Path: utzoo!mnetor!uunet!portal!atari!apratt
From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: about ST disk drives (and media change)
Message-ID: <914@atari.UUCP>
Date: 22 Dec 87 18:43:50 GMT
References: <8712211937.AA01474@ucbvax.Berkeley.EDU>
Organization: Atari Corp., Sunnyvale CA
Lines: 33

in article <8712211937.AA01474@ucbvax.Berkeley.EDU>, 
gjwelych@SUNRISE.ACS.SYR.EDU ("Welych, Gregory J.") says:
>    What in the Atari drives senses the change of media?
>    And how does the drive signal this to the ST?

The media-change sensing is done using the write-protect signal from the
drive.  The assumption is that when you take a write-enabled disk
out of the drive, the write-protect sensor will pop up, and the drive
will signal "write protect" to the controller.  When you put another
disk (or even the same disk) in the drive, the write-protect sensor
will be pushed down again and signal "not write protect."

When a disk is really write protected, of course, there will be no
transition on the line.  Because of this, write-protected disks go
into the "maybe changed" state 1 sec (I think) after the last disk
access.  (The assumption is that you won't change from one write
protected disk to another within one second).

When a disk is in the "maybe changed" state, the next access (through
RWABS or maybe Floprd) will check the serial number of the disk in the
drive.  If it hasn't changed, then the disk hasn't changed, either. 
This is another assupmtion: you should not have two disks with the same
serial number.

If your drive suppresses write protect changes when there is no disk
present, then switching from one write-enabled disk to another will not
cause the transition, and the BIOS will not sense media change. 

Sorry.  Nobody said it would be easy.

============================================
Opinions expressed above do not necessarily	-- Allan Pratt, Atari Corp.
reflect those of Atari Corp. or anyone else.	  ...ames!atari!apratt