ST-506 drives with a SCSI to ST-506 (or, in some cases, ESDI) controller on the bottom of the drive; this continued until the early 1990s, when single-chip ATA and SCSI host adapters from Western Digital, Adaptec and Emulex/QLogic (as well as custom ones designed by the hard drive makers themselves) became commonplace.

[edit] Connector Pinouts

From ST506/ST412 OEM Manual^[6]

In the following table, "~" denotes a negated (active low) signal.

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 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

[edit] Notes

1. ^ "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. ^ op. cit. EDN
5. ^ "Xebec Lands Key IBM Controller Pact", Computer System News, November 29, 1982, pg. 1, 29.
6. ^ http://www.bitsavers.org/pdf/seagate/ST412_OEMmanual_Apr82.pdf