Harddrive Controllers

From: Atari SIG (xx004@cleveland.Freenet.Edu)
Date: 03/02/91-11:00:00 PM Z

From: xx004@cleveland.Freenet.Edu (Atari SIG)
Subject: Harddrive Controllers 
Date: Sat Mar  2 23:00:00 1991

      This is a continuation of the discussion about Hard Disk
controllers and the ST.  It seems that I may have caused some
confusion concerning SCSI, MFM, RLL, and how all of this relates
to the ST.  I hope I can clarify all of this.

      First, a hard drive system for the ST will normally contain
three components: a host adapter, a controller, and the hard drive
itself.  The host adapter is a device which allows the ST's DMA
port (Direct Memory Access) to speak to a SCSI (Small Computer Systems
Interface, or 'Scuzzy'). The host adapter converts the 'language' 
spoken by the Atari's DMA port into the SCSI standard.  The controller
is a device that 'controls' the hard drive.  This part of the interface
can receive and send over the host adapter's SCSI interface.  Its function
is to 'talk' to or control the hard drive mechanism in a predefined way.
These predefined ways are what is know as density or data encoding
schemes.  Every harddrive stores information on the platters in some form
of data encryption, that is series of 1's and 0's.  The two most
common are MFM (Modified Frequency Modulation) and RLL (Run Length
Limited).  The boards that handle such encoding usually are compatible
with the ST506/ST412 interface standard - actually it isn't so much
a standard, but a defacto standard to be compatible with most drives
on the market.  Lastly, if you have what is known as an embedded drive,
which means that the controller is part of the hard drive itself, then
the host adapter's SCSI interface will connect directly to the drive.

      The origninal question asked whether or not a RLL controller can
be connected to a MFM drive mechansim and whether or not a PC compatible
controller can be used.  I originally answer both of these as no, you 
can't.  I was, it was pointed out, wrong.  Well, after further 
investigation, I will have to take exception with this.  First, of three
harddrive manufactures I spoke with, they said yes it was possible to
connect a RLL controller to a MFM drive mech, but, and this is a big
but, some data integrity will be lost and their warenties for the
drive will be void.  Also, consulting a Adaptec catalog and reference
guide, the warn against connect a RLL controller to a MFM drive, stating
the same reasons that the drive manufactures did.

      RLL does give more storage to a MFM drive, 1.5 times more.  This
is do to the way the controller 'converts' or store the data on the

      As far as the PC comatability goes, as I stated in the original
response, controller card for PC comaptibles that are meant to
plug in to the XT or AT bus will not work - for obvious reasons.
As far as general controllers, the Adaptec 5500, 4000, and 4070 controller
will work with the Atari. These controllers are 'generic' and do
not require use of an XT/AT bus.

      I hope that this clears up an problems with the original response.
I am glad that the answers I give are not just 'believed, and I
welcome all critisisms.  I would truly hate to mislead anybody with
bad or partially bad information.

                                    Peter Haller
                                    Hallsound System & Software

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