scott@hpcvca.CV.HP.COM (Scott Linn) (11/12/90)
This may have already been covered earlier, but I missed it. What are the main differences between RLL, ESDI, SCSI, etc. disk interfaces? Thanks, Scott Linn
scjones@thor.UUCP (Larry Jones) (11/16/90)
In article <25730002@hpcvca.CV.HP.COM>, scott@hpcvca.CV.HP.COM (Scott Linn) writes: > What are the main differences between RLL, ESDI, SCSI, etc. disk > interfaces? It's been a while since the last time I posted this, so here we go again: 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
dd2x+@andrew.cmu.edu (David Eugene Dwiggins) (11/19/90)
Please stop generalizing about IDE drives. My IDE drive uses sector translation (it has up to 50 sectors/track) and is faster than some ESDI drives. Generally, in the past one could expect MFM or RLL controllers on IDE drives, but that assumption doesn't hold anymore. Make sure to check what kind of controller the IDE drive you are considering uses before you buy. David Dwiggins