djh@neuromancer.metaphor.com (Dallas J. Hodgson) (09/21/90)
In article <Sep.19.12.05.42.1990.26653@dimacs.rutgers.edu> srm@dimacs.rutgers.edu (Scott R. Myers) writes: > >Okay I'm tired of being in the dark about all of this different Hard >Disk interfacing. I've worked with IBM, Mac and Amiga and everyone >seems to do it different. I've got IDE MFM ESDI Hocus Pocus on the >brain. Can some of the netlanders shed some light on What all of >these Acronyms mean but more importantly for my knowledge which is the >better performer and why. Thanx in Advance!!! ST506: The standard IBM-PC controller standard, invented by Seagate (or was it Shugart?) Basic Premise: Cheap; the hard drive smarts are in the controller card. RLL : "Run Length Limited", a slightly different version of the above that uses smarter compression technology to increase the recorded bit- density. RLL drives are identical, but "certified" as RLL. The controller card makes the difference. ESDI : The highest-performance hard drive standard in popular PC use. A much more thought-out, higher-performance standard. Twice the bit density of standard ST506 drives. Up to 1.5mb/sec transfer rate. The drives are a bit smarter. Special controller card required. "ESDI = Enhanced Systems Data Interface", I believe. SCSI : A kind of generic hardware protocol where many devices of different types can be daisy-chained together and communicate with each other in a standard fashion. SCSI-II is the latest improved spec, which allows SCSI hardware to talk amongst themselves, among other things. Most of the smarts are in the peripherals themselves; the controller cards are simple in comparison. Max transfer rates can be comparable to ESDI, but this depends on the implementation. ST506 (PC) and SCSI (Non-PC) are the most popular microcomputer drive formats. There are more formats than this, but you don't see 'em in the (comparitively low-performance) microcomputer industry. Anybody remember the first Amiga hard drive? The A-1000 SASI interface drive from Tecmar. What a $1000 20-MB failure! +----------------------------------------------------------------------------+ | Dallas J. Hodgson | "This here's the wattle, | | Metaphor Computer Systems | It's the emblem of our land. | | Mountain View, Ca. | You can put it in a bottle, | | USENET : djh@metaphor.com | You can hold it in your hand." | +============================================================================+ | "The views I express are my own, and not necessarily those of my employer" | +----------------------------------------------------------------------------+
scjones@thor.UUCP (Larry Jones) (09/21/90)
In article <Sep.19.12.05.42.1990.26653@dimacs.rutgers.edu>, srm@dimacs.rutgers.edu (Scott R. Myers) writes: > Okay I'm tired of being in the dark about all of this different Hard > Disk interfacing. I've worked with IBM, Mac and Amiga and everyone > seems to do it different. I've got IDE MFM ESDI Hocus Pocus on the > brain. Can some of the netlanders shed some light on What all of > these Acronyms mean but more importantly for my knowledge which is the > better performer and why. Thanx in Advance!!! OK, one more time: There are two separate concepts that need to be addressed to understand hard disks -- the recording technology and the disk interface. FM, MFM, RLL, ARLL, ADRT, and ZBR and all recording technologies. ST412, ST506, ESDI, SCSI, IDE, and AT are disk interfaces. Any recording technology can be combined with any interface, so it takes a pair of these to completely describe a disk. The recording technology is actually determined by the controller which may be an integral part of the disk or may be a completely separate piece of hardware depending on the interface. Recording Technologies ---------------------- FM is Frequency Modulation. This is the recording technology used for single density floppies. It is not currently in use for hard disk. MFM is Modified Frequency Modulation. This is the recording technology used for double density floppies and many hard disks. It has twice the capacity of FM and results in the traditional 17 sectors of 512 bytes each per track on a typical disk. RLL is Run Length Limited. There are actually infinitely many RLL recording schemes including FM (RLL 0,1) and MFM (RLL 1,3). When used all by itself, it refers to RLL 2,7 which has three times the capacity of FM (1.5 time MFM) and results in 26 sectors per track. ARLL is Advanced RLL (also known as ADRT for Advanced Data Recording Technology). This is another RLL method which is used by Perstor to achieve nearly four times the capacity of FM. ZBR is Zone Bit Recording which means that different recording methods are used on different parts of the disk. This allows many more sectors per track on the large outer tracks than on the small inner tracks. Disk Interfaces --------------- ST412 and ST506 are the traditional hard disk interfaces. The controller is completely separate from the disk; it typically plugs into a bus and is connected to the disk by a cable. These interfaces are nearly identical (the names are currently used interchangably) and are named for the original Shugart disks that had them. IDE is Integrated Drive Electronics (which is also known as AT for the IBM PC-AT). IDE puts a traditional disk controller on the disk drive. The controller can then be connected by a cable directly to the AT bus (although a special connector or adapter card is required). SCSI is the Small Computer System Interface. The SCSI is a separate bus which is defined to allow all sorts of peripherals to be connected -- disks, tape drives, even printers. A SCSI disk drive has an integral controller which can completely hide the actual geometry of the disk which allows for things like ZBR. The computer system also needs an interface to the SCSI bus -- this can be as simple as an adapter card that lets software read and write the individual bus lines of as complex as an intelligent controller that supports multiple outstanding requests and bus master DMA access to memory. ESDI is the Enhanced Small Device Interface, another bus similar to SCSI but optimized for disks only. An ESDI drive has the most critical parts of the controller on the drive and the rest of the controller on a separate card which is connected to the drive by a cable. Advantages and Disadvantages ---------------------------- Recording technologies are easily summed up -- higher densities give you higher capacity and speed and lower reliability. Drives with integrated controllers increase reliability since the low-level signals from the disk don't have nearly as far to go. Thus, a high density drive with an integral controller should be as reliable as a lower density drive with a separate controller. Drives with integral controllers are more expensive than drives without (for obvious reasons), which can be important if you're buying more than one. ESDI is a nice compromise here since some of the controller logic is shared. SCSI is nice if you want to support lots of devices (SCSI allows up to 7 devices on the bus, the other interfaces support only two disks) or a number of different devices. Performance is very difficult to generalize. It depends as much on what you are doing and what kind of software is driving the hardware as it does on the actual hardware. As a very rough rule of thumb, ST506 and IDE interface drives are slowest, SCSI and ESDI fastest. ---- Larry Jones UUCP: uunet!sdrc!thor!scjones SDRC scjones@thor.UUCP 2000 Eastman Dr. BIX: ltl Milford, OH 45150-2789 AT&T: (513) 576-2070 There's a connection here, I just know it. -- Calvin
monty@sagpd1.UUCP (Monty Saine) (09/22/90)
In article <1449@metaphor.Metaphor.COM> djh@neuromancer.metaphor.com (Dallas J. Hodgson) writes: >SCSI : A kind of generic hardware protocol where many devices of different > types can be daisy-chained together and communicate with each other > in a standard fashion. SCSI-II is the latest improved spec, which > allows SCSI hardware to talk amongst themselves, among other things. SCSI devices have always had the ability to talk among themselves. It's just that all manufacturers did not implement the feature! Monty Saine
d6b@psuecl.bitnet (09/22/90)
In article <184@thor.UUCP>, scjones@thor.UUCP (Larry Jones) writes: > RLL is Run Length Limited. There are actually infinitely many RLL > recording schemes including FM (RLL 0,1) and MFM (RLL 1,3). When used Just to clarify: what do the two numbers mean? Take MFM as an example. I assume that the 1 means at least 1 zero (lack of transition) after a one, and the 3 means up to 3 zeros in a row, but I'm not sure...what's the correct interpretation? Also, what's the relative (raw) bit frequency (with an equivilent transition density) for MFM and RLL (I assume 1.5)? And ARLL? Thanks for the article...very nicely done! -- Dan Babcock
mk59200@korppi.tut.fi (Kolkka Markku Olavi) (09/24/90)
In article <184@thor.UUCP> scjones@thor.UUCP (Larry Jones) writes: >ESDI is the Enhanced Small Device Interface, another bus similar to SCSI >but optimized for disks only. Actually ESDI is an improved version of ST506/ST412 with the encoding/decoding circuitry moved to the drive and some other modifications. It's not at all similar to SCSI. >(SCSI allows up to 7 devices on the bus, the other interfaces support only two >disks) ST506 and ESDI allow 4 disks, but most IBM-PC controllers support only 2 disks. (Otherwise a very good summary. Thanks.) -- Markku Kolkka mk59200@tut.fi
lupe@alanya.Germany.Sun.COM (Lupe Christoph - Sun Germany Consulting - Munich) (09/26/90)
I'd like to add a few things to this very good summary. First of all, with disk interfaces, you may have different widths of the data path. ST506 takes the bitstream from the disk, as far as I'm aware, with the clock bits, and lets the controller in the host decode. This is why you can buy MFM and RLL controllers for ST506 disks. ESDI (I believe) does data separation on the disk side, so you have no choice of encoding.. I also *believe* you still have a bitstream. Maybe not. SCSI has an 8 bit wide data path, with 16 bits coming as an option in SCSI-2. The "ATbus" (that for disks, not that for controllers) has 16 bits, I believe. SCSI and AT use a high-level interface that works with command packets. ST506 and ESDI toggle lines. E.g. to read a sector from some track on some cylinder, you twiddle lines to step the head, then set some others to select the head, the take the bitstream until you got the sector. In SCSI, you tell the disk to get you logical sector #so-and-so. This number counts the sectors on the disk from cyl 0, trk 0, sec 0 up. There are also some interface standards that are not (currently ?) used in the micro world. That's SMD, ESMD, and IPI. SMD is much like ST506 for discussions like these, just faster. ESMD is the same feature-wise, just even faster. BTW, SMD stands for Storage Module Drive, a line of disks manufactured by CDC (today, Imprimis) way back in the 70s. ESMD is Extended/Enhanced or some other noise word SMD. Oh yeah, ST506 is Shugart Technology drive type 506. SCSI means Small Computer Systems Interface. AT is Advanced Technology. I don't know what IPI stands for, maybe Intellegent Peripheral Interconnect. It is somewhat like SCSI, but faster. Haven't seen much of it myself yet. Sun is using it in the high-end machines. Disks have around 1GB, typically. Transfer rate is 3 MB/sec or even 6 MB/sec. -- | lchristoph@Sun.COM (Internet) | Disclaimer: | | ...!unido!sunmuc!lupe (German EUNet, "bang") | My employer has a | | lupe@sunmuc.UUCP (German EUNet, domain) | non-exclusive license | | ...!suninfo!lchristoph (Sun Germany customers) | to my opinion. |
andrey@against.cs.caltech.edu (Andre T. Yew) (09/26/90)
>>>>> On 25 Sep 90 20:57:58 GMT, lupe@alanya.Germany.Sun.COM (Lupe Christoph - Sun Germany Consulting - Munich) said:
L> I don't know what IPI stands for, maybe Intellegent Peripheral Interconnect.
L> It is somewhat like SCSI, but faster. Haven't seen much of it myself yet.
L> Sun is using it in the high-end machines.
I've seen IPI used in some high-end Silicon Graphics machines (>$200K).
Actually they were using IPI-2. I can't remember what IPI stands for, but
I can say that it was extremely fast. One of the demos played back from the
IPI disks had about 1 or 2 minutes digitized from "Batman". They were playing
it back in a window the size of an overscanned NTSC picture at real-time
(30 fps, not 60 fps, as some people around here think it is). So what, you
say. Well, the entire sequence was not compressed. As in, they just stored
each pixel as a 24 bit number or 3 bytes (I don't know which) and read it back
and drew it on to the screen. According to SGI reps, the code is as simple as
something just looping over the file and drawing each line or pixel (not clear)
to the screen.
L> --
L> | lchristoph@Sun.COM (Internet) | Disclaimer: |
L> | ...!unido!sunmuc!lupe (German EUNet, "bang") | My employer has a |
L> | lupe@sunmuc.UUCP (German EUNet, domain) | non-exclusive license |
L> | ...!suninfo!lchristoph (Sun Germany customers) | to my opinion. |
--
Andre Yew
andrey@through.cs.caltech.edu
(131.215.128.1)