ST-506
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The ST-506, introduced in 1980[1] by the then named Shugart Technology (today Seagate Technology) was the first full-height 5.25 inch drive. It stored up to 5 MB after formatting. The similar (but more expensive) 10 MB ST-412 was introduced in late 1981. Both used MFM encoding (already widely used in floppy disks). A subsequent extension of the ST-412 used RLL for a 50% boost in capacity and data rate.
The ST-506 (and the ST-412) 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]
For example, a controller card designed for the S-100 bus microcomputers could control up to four drives. Unlike modern systems where the data is sent to the drive and the drive itself is supposed to deal with it, in the ST-506 the entire system was controlled by the controller and the disks were "dumb", connected to the controller with two cables. The 34-pin control cable would control the mechanical motions of the drive with pins such as "head select 2" which it would use to position the head, and then data could be read or written serially using only 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.
A number of other companies quickly introduced drives using the same connectors and signals, creating an ST-506-based hard drive standard. By the time the IBM PC was being introduced in the 1980s it was universal. IBM chose to use it, building an adaptor card for their expansion system in the PC. As a consequence of IBM's endorsement, almost all 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
In the following table, "~" denotes a negated (active low) signal.
| GROUND | 1 | 2 | ~HD SLCT 3 |
| 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 | No connection |
| 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 | No connection |
| GROUND | 31 | 32 | No connection |
| 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 |
[edit] Notes
- ^ "Disc-storage innovations keep coming while manufacturers ponder user needs," EDN, May 20, 1980, pg 59
- ^ the principal difference was that the data rate was increased from 4.34 to 5.00 Mbits/sec.
- ^ "Simplify system design with a single controller for Winchester/floppy combo," Electronic Design, October 25, 1979, pg 76-80.
- ^ op. cit. EDN
