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From: jms@tardis.Tymnet.COM (Joe Smith)
Newsgroups: comp.sys.amiga.tech,comp.sys.amiga.hardware
Subject: Re: Clicking Drives II
Summary: Have to step heads to detect new floppy.
Message-ID: <1412@tardis.Tymnet.COM>
Date: 12 Jan 91 09:06:05 GMT
References: <1990Dec19.164559.19792@engin.umich.edu> <135@dogmelb.dog.oz.au> <1990Dec20.211301.28307@msuinfo.cl.msu.edu> <7357@sugar.hackercorp.com> <1990Dec31.091243.470@phoenix.pub.uu.oz.au>
Reply-To: jms@tardis.Tymnet.COM (Joe Smith)
Followup-To: comp.sys.amiga.hardware
Organization: BT Tymnet, San Jose, CA
Lines: 35

In article <1990Dec31.091243.470@phoenix.pub.uu.oz.au> proff@phoenix.pub.uu.oz.au (Frederick Solidus) writes:
>In <7357@sugar.hackercorp.com> peter@sugar.hackercorp.com (Peter da Silva) writes:
>>It's a design flaw in the standard 3.5" interface. Apple doesn't have the
>>problem because they don't use standard drives, and IBMs don't have
>>the problem because they don't detect disk changes at all.
>
>Still, I don't see why the read/writte status just carn't be scanned. That
>would inform if there was a disk change.

The problem is in the way that the original floppy manufacturers decided to
implement the "disk changed" signal.  To quote the Amiga Hardware Manual:

    CHNG-(Pin #11)
	A selected drive will drive this signal low whenever its internal
	"disk change" latch is set.  This latch is set when the drive is
	first powered on, or whenever there is not diskette in the drive.
	To reset the latch, the system must select the drive, and step
	the head.  Of course, the latch will not reset if there is no
	diskette installed.

[The signal is latched in case one floppy is removed and another one inserted
while the Operating System was not looking at the disk drive.  (These 3.5
inch drives were not designed specifically for the Amiga.)  The WRPRO
(write-protect) signal is not an accurate indicator of disk presence.]

So, the Amiga can detect immediately when the floppy is removed.  However,
there is no signal from the disk drive that says when a new floppy is
inserted.  The only way to detect the presence of a floppy in the drive
is step the heads to the next track and see if that operation has caused
the CHNG signal to become de-asserted.
-- 
Joe Smith (408)922-6220 | SMTP: jms@tardis.tymnet.com or jms@gemini.tymnet.com
BT Tymnet Tech Services | UUCP: ...!{ames,pyramid}!oliveb!tymix!tardis!jms
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San Jose, CA 95161-9019 | humorous dislaimer: "My Amiga 3000 speaks for me."