Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!mcsun!hp4nl!eutrc3!euteal!blitter 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!