Date   : Sat, 18 Nov 2006 13:40:54 -0000
From   : dl.harper@... (David Harper)
Subject: 5 1/4" disc on PC

Jonathan Graham Harston wrote:

>> >> When the BIOS was set to 360k 51/4 disc the POST (floppy seek) failed
>> >> and the drive only showed up as 40T in omniflop
>> >> When BIOS set to 1.2M it passed POST, showed up ok in Omniflop but
>>
>> It sounds to me like a stepping-speed problem. A 5.25" drive cannot be 
>> brand
>
> There are different issues with 5.25" drives on PCs. A 40 track
> 5.25" "360K" PC drive rotates at 300rpm, whereas a 80 track 5.25"
> "1.2M" PC drives rotates at 360rpm. "Proper" 80 track 5.25" drives
> for BBCs rotate at 300rpm. Drives for BBCs were often 1.2M drives
> with links or solder pads that could be set to make them rotate at
> the correct 300rpm.
>
> I'm doing this from memory at the moment, but with a PC you need
> to trick it to use a 5.25"/80trk/300rpm drive correctly by telling
> the BIOS that it's actually a 3.5" drive.

This is true, of course, but I have never actually found the rotation speed 
to be an issue in practice. The FDC in the PC usually seems quite happy to 
handle data at 360 and 300 rpm. You may have different experience, of 
course.

As many here will know, this weird difference in speed (why are there so 
many oddities with the design of the PC??!) is the reason for the different 
capacities of 5.25" and 3.5" HD floppies. 15 sectors per track at 360 rpm 
gives the same data transfer rate as 18 sectors at 300 rpm. 15 sectors per 
track works out as 1.2Mb for the whole disk; 18 sectors per track totals to 
1.44Mb for the disk.

David Harper