Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!ncar!gatech!ncsuvx!ece-csc!mcnc!ecsvax!mjg
From: mjg@ecsvax.uncecs.edu (Michael Gingell)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Hard drive / controller problems
Summary: The twist makes both drive connectors the same
Message-ID: <5413@ecsvax.uncecs.edu>
Date: 15 Jul 88 16:42:26 GMT
References: <222@psuhcx.psu.edu> <16800327@clio> <230@psuhcx.psu.edu> <4452@sphinx.uchicago.edu>
Organization: UNC Educational Computing Service
Lines: 32

In article <4452@sphinx.uchicago.edu>, mobo@sphinx.uchicago.edu (Samuel Wilson) writes:
> 
> I put a second ST225 in an ATT6300.  I was using a Western Dig. WX2
> controller.  I was also using a cable with a twist between the 
> connection for drive c and d (or a and b, as the case probably was). 
> the controller could talk to one drive, or the other, but never both
> at once.  I called Everex (who, for some reason known only to them had
> marketed the WD-Seagate combination), and the (very clued-in) tech
> guy said, "well, you have both drives set to drive 2 don't you?"
> (silence on my end).  I finally said "Yea, sure I do, course I do, 
> waddya think, I'm a dummy, that's the ticket" and went and set both
> seagates to be drive 2 and the thing works great.  
> 
> I have some idea how controllers work, and some idea how drives work, 
> but no idea why *that* worked.
> 
On both floppy and hard disk controllers the cables have separate
control lines for each of two drives. There is a drive select and a
motor on line for each drive. The lines are so placed so that the
first connector is for drive two while the lines that go through to
the end connector for drive one correspond to unused pins on the
first drive.  The twist reverses this so that now drive one has
its line connected and drive two's wires are unconnected. The only
way this works is if you choose drive two in the first place.

IBMs wiring scheme is a clever solution to how to get separate Motor
Control and Drive select lines to each drive using the existing
Shugart drive connector on the drive end and their own "proprietary ?"
wiring scheme on the controller end of the cable.
What's the betting IBM has this on its long list of patents it has
threatened to sock the competition with.

Mike     ..ecsvax!mjg