Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!gem.mps.ohio-state.edu!ginosko!xanth!rlb
From: rlb@cs.odu.edu (Robert L. Bailey)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Low Density Disks in High Densi
Message-ID: <9828@xanth.cs.odu.edu>
Date: 26 Aug 89 04:58:26 GMT
References: <291@bilver.UUCP> <45900267@uxe.cso.uiuc.edu>
Reply-To: rlb@cs.odu.edu (Robert L. Bailey)
Organization: Old Dominion University, Norfolk Va.
Lines: 32

In article <45900267@uxe.cso.uiuc.edu> coleman@uxe.cso.uiuc.edu writes:
>
>/* Written  9:13 am  Aug 23, 1989 by bill@bilver.UUCP in uxe.cso.uiuc.edu:comp.sys.ibm.pc */
>/* ---------- "Re: Low Density Disks in High Densi" ---------- */
>>>This raises yet another question: I have a 1.2M 5-1/4" drive in my AT clone.
>>>It is able to read/write low density 360K diskettes (using the 360K format)
>>>with no problems, and I swap 360K diskettes with my XT clone all the time
>>>with no problems. How does the 1.2M drive know to change its write current
>>>in order to successfully write a 360K floppy? 
>
>>That's simple.  Part of the formatting routine.  You format HD and it goes to
>>the higher current.  DD and it uses the lower.   If the disks had been made
>>different for hd/dd the problems might not exist. 
>/* End of text from uxe.cso.uiuc.edu:comp.sys.ibm.pc */
>
>That doesn't explain why I am able to successfully read/write 360K diskettes
>FORMATTED WITH A 360K DRIVE in my 1.2M drive! How does the 1.2M drive know not
>to use high write current when writing data to a 360K disk?

Simple.  When DOS logs in a new disk for the first time, it reads track 0.
It then steps in 1 track and reads again.  If it is a high density drive
with a 360K disk, the track number will still be 0.  If it is a HD drive
with a 1.2M disk the track number will be 1.  This difference occurs
because the 360K tracks are roughly twice the size of the 1.2M tracks.
When it has been determined that the disk is a 360K, the drive is forced
to write at the lower current level.  It also double steps the drive
to position the head over the proper track. This is because in a HD drive
track 1 is twice as far from track 0 as it is for a 1.2M diskette.

Hope this clears up the confusion.

Bob Bailey