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From: dlm@druwy.ATT.COM (Dan Moore)
Newsgroups: comp.sys.atari.st
Subject: Re: Problem with TOS recognizing media changes
Message-ID: <3938@druwy.ATT.COM>
Date: 27 Mar 89 17:03:51 GMT
References: <493@ultb.UUCP>
Organization: AT&T, Denver, CO
Lines: 29

in article <493@ultb.UUCP>, clf3678@ultb.UUCP (C.L. Freemesser) says:
> The actual problem is that the disk-eject device the drive has, which
> tells the computer that a new disk has been inserted), has failed.  

	The ST does not use the media change line from 3.5" drives.  If
they did there wouldn't be problems with the ST not detecting disk
changes.  Instead of using the media change signal that most 3.5" drive
produce Atari checks the write protect line every vblank.  If the status
of that line changes the BIOS sets the drive status to "maybe changed". 
The next access to the drive will read the boot sector and compare
the serial number on the disk to the one it thinks should be there.

	In order to check the write protect status of the drive the
vblank code must select the drive, read the status and then deselect the
drive.  Unfortunately many drives do not return valid write protect
status for several milliseconds after they are selected, which is many
times longer than the vblank code takes.  So these drives have problems
on the ST since media changes aren't always detected.

	This has almost nothing to do with the quality of the drive,
I've got some very good Epson drives that miss media changes on the ST
and some garbage Chinons that do detect the media change.



				Dan Moore
				AT&T Bell Labs
				Denver
				dlm@druwy.ATT.COM