On Sat, 2008-02-02 at 13:39 -0500, William Case wrote: > Can someone briefly explain On this list? ;-) > to me the difference between an IDE (ATA) and a SCSI device. They're different systems. At the simplest description, they use different connectors, and you can't use a IDE device on a SCSI bus, nor vice versa. This is square peg and round hole territory. Getting more technical, it's also about the methodology of talking to the device. So it's not just a case of having the same signals in different connectors. If you can remember back to when there were at least four or five different types of CD-ROM connections, none compatible with each other, and some hard to tell which they were, we finally got a mostly standardised ATAPI system. And what's that? An IDE style of connection to a drive that's pretending to be SCSI (it uses the commands that SCSI does, and acts like a SCSI drive, but using the IDE connection). And how did Linux handle that? Sometimes you talked to your CD-burner as if it were an IDE (ATAPI) device, other times as if it were SCSI. Then the high speed UDMA IDE drives started coming out (the ones that use the 80-wire connection), and we got computers with new interfaces to handle them. They, mostly, show up on a system as SCSI devices. Then we had things like IDE hard drives in an external box, that connects to the computer using a USB connector, and appearing to the computer as if it were a SCSI device. Now we have SATA, which is a move away from the parallel ATA to a serialised ATA system, and again, the drive is acting in a SCSI manner. You didn't expect things to get less confusing, did you? :-\ > Given below are some questions that spring to mind. They may be > mis-formed questions and therefore need not be answered, but they may > demonstrate where my confusion and misunderstanding are coming into > play. > > e.g. > Does IDE refer to the physical device? > Or, specifically just to the bus used? > Or, to the driver for the device? > Or, the type of interface (plug)? I'd say, to explain it simply, all of the above. You use an IDE drive on an IDE drive interface, using IDE leads, etc. > Does SCSI refer to a set of protocols used when designing the device? > Or, to a specific driver design? As a simple explanation, there's two aspects to SCSI. The commands used to use the drive, and the bus used to connect it. They go together, mostly. Though, now, there's more and more devices that talk SCSI, but are connected in different manner than traditional SCSI devices. > Can you have an IDE device without SCSI? > Or, can you have a SCSI device without it being IDE? Yes, they're two different systems. They're not interchangeable, and one doesn't depend on the other being present. And, at least in the olden days, it wasn't too common to find a personal computer that had both types at the same time. Generally speaking, if someone talks about a SCSI drive, they're talking about a SCSI drive that connects via a SCSI bus. Likewise, for IDE. Unless they're odd, they won't be meaning a SATA drive, or any other drive, simply because of the commands it might use. Most people call the drive for how you connect it up. Just to be confusing, there's two ways to connect an IDE drive, to a 40 wire IDE port (whether using a 40 or 80 wire cable), or the 44 pin connector inside laptops (it has power, as well as data in the lead). Just one way to connect a USB drive, to a USB connector (though I don't ever recall seeing a genuine USB harddrive, they're supposed to exist). Just one way to connect firewire drives. There's two ways to connect a SATA drive, internal and external SATA connectors are physically different. And there's several ways of connecting SCSI drives - internally, a 50-wire ribbon is usual, but externally there's the 25 & 27 pin D connectors, small 50 pin connectors, large 50 pin connectors, and several other connector types. I would have though that this, would have answered some of your questions about SCSI: > SCSI (Small Computer System Interface) is a set of standards for > physically connecting and transferring data between computers and > peripheral devices. The SCSI standards define commands, protocols, and > electrical and optical interfaces. SCSI is most commonly used for hard > disks and tape drives, but it can connect a wide range of other devices, > including scanners and CD drives. The SCSI standard defines command sets > for specific peripheral device types; the presence of "unknown" as one > of these types means that in theory it can be used as an interface to > almost any device, but the standard is highly pragmatic and addressed > toward commercial requirements. It's a physical and technical standard. -- (This computer runs FC7, my others run FC4, FC5 & FC6, in case that's important to the thread.) Don't send private replies to my address, the mailbox is ignored. I read messages from the public lists.
