Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!wuarchive!mit-eddie!rutgers!cbmvax!jesup
From: jesup@cbmvax.commodore.com (Randell Jesup)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Clicking Drives II
Message-ID: <16719@cbmvax.commodore.com>
Date: 21 Dec 90 06:02:01 GMT
References: <1990Dec19.164559.19792@engin.umich.edu> <135@dogmelb.dog.oz.au> <1990Dec20.211301.28307@msuinfo.cl.msu.edu>
Reply-To: jesup@cbmvax.commodore.com (Randell Jesup)
Organization: Commodore, West Chester, PA
Lines: 42

In article <1990Dec20.211301.28307@msuinfo.cl.msu.edu> dailey@buster.cps.msu.edu (Chris Dailey) writes:
>>> question, why does trackdisk default to clicking?  There seems to be
>>Because that is what it has always been. (compatible!!)
>
>That doesn't seem to be a logical reason.  HOW is it compatible, and why
>is it important to remain compatible in whatever respect it is
>compatible in?

	because older machines (pre-a3000's) do not all have drives with a
track-0 disable on the step-out pulse.  On drives without it, the head gets
to 0, and then pounds against the stop if you use nockick.  This a) makes
a lot more noise than clicking, and b) can blow the alignment of the drive.
Since (for various reasons) trackdisk is not conditionally compiled for
different machines yet, we do not default to noclick.  Distributing something
that can physically damage a user's hardware if he sets it wrong is not
a real good idea, so there is no "noclick" prefs at this time.

>How does clicking prevent the drive from being stepped backward past
>track zero???

	the clicking is the drive stepping in and out alternately.  Noclick
type things always step out, and when they get to track 0 the track-0 detect
stops it from stepping any more (if they have such a lockout).

>And why do Commodore drives require the click and why can't it be
>eliminated, as other companies' machines do no have or need it?

	Most other computers either a) don't notice disk insertions/removals,
b) require the OS to do removal.  Some others may specify drives that are
'noclick-safe', or have non-standard interfaces.  Note that the reason for
the click is  because the disk insertion signal is gated by the step line
when the drive is empty.  Some controllers and drives may be able to use
a separate signal to check for disk insertion instead of using STEP (which
is the default).  The Amiga doesn't have any unused port bits for this, nor
do we specify that drives support this (I think).

-- 
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.commodore.com  BIX: rjesup  
The compiler runs
Like a swift-flowing river
I wait in silence.  (From "The Zen of Programming")  ;-)