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From: wtm@neoucom.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: AT vs. 360K drive
Message-ID: <666@neoucom.UUCP>
Date: Tue, 18-Aug-87 10:04:17 EDT
Article-I.D.: neoucom.666
Posted: Tue Aug 18 10:04:17 1987
Date-Received: Fri, 21-Aug-87 05:47:42 EDT
References: <1918@isis.UUCP> <2557@tekgvs.TEK.COM>
Sender: wfd@neoucom.UUCP
Organization: Northeastern Ohio Universities College of Medicine
Lines: 39
Summary: What pin 34 does


On 360 K drives, pin 34 is the select lead when the drive is
configured as unit 03.  Most PeeCompatibles configure the drives as
unit 00, and flip-flop a few wires in the cable to differentiate
between drives A and B.  On such a drive, pin 34 is intended to be
an input, and the appearance of this lead to an AT computer's
connector is indeterminate, and can cause unreliable operation.

On 1.2 meg drives (and 720 K micro floppies too), pin 34 is a media
change output to the computer.  Every time you insert or remove a
disk, a logic pulse appears on this lead.

To get a 360 K disk to work on your AT, you have to either put a
small piece of tape between the pin and the drive connector or else
slice the trace on the drive circuit board leading to pin 34.  Be
sure you know what you are doing in the latter case!  (Actually I
prefer the latter method.)  Second, you have to run your computer's
diagnostic program to inform the non volitile RAM what sort of
drive(s) is/are connected.  At this point, everything should be OK.

Note that the AT BIOS watches the media change lead on the drive to
see when you swap disks.  Normally, the disk's File Allocation
Table (FAT) is cached in RAM.  Swapping a disk necessitates
re-reading the FAT.  On 1.2 meg drives, pin 34 does the deed.  On
360 K drives, there is no media change signal, thus the BIOS
re-reads the FAT regardless, whenever more than 1/2 sec. has
elapsed since the last disk access.  If you were some sort of
super-human and could change diskettes faster than 1/2 sec, you
could wind up with a hoplessly scrambled 360 K disk if DOS wrote
with the wrong FAT in the RAM cache.

Hope this is helpful,
Bill

(wtm@neoucom.UUCP)

Note that Teac, Toshiba, and probably other companies make 360 K
dirves  that can be used on an AT without any slicing and dicing of
pin 34 required.