Hard Drive Controller

From: Robert Berry (ap241@cleveland.Freenet.Edu)
Date: 02/28/91-11:17:10 AM Z

From: ap241@cleveland.Freenet.Edu (Robert Berry)
Subject: Hard Drive Controller
Date: Thu Feb 28 11:17:10 1991

   I have the chance to buy a 65 meg hard drive from another local ST
user.  He said it has everything with it except the controller; it was
originally an Atari 20 meg drive, but he upgraded it to a 40 meg MFM.
The existing controller got zapped by lightning, so he replaced it
with an RLL controller, which he says allows the drive to be formatted
to 65 megs.  He's keeping the controller to use with his Mega 2 that he
plans to buy when he sells his present system.  My question is:  Is
this method reliable?  What kind of controller would I need (i.e., one
that works with an XT, AT, SCSI, etc.; I found these different models
while searching through an issue of Computer Shopper)?  I can't make
heads or tails out of those ads in Computer Shopper, but I was just
wondering if the Atari drives use the same kind of controllers that
IBM-compatible hard drives use.

**********Answered by Peter Haller: 02/28/1991****************

First a little clarification:

        A harddrive (HD), by itself is dumb.  It is able to
physically write to its media and that is all.  In order for
a HD to talk to a computer an interface is requried.  This
interface is two fold: 1) a means for the computer to talk to the
HD, and 2) a 'protocol' that the drive will store the information
on to the HD.

        MFM and RLL are 'protocols'.  SCSI ( pronouced 'SCUZZY')
is an interface to the protocol, and many other things as well.  Two 
types of drives exist on the market today:  1) HD that have imbedded SCSI
2) drives that are either RLL or MFM. (There are other types, ESDI for
example, but as far as I know no interface for the ST for this type.) 
A SCSI 'embedded' drive needs no extra card in between the host
SCSI adapter (ie ICD host adapter).  Most of these drives are RLL
now, though don't hold me to this.  The second type requires a 
extra card between the host SCSI adapter and the drive - a 
translator, if you will.

        Now, with everything a little clearer, I can tell you
what you want to know....

        Bad news:  NO, an IBM (PC) compatible RLL, MFM, or SCSI
adapter will not help you here.  (Read not compatible.)  This
is simply that most controllers for the PCs are designed for
the XT/AT bus - expansion slots. 

        Good News: A HD that works on a PC will work on the ST,
if you have the correct hardware to interface to it - and it is
either MFM, RLL, or SCSI.

        Now, what you need to do is, buy a host adaptor for the ST: ICD
makes what has become a defacto standard for Atari, though Supra also makes
one.  This host adaptor is SCSI based, and connects to the DMA (Direct
Memory Access) port on the ST - you know the connector that doesn't
get used by anything.  With this adaptor connected to the ST any SCSI 
embedded HD can be connected to the ST.  If your HD is not SCSI embedded, 
you will need that 'inbetween' card that connects the HD to the SCSI card.
Again, ICD sells these cards.

        Simple, right?  Well, it really is, though I haven't gotten into
power supplies and such.  As far as using an RLL controller with a MFM
drive, well I have never heard of that - it sounds wrong, but then again
somebody may have figured something out.  I will investigate this further.
Look in the Programmers and Developing Forum in this Sig under the 
Information & Ideas menu selection for added information.

                                        Thanks for the Question!
                                          Peter Haller [ae302]
                                        Hallsound Systems & Software


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