Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!husc6!cfa!ward From: ward@cfa.harvard.EDU (Steve Ward) Newsgroups: comp.sys.ibm.pc Subject: Re: Crooked mail-order hard disk company Summary: HDA is only part of a disk drive Message-ID: <1086@cfa.cfa.harvard.EDU> Date: 14 Sep 88 18:11:59 GMT References: <6522@megaron.arizona.edu> <211@pigs.UUCP> <677@ttrde.UUCP> <11 <574@dasher.SanDiego.NCR.COM> Organization: Harvard-Smithsonian Ctr. for Astrophysics Lines: 29 A disk drive generally consists of two major parts: the head-disk assembly and the drive read/write and control electronics. It is true that the same HDA (head-disk assembly) is commonly used on multiple models of disk drives within many manufacturers' disk drive product lines. Sometimes the electronics read/write and control electronics are quite different, though. On ST506, ESDI, and SCSI disk drives the entire drive electronics is commonly on a single board attached to the HDA. The differences between an ST506 RLL and MFM electronics board can be major in that the data rates are different and encoding/decoding methods are different. The same HDA on two drives does not make them the same drive. It is possible to make a drive (HDA + electronics) that will operate on the ST506 interface for both RLL and MFM encoding methods, but most disk drive manufacturers include optimizations in their electronics for either RLL or MFM, making the drive at least better suited for the recommended encoding method, if not rendering operation for the alternate method impossible. It would be desirable if the ST506 drives were all designed for equally good MFM and RLL performance, but I have been told by disk drive manufacturers that this is not generally the case. At any rate, if the electronics boards are different, even with the same HDA, the drives are not the same. Note that some drives use the same HDA but the manufacturer may use a smaller number of heads/platters in one model. This approach gives the maker commonality of parts and allows different capacity models.