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Old 5th Jul 2019, 8:10 am   #1
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Default Using 5.25-inch BBC Micro Floppy Drive with a PC

A while ago I picked up a dual 5.25-inch floppy drive off ebay, manufactured by Watford Electronics and intended for use with a BBC Micro. I stumbled across it again yesterday and in the interim I have bought a complete BBC B with a whole plethora of accessories, including another 5.25-inch floppy drive.

Since I now don't have a specific purpose for the first drive (I bought it because I knew I'd get a BBC Micro one day!), I am now wondering about the possibility of connecting it to a more modern PC (Windows XP, currently with a 3.5-inch drive). I know that basically doing the opposite - connecting a modern 3.5-inch floppy drive to a BBC Micro - can be problematic, but I was wondering if there are any issues that could be encountered when connecting the 5.25-inch floppy drive to the PC. It's got it's own power supply, so I'm not worried about it drawing an excessive current from the PC supply.
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Old 5th Jul 2019, 8:58 am   #2
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Default Re: Using 5.25-inch BBC Micro Floppy Drive with a PC

Hi Andrew,
a Make and full Model number may help.
also maybe some photos.
By 'Dual' is it 2 x 5.25" Drives?

If is standard and with standard Connectors there should be no problem connecting to a PC which supports Floppies, providing the bios includes 5.25" floppies.

A Ribbon Cable / Stripline is required that has a connector or connectors for 5.25" is needed, as 3.5" differ. Some had as many as five Sockets to cover all possible configurations.
1 - mobo, 2 - drive A: (3.5" + 5.25"), 2 - drive B: (3.5" + 5.25")

Watford Electronics, there is a Blast from the Past, now sadly gone!

Regards - Mike
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Old 5th Jul 2019, 9:36 am   #3
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Default Re: Using 5.25-inch BBC Micro Floppy Drive with a PC

Hmm, your answer makes me think I am mis-remembering things.
From memory I recall the cable coming out of the drive looking like the standard PC FDD connector...
I will check when I get back tonight!

A quick search online brings up an image of the various ports on the BBC Micro (attached). This shows the floppy connector as a 17-pin cable that looks practically identical to the PC connector.

Of course, whether it can just be plugged in is a completely different question....
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Last edited by andrewferguson; 5th Jul 2019 at 9:41 am.
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Old 5th Jul 2019, 10:25 am   #4
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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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Old 5th Jul 2019, 2:10 pm   #5
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Default Re: Using 5.25-inch BBC Micro Floppy Drive with a PC

Disk drives with a 34-way data connector and 4-way power connector are pretty much the same. You can use drives from IBM PCs on a BBC, but not necessarily the other way around. Well, they'll work, sort of, but might not be readable anywhere else .....

IBM PC drives are double sided, 40 tracks. BBC Micro drives can be single or double sided, and 40 or 80 tracks. You shouldn't write 40-track disks on a 40/80-track switchable drive.

The BBC expects drives to be jumpered as 0 ("drive 2" is the other side of drive 0, if it has two heads) and 1 (similarly 3) whereas the PC expects both drives to be jumpered as 1 and uses a twist in the ribbon cable to swap over the select signals.

Also the BBC disks are formatted as ten sectors of 256 bytes, whereas MS-DOS uses 9 sectors of 512 bytes.
If I have seen further than others, it is because I was standing on a pile of failed experiments.
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Old 8th Aug 2019, 11:10 pm   #6
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Default Re: Using 5.25-inch BBC Micro Floppy Drive with a PC

You might need to check/jumper the correct use of pin 34. On the IBM-PC this is disk change. Most non PC systems use pin34 as a ready signal.

Older drives usually have a jumper to select this. New drives are usually for a PC and have pin 34 as disk change. As your drives started on the BBC micro they will be set for READY and need to be changed to DISK CHANGE. There is probably a jumper to do this.

I also think that the PC uses a stepping rate of 3ms for 80 track drives and 6ms for 40 track drives. Some BBC drives might not be able to step at this speed.
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