Xref: utzoo comp.periphs:1135 comp.sys.atari.st:10852 Path: utzoo!utgpu!attcan!uunet!peregrine!elroy!ames!husc6!purdue!decwrl!vixie From: vixie@decwrl.dec.com (Paul Vixie) Newsgroups: comp.periphs,comp.sys.atari.st Subject: Re: ESDI questions Message-ID: <35@volition.dec.com> Date: 13 Aug 88 01:39:03 GMT References: <168@cstw01.UUCP> <1988Aug10.165519.20257@utzoo.uucp> Organization: DEC Western Research Lab Lines: 46 henry@utzoo.uucp (Henry Spencer) writes: # ESDI is sort-of-kind-of an improved ST506/412. It was designed to be a # better and faster drive-to-controller interface. Its relationship to # ST506/412 is as a (superior) competitor. It is unrelated to SCSI, which # is a controller-to-host interface. This is a visual and semantic problem that causes a great deal of confusion. In ST506 and ESDI, there is a board on the system IO bus (or some chips and a port on the motherboard) to which one attaches a cable; the other end of this cable is attached to a disk drive. In SCSI, there is STILL a board on the system IO bus (or some chips and a port on the motherboard) to which one attaches a cable; the other end of this cable is attached to a disk drive, which has an on-drive disk controller. It's just that in ST506 and ESDI, we call the board-on-the-bus or chips-on-the-motherboard a "controller" and the cable running to the drive(s) is called a "drive-to-controller interface", while in SCSI, we call the board-on-the-bus or chips-on-the-motherboard a "SCSI interface" and the cable running to the on-drive disk controller is called a "controller-to-host interface". To most people, there's still just a board in the machine and a cable going to a disk drive. The fact that the board is called an "interface" in SCSI because the controller is in the drive doesn't change the common terminology of calling the board in the CPU cabinet a "controller". It is more accurate to call it an "interface", of course, because the controller (the thing that deals with the MFM or whatever modulation the drive uses) is in fact elsewhere. It all hangs on calling the board a "controller" when in SCSI, it's an "interface". I can't repeat that too many times -- us board-swappers that don't learn the details of things are easily confused by details like this one. (I don't mean that _you_ are confused, Henry -- it's just that your answer didn't really ease the confusion that had prompted the question.) -- Paul Vixie Digital Equipment Corporation Work: vixie@dec.com Play: paul@vixie.UUCP Western Research Laboratory uunet!decwrl!vixie uunet!vixie!paul Palo Alto, California, USA +1 415 853 6600 +1 415 864 7013