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Old 5th Jul 2019, 10:25 am   #4
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,157
Default Re: Using 5.25-inch BBC Micro Floppy Drive with a PC

BBC Micro 5.25" drives are basically compatible with the PC. I've used the same drives with both PCs and BBCs. However, there are a few things to bear in mind:

To appear as drive A: on a PC, the 34-way ribbon cable needs the 'twist' in it because PCs address their drives differently. With a straight-through cable, the drive will appear as drive B:. This article explains the problem:

Floppy drives for the BBC generally came in 40-track and 80-track varieties, both single-and double-sided. Only some combinations are valid as PC drives.

PC drives come in 360k (5.25" 40 track), 720k (3.5" 80 track), 1.2M (5.25" 80 track high density) and 1.44M (3.5" 80 track) plus a few other rarely-encountered oddities. Generally the BIOS settings have to be set to the right type of drive.

A PC's '360k' drive needs to be 40-track, double sided. Nothing else will work.
A BBC's 80-track, double-sided drive might work on a PC if you tell the PC that it's a 720k 3.5" drive, though would result in a non-standard disc. Though the geometry is technically the same, I don't think I've ever got this to work.
A PC's '1.2M' drive is 80-track, double-sided, high-density, and the PC's controller knows how to double-step the drive to read and write 40 track 360k discs. It might be possible to tell the PC that a BBC's 80-track drive is a '1.2M' type. However, there are some problems with this:
- the disc rotation speed is different ('1.2M' on the PC uses 360rpm, all other modes use 300rpm) so the controller might not do the right thing.
- 1.2M mode definitely won't work - the drives and discs are magnetically different for this mode, and BBC drives won't support it.
- even if it does work, it will only read and write PC 360k discs.

If you want to read BBC micro discs on the PC, that's a whole new can of worms. BBC discs (DFS ones, anyway) are single-density FM format, which should technically be readable using special software. However, many PC disc controllers just don't have the hardware to deal with it. Generally the older the PC, the greater the chances of success. I got it to work using an Abit KT7 Athlon motherboard and a really old Shugart SA400 full-height drive.

BBC ADFS discs should be easier, being double-density MFM format, but I haven't tried that.

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