Path: utzoo!attcan!uunet!tektronix!dadla!amadeus!jamesa
From: jamesa@amadeus.TEK.COM (James Akiyama)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Hard Disks: XT vs. AT
Keywords: hard disk xt at
Message-ID: <1361@amadeus.TEK.COM>
Date: 14 Jul 88 23:23:56 GMT
References: <6201@megaron.arizona.edu>
Organization: Tektronix Inc., Beaverton, Or.
Lines: 60

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.
> 
> So my question: what differences are there between the XT (e.g. WD WX1)
> controller and the AT (e.g. WD WA2) controller?  Don't most ST 506 drives
> work with either?  Are there any little jumpers I need to pull?
> 
> Any help would be greatly appreciated!
> -- 
> | Clint Jeffery, University of Arizona Department of Computer Science
> | cjeffery@arizona.edu -or- {ihnp4 noao allegra cmcl2}!arizona!cjeffery
> --

There are actually several differences between an XT (e.g. WD WX1) and AT 
(e.g. WD WA2) controller.  This is a list of some:

    1.	XT controllers contain on board BIOS while the AT controller uses
	the AT BIOS.  This is because the original PC did not support fixed
	disk and used an adaptation of the Xebec controller.

    2.	The XT uses DMA to transfer data between the fixed disk buffer and
	system memory while the AT uses the CPU.

    3.	The XT is a 8-bit controller while the AT is a sixteen bit controller.
	Since the WD1010 (or WD2010 in newer AT controllers) is only 8-bits,
	the controllers assemble two 8-bit values into one 16-bit word.  This
	is part of the reason why most AT's can get by with a 3:1 (or 2:1)
	interleave.

    4.	Most AT controllers also contain the floppy disk controller.

Either controller should work with a standard ST506 (ST412) drive.  Note that
the ST412 drives support "buffered seeks" while the original ST506 drives did
not.  All current drives support buffered seeks (at least those I'm aware of).

Note that the standard AT controller, as shipped, requires the drive set to
DRIVE #2 and requires that the 34-pin cable have four line twisted.  Many third
party controller have a jumper to make the controller "IBM compatible" or
standard "ST412" compatible.  Check with the OEM.  I'm not sure whether this
was true on the XT--part of the problem being that IBM had several variations
to the XT controller (I believe).  If you need more information, and have the
WD1003-WA2 AT controller, let me know, since I have the OEM technical manual
for that controller.

One final note is that the WD1003-WA2 has two 20-pin connectors, one for each
fixed disk.  You must use the 20-pin connector closer to the 34-pin connector
for the first drive.  It is the one with the higher "J" number (I believe the
two "J" numbers for the 20-pin connector are J30 and J40--you need to use J40).
If you use the wrong connector, the controller will not be able to read any
data from the fixed disk.

Hope this helps.

James E. Akiyama
jamesa@amadeus.LA.TEK.COM
UUCP: ....!tektronix!amadeus!jamesa
ARPA: jamesa%amadeus.LA.TEK.COM@RELAY.CS.NET