From: scjones@thor.sdrc.com (Larry Jones)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: IDE, RLL, SCSI, EDSI, MFM, SCSI-2 ?
Summary: Disk Interfaces Explained
Date: 6 Nov 92 21:23:41 GMT

In article <92308.202512U35334@uicvm.uic.edu>, U35334@uicvm.uic.edu (Juan A. Varela F.) writes:
> I have been reading in this group a lot about the different types of hard
> disks.  What is the difference among all these?  Which one is technologically
> superior?  Why?  How are they compatible with each other?  I basically
> understand nothing about hard drives and any information would be greatly
> appreciated.

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, SCSI-2, 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 disks.

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.  RLL packs
more data into the same disk space as MFM but requires tighter
tolerances in the disk mechanism and electronics.

ARLL is Advanced RLL (also known as ADRT for Advanced Data Recording
Technology).  This is Perstor's name for their RLL recording scheme
which achieves nearly twice the capacity of MFM.

ZBR is Zone Bit Recording which means that different data rates 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.  These controllers rarely have
any intelligence, although there are a few that can support sector
translation.  Sector translation allows you to circumvent limits like
the PC BIOS limit of 1024 cylinders by pretending that the disk has more
heads and/or more sectors per track than it really does and fewer
cylinders; the controller translates from this virtual geometry to the
real geometry as required.  Most (all?) of these controllers support two
disks, and you usually can't have more than one in a system.

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).  Since the
controller is a traditional controller, you usually can't have more
than one in a system, but there is usually a way to disable the
controller on an IDE disk and connect it to a controller on another IDE
disk.  Again, you are usually limited to two disks and, since the
controller is integrated with the disk, there are frequently
incompatibilities between different manufacturers so you may well need
both disks to be from the same manufacturer.

SCSI is the Small Computer System Interface.  This 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.  The SCSI bus supports up to 8
devices, one of which is usually the adapter card.  SCSI-2 is an
improved version of the original SCSI specification; there is usually no
problem mixing SCSI and SCSI-2 devices on the same bus.  SCSI adapters
come in two varieties: bus mastering, and non-bus mastering.  Bus
mastering adapters actually take over your system bus and handle
transferring data to and from main memory themselves.  Non-bus mastering
adapters let the CPU transfer the data.  Bus mastering is potentially
faster and, if the CPU has cache memory, allows the CPU to continue
processing at the same time as data is being transferred.

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 somewhat lower reliability because of
the tighter tolerances required.  As long as the drive is designed for
use with the recording technology, you shouldn't have any problems. 
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 interface drives are slowest, IDE, SCSI, and ESDI fastest.  For
multiple disks on a multitasking operating system, nothing beats a
top-of-the-line bus mastering SCSI controller.
----
Larry Jones, SDRC, 2000 Eastman Dr., Milford, OH  45150-2789  513-576-2070
larry.jones@sdrc.com  or  ...uunet!sdrc!larry.jones
Mom would be a lot more fun if she was a little more gullible. -- Calvin