Path: utzoo!attcan!uunet!cs.utexas.edu!rutgers!ucsd!ucsdhub!hp-sdd!apollo!weber_w
From: weber_w@apollo.HP.COM (Walt Weber)
Newsgroups: comp.sys.atari.st
Subject: Re: Hard Drive
Message-ID: <4894e969.20b6d@apollo.HP.COM>
Date: 11 Feb 90 15:34:00 GMT
References: <7500014@m.cs.uiuc.edu>
Sender: root@apollo.HP.COM
Reply-To: weber_w@apollo.HP.COM (Walt Weber)
Organization: Hewlett Packard NARC @ Apollo Systems Division
Lines: 40

In article <7500014@m.cs.uiuc.edu> totty@m.cs.uiuc.edu writes:
>
>	What exactly does the host adapter need to do?  How similar
>	are the protocols between the Atari DMA interface and a standard
>	disk controller?  How about between DMA and SCSI controllers?

The Atari DMA interface interface implements what some have called a "modified
SCSI interface", since (1) the connector and interface specs regarding cable
lengths, etc. is different from SCSI, and (2) the command descriptor block used 
to transmit commands to the device differs in that the opcode and controller
number are both in byte 0 of AtariDMA but in different bytes within SCSI (there
are other differences, as well).  The host adaptors which I have seen convert
AtariDMA <--> SCSI, and then will talk directly to a drive with an embedded
SCSI controller, or will talk to a controller card (Adaptec or Omti are two
popular choices) which does SCSI <--> ST506.

>	what would be involved in actually building one of these interfaces?
>	Would it be a simple FSM protocol converter with some buffering or
>	are there more complicated things necessary?

Unknown by me.  Looking at the ICD and BMS (Berkeley Micro Systems) boards,
there appear to be three "sections" - one to do command restructuring, one
to drive each side at the appropriate signal levels and handle arbitration,
and an third which puts a clock at one of the AtariDMA bus addresses,  since
the early ST's didn't have an on-board clock, and there are other uses for
the cartridge port (buy a Spectre GCR!!).

>	Also, where can I get technical information on controller details
>	and the Atari DMA port?

Become a registered developer with Atari, and you can get the docs.  You will
also have spent more than the cost of an ICD or BMS board (approx $125).  But
you did say it was for "fun", so perhaps the cost isn't an obstacle.

...walt...

Walt Weber               Hewlett Packard NARC @ Apollo Systems Division
-The views expressed herein are personal, and not binding on ANYONE-
   "The power of accurate observation is commonly called cynicism
    by those who have not got it" -George Bernard Shaw