Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!amdahl!pyramid!prls!philabs!micomvax!ray
From: ray@micomvax.UUCP (Ray Dunn)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Hard Disks: XT vs. AT
Keywords: hard disk xt at
Message-ID: <1193@micomvax.UUCP>
Date: 14 Jul 88 21:55:17 GMT
References: <6201@megaron.arizona.edu> <11517@steinmetz.ge.com>
Reply-To: ray@micomvax.UUCP (Ray Dunn)
Organization: Philips Electronics Ltd. (TDS - Montreal) St. Laurent QC, Canada
Lines: 48

In article <11517@steinmetz.ge.com> davidsen@crdos1.UUCP (bill davidsen) writes:
>In article <6201@megaron.arizona.edu> cjeffery@arizona.edu (Clinton Jeffery) writes:
>| Hi people: for a year now I have been running an Atasi 3046 drive in my
>| XT clone.  Recently I purchased an AT clone kit, planning on using the
>| Atasi as its hard drive.  So far, the hard disk "fails initialization"
>| and has resisted any attempts at low-level formatting in the AT.
>
>  I believe that AT disks are *not* ST506, they are .....

Hmmm...  A common misconception here.

The "ST506" refers to the *low level* interface between the disk controller
and the drive, and commonly is used for both XT's and AT's.  The interface
between hardware and software (i.e. to the BIOS, or to any BIOS ignoring
badly behaved software) is an "XT interface" or an "AT interface"
respectively.

This software interface is independant of the low level interface, so that,
for example, the higher capacity RLL drives still use the same
software/hardware interface, but, if you like, the protocol going over the
ribbon cable from controller to the disk drive is different.

The XT and AT interfaces are very different, for example:

The XT uses DMA ch 3, IRQ 5, I/O addresses 320-323 and an 8-bit data
transfer.

The AT does *not* use the DMA (fun eh?), uses IRQ 5, I/O addresses 1F0-1F7
and 3F7, and uses 16-bit data transfers.

You can read about this in various places in the IBM manuals, which have
changed over time, originally in the straight "Technical Reference Manual",
but now, I think, in the "Options and Adapters" section.

The situation is further confused by the fact that the BIOS interfaces to
the disks are different in the XT and AT, and provide different services.

Lower cost solutions are now quite common for clones, with the controller
built into the drive (i.e. no separate controller card, with the disk
connected directly to a special connector on the motherboard, or to a
"dummy" board plugged into an option slot.  This first happened with XT's,
but is now available for AT drives as well.

Now, why doesn't your XT drive work on the AT....
-- 
Ray Dunn.                      |   UUCP: ..!{philabs, mnetor}!micomvax!ray
Philips Electronics Ltd.       |   TEL : (514) 744-8200   Ext: 2347
600 Dr Frederik Philips Blvd   |   FAX : (514) 744-6455
St Laurent. Quebec.  H4M 2S9   |   TLX : 05-824090