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From: blitter@ele.tue.nl (Blitter_stagairs_Frans)
Newsgroups: comp.sys.ibm.pc
Subject: Re: External 3.5" drive for XT
Message-ID: <116@euteal.ele.tue.nl>
Date: 3 Oct 89 09:29:18 GMT
References: <89100103302956@masnet.uucp>
Organization: Eindhoven University of Technology, The Netherlands
Lines: 37

OK, now for the total sum up:
Standard XT (PC) floppy controllers support only 1 transferrate: 250 kbit/s
(Yes, I know they also support 125 kbit/s single density format but this
died long ago)
With this transfer rate two types of floppys are supported: 360 kb 5.25"
and 720 kb 3.5". Also 720kb 5.25" is supported but this never made it in
the PC world.
Standard AT floppy controllers support 3 transfer rates:
250 kbit/s for 5.25" 360kb and 3.5: 720kb (same as XT)
300 kbit/s for 360kb floppy in 1.2 MB drive
500 kbit/s for 1.2 MB 5.25" and 1.44 MB 3.5"

Now the question arises why there is only 1.2 MB on a 5.25" floppy and
1.44 MB on a 3.5" floppy. This is because 1.2MB drives have 360rpm
motors and 1.44MB drives turn 300 rpm. All drives except 1.2 MB turn
at 300 rpm. This results in 15 sectors/track on a 1.2MB drive and
18 sectors/track on a 1.44MB drive.

Now it also becomes clear why you need the 300kbit/s transfer rate in an
AT. The normal 360kb floppy in an 1.2MB drive turns at 360 rpm, this 
means that the you must multiply the standard 250 kbit/s transfer rate
with a factor 360/300 and then you get exactly 300 kbit/s.

To make matters even more complicated there are certain 1.2MB drives
that change speed from 360 rpm to 300 rpm if they are set to 
double density (360kb). In this case the controller doesn't have 
to support 300 kbit/s transfer rate.

All this means that in principle you can't use 1.2MB or 1.44MB drives in
an XT because the controller won't support it. BUT, there are new
(and improved :-)) controllers which do support these drives and
transfer rates in an XT. These controllers must be accompanied by 
software, either on floppy (device drivers) or on ROM on the controller.

Paul Derks 

Disclaimer:  Anything I say can not and wil not be used against me!