ST-506
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The ST-506 was the first hard disk intended for use specifically on microcomputers, introduced in 1979 by Seagate Technologies. The ST506 was a full-height 5.25" drive that stored up to 5MB after formatting, partnered by the otherwise-similar (but much more expensive) 10MB ST412. Both were introduced using MFM encoding (already widely used in floppy disks), but later switched to RLL for a 50% boost in capacity and data rate.
The ST506 was interfaced to the microcomputer using an S-100 bus card which 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 ST506 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 ST506-based hard drive standard. By the time the IBM PC was being introduced in the 1980s it was universal. IBM wisely chose to use it, building an adaptor card for their expansion system in the PC.
Almost all drives in the 1980s were ST506-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 ST506 drives with a SCSI to ST506 (or, in some cases, ESDI) adaptor on the bottom of the drive; this continued until the early 1990s, when single-chip ATA and SCSI controllers from Western Digital, Adaptec and Emulex/QLogic (as well as custom ones designed by the hard drive makers themselves) became commonplace.