Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!uwvax!puff!plocher From: plocher@puff.wisc.edu (John Plocher) Newsgroups: net.micro.pc Subject: Re: Reading/writing 360k diskettes on an AT (long) Message-ID: <273@puff.wisc.edu> Date: Tue, 28-Oct-86 02:16:33 EST Article-I.D.: puff.273 Posted: Tue Oct 28 02:16:33 1986 Date-Received: Tue, 28-Oct-86 06:43:51 EST References: <564@bucsb.bu.edu.UUCP> Reply-To: plocher@puff.WISC.EDU (John Plocher) Organization: U of Wisconsin CS Dept Lines: 100 Keywords: AT 360K 1.2M Hi Capacity Disk Drive Summary: _WHY_ IBM says not to write on a 360K disk with a 1.2M drive... In some article Jim Frost replies to: > spence@utcsstat.UUCP who asks: >> I use an AT as well as several PCs. The AT has a 1.2 Mb drive >>in addition to its hard disk. I don't want to add a 360K drive, but I do >>want to be able to write 360K diskettes as well as read them. > >Any 360k diskette written by any reasonably compatible computer will >operate correctly (ie read and write). ^^^^^ Not really! A small illustration: [###] Width of head on 360K drive [*] Width of head on 1.2 Meg drive [ ] Gaps between cylinders of data on a disk 1) ### ### ### ### cylinders of data on 360K disk ### ### ### ### (a top down look onto the disk surface) ### ### ### ### ( <- to outer edge: ### ### ### :to inner hub -> ) 2) * * * * * * * cylinders of data on a 1.2 Meg disk * * * * * * * * * * * * * * 3) * * * * cylinders written on a BLANK 360K disk by 1.2M drive * * * * (Note that every OTHER cyl is used) * * * * 4) #*# #*# #*# #*# cylinders of 360K data overwritten by 1.2M drive #*# #*# #*# #*# (note that not all the 360K data is erased...) #*# #*# #*# #*# > Regardless of what IBM says >about diskettes being readable only on AT's after an AT has written >on them, this hasn't been my experience. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Most people will not have problems when swapping disks between drives because the data on the disk is never overwritten (I'm talking about DOS reusing clusters on the free list, NOT about updating a file with an editor...) Partially filling a disk with 360K stuff and then adding some 1.2M stuff and reading it all back on a 360K drive should work. In that case, you only cover cases 1 and 3 above. BUT if you write 360K, erase some of it, then write some new stuff on it with the 1.2M drive, you can get case 4 above. Since the head in the 1.2M drive is smaller than that in the 360K drive, it writes a smaller path of data on the disk. Unfortunately, it is right down the middle of the existing 360K swath, and not all the 360K swath is rewritten. Still no problem on a 1.2M drive, since it NEVER sees the overlap (the #'s in case 4). But if you take this disk back to a 360K system, the drive sees the old 360K data mixed in with the new 1.2M data and doesn't know what it's seeing! Also, a point to remember is that the data swath on the 360K disk is that wide for a reason: The 360K drives *need* that much amplitude so that the flux transitions can RELIABLY be recovered from the disk surface. The swath laid down by the 1.2M drive may not have enough amplitude for some 360K floppies to read! >You can force the AT to format a 360k diskette by specifying >FORMAT A:/4 >I have used this extensively. It works fine. The format /4 command produces case 3 above; if the disk was orig used on a 360K system, case 4 might come into play. Note: IBM MIGHT be formatting as case 2 to really erase the disk, then redoing as case 3 to generate proper 360K format... >Now, I had people tell me that once a 1.2Mb floppy drive writes on >a 360k diskette, it would be unreadable by 360k drives. I also found ^^^^^ >this in one of IBM's manuals. A better word is 'might'. IBM is saying 'would' to keep people from coming back to them with "I did it the way that you said should work, and now it doesn't work..." >The point of all this is that you don't NEED a special copy program. >... the COPY command works fine. Reading a 360K disk on a 1.2M drive will NEVER give problems. Reading on a 360K drive: - Using a disk only used in a 360K drive will ALWAYS work :-) - Using a disk formatted on a 1.2M drive and only written to by a 1.2M drive should give you no problems. ^^^^^^ - Using a disk formatted on a 360K drive and written to by 360K and 1.2M drives (but without files being erased or modified) should still be no problem. ^^^^^^ - Using a 'working disk' (edit/compile/edit...) interchangably between 360K and 1.2M systems is bound to cause problems when you can least afford them! ^^^^^^^^^^^^^^^^^^^^^^^ Hope this clears up some of the confusion... -- "Never trust an idea you get sitting down" - Nietzche ------------ {harvard,seismo}!uwvax!uwmacc!uwhsms!plocher (work) John Plocher {harvard,seismo}!uwvax!puff!plocher (school) ------------ decvax!encore!vaxine!spark!121!0!John_Plocher (FidoNet)