What is the design of the IDE (Integrated Drive Electronics)
          interface and how should it be used with SCO XENIX and SCO UNIX
           System V/386?
            

           IDE is an acronym for Integrated Drive Electronics.  Many of the
           latest generation hard disk drives are of this type.  This
           interface design is characterized  by the connection of a low
           cost IDE adapter (frequently included on the motherboard)
           to one or two IDE hard drives. 

           The drives are connected to the adapter by a single 40 pin cable
           which carries both control and data information.  The adapter
           is very low cost and easy to build because the intelligence
           controlling the hard drive is located in a controller
           which is part of the hard drive itself.

           Each of the two IDE drives which may be connected to a single
           adapter plays one of three rolls:

           1. If only one drive is connected it must be jumpered
              for single drive mode.

           2. If two drives are connected, one drive must be jumpered
              for the two drive master mode.

           3. If two drives are connected, one drive must be jumpered
              for the two drive slave mode.

           You cannot take a one drive system and just add another drive
           without changing the jumpers.

           IDE drives, as commonly marketed today, emulate the ST506 interface
           common to all MFM and most ESDI controller/drive combinations.  When
            installing an IDE drive, the ST506 interface selection should be 
            used.
 
           Marking bad spots on an IDE drive is unnecessary.  The embedded
           controller watches the disk and will map out the bad tracks
           automatically.  When installing SCO XENIX or SCO UNIX System V/386 
            the bad track scanning program badtrk(ADM) is not necessary.  In 
            fact, if you do start to see error messages indicating that bad 
            spots are being found on the hard disk as you are running it, the
           IDE hard drive itself should be replaced.

           Another feature of most IDE drives is that the embedded controller
           can do sector translation automatically.  This allows the drives
           to be configured with several different head/cylinder/sectors per
           track settings.  You may want to use some settings other than the
           factory defaults if your drive has more than 1024 cylinders or
           if the new settings more closely match some drive type in the
           ROM BIOS (an important issue if you are also installing DOS).

           This advanced automatic translation feature sometimes has
           an associated cost.  Many IDE hard drives cannot be low-level
           formatted by normal formatting utilities.  The utility
           program will not understand the underlying hardware of the
           IDE drive and may render the drive useless. It is important
           to check with the manufacturer of any IDE hard drive before
           attempting to low-level format the drive.

           Perhaps the most confusing thing about IDE hard drives is that
           the IDE interface is not yet a uniform standard.  Because of this,
           each manufacturer may build their IDE drives differently.  
           Sometimes, one manufacturer may even build different drives
           within their own product line differently.

           Most IDE drives work the same way and most will work with SCO
           XENIX and SCO UNIX System V/386 without any special effort.

           SCO is in the process of qualifying IDE hard drives so that we 
            can track which drives may be troublesome.  So far, we do not 
            have any IDE hard disk drives which we know for sure will not 
            work with SCO XENIX or SCO UNIX System V/386.

           When running SCO XENIX 2.3.2 GT some IDE hard drives require
           Support Level Supplement (SLS) xnx259b in order to be installed.
           This SLS is free of charge to any SCO customer requiring it.
           
           The following IDE drives and hard disk controllers have been
           tested and are known to work with SCO XENIX 386 Operating System 
            Release 2.3.2 GT.  These drives fall into two categories:  those
           IDE hard drives that require SLS xnx259b in order to install; and 
            those that do not require SLS xnx259b.

           A. Drives that require SLS xnx259b

              Older model COMPAQ MFM hard disk controllers
              Seagate 157A  
              Rhodine RO 3000A series *

           B. Drives that work without SLS xnx259b

              Rhodine                 RO 3000AP series *
              Microscience               7070   series *
              Microscience               7100   series *
              Plus Development 120-AT 
              Seagate ST1144A
              Conner  CP3104
              Conner  CP3204F

           * The word "series" indicates that a vendor is selling a group of 
              drives that have sequential identification numbers.  For example,
              the Rhodine RO 3045A drive is a member of the Rhodine RO 3000A 
              series.

           SLS xnx259b is available from the SCO Support Media Department.