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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 21st Aug 2011, 11:14 pm   #1
McMurdo
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Default questions about DD and HD floppies

Hello, got a couple of queries about old 3.5in double density discs and their drives.

I've heard you can use 2HD high density (normal) diskettes in an old double-density 3.5in drive by covering up an ID hole and formatting at the correct level. Can anyone confirm it and elaborate?

Secondly, I have a machine (non-PC) here that uses old 3.5in DD drives, I suspect the drives are worn out. Can anyone think of a way to fit a new drive that will read the original DD-formatted discs & program? The files are in bespoke non-fat format.
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Old 22nd Aug 2011, 12:46 am   #2
paulsherwin
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Default Re: questions about DD and HD floppies

Old 3.5" DD drives don't have the sensor pin to detect the HD hole, so you should be able to use HD disks without modification subject to suitable formatting. I don't remember actually doing this though - it would have been a long time ago.

To answer the second point, if the machine in question uses a standard FD interface it should be possible to just plug in a later drive and it will 'just work'. Again, I haven't actually done this though. There are certainly problems using IBM proprietary 2.88MB drives with standard FD interfaces.
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Old 22nd Aug 2011, 8:47 am   #3
cmjones01
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Default Re: questions about DD and HD floppies

It's certainly possible to format 3.5" HD discs as DD by covering up the ID hole (it's the one that isn't the write protect hole) or, as Paul says, simply using an older drive which doesn't know about HD discs.

However, be aware that the magnetic material on the discs is different: if I remember rightly, HD 3.5" discs have a thinner layer of magnetic coating on them than proper DD 3.5" discs, which results in a weaker signal read off the disc, which can make them unreliable when used this way. There is a similar problem with 5.25" discs, though in this case I think HD 5.25" discs have a higher coercivity coating than DD ones.

As for using a newer drive to replace a DD one, it ought to work, but be aware that many modern cost-reduced 3.5" drives support only the bare minimum of signals on their interfaces to work with a PC. It tends to be impossible, for example, to set them to respond to drive IDs other than 1. This can be a problem in old systems which expect a drive ID of 0 for the first drive.
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Old 22nd Aug 2011, 10:15 am   #4
richrussell
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Default Re: questions about DD and HD floppies

Also be aware that if the discs are old the actual data or even the magnetic medium itself on the discs may have degraded. I've got loads of Atari ST and Amiga 3.5" floppies that just have lost data and even when formatted won't store data reliably any more.

HD floppies have a very thin magnetic layer, so you can find that older DD drives can corrupt data on the other side if they write to them. Will be fine for reading only though.
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Old 22nd Aug 2011, 3:19 pm   #5
dave cox
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Default Re: questions about DD and HD floppies

I've seen a very good discussion of using modern 3.5 HD drives / HD medium for DD applications on "stairway to hell". I'll post a link later ...

dc

Ahhh, easier than i thought, its in the hardware section as you would expect ...

stair way to hell
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Old 22nd Aug 2011, 9:11 pm   #6
McMurdo
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Default Re: questions about DD and HD floppies

Well, I spent the best part of the afternoon and part of this evening trying to get a modern floppy drive to work.

The unit in question is an early 90's portable, mains driven floppy drive that is supposed to take the place of your old punched paper tape machines.

First, I blocked up the density ID holes on 2 HD discs and formatted them to 720K in an XP command window on my home PC. My PC at work repeatedly crashes BSOD if I insert said discs in its floppy drive at any time.

I read a forum article on a certain type of Korg synthesiser that uses similar drives and discs. The feeling was that some modern 1.44M drives worked, most didnt.

I tried a modern Teac floppy drive in the old reader. It didnt work, reporting on its 2-line LCD display a 'floppy disc drive fault'. I examined the original full-width 3.5in drive and decoded the numerous jumper settings and their bamboozling mnemonics with the aid of google and several datasheets; none of which related directly to this old drive. I ascertained that it was configured as DS0 (drive select no. 0), it had pin 2 defined as a 'disc present' flag and pin 34 as a 'drive ready' flag; all of which are different on a standard PC floppy drive. Modern drives have pin 2 as 'DD disc detect' and pin 34 as 'disc change' and are usually hard wired as 'DS1' ie drive select no1/cable select.

To cut an already long story short, I tried 'faking' the control signals with pushbuttons to see if I could fool the reader into using the new disc. It worked; so I set about cutting various tracks inside the drive, tracing chip pins etc. I had just about got the thing to work when all of a sudden there was a 'ping' and most of the FDD head assembly sprung free and shot all over the floor; I still dont know where it all went.

I found another drive in the scrap bin, this time an Alps, and did the whole tedious track tracing again. This time, and with a more robust chassis, I managed to get it to work.

Needless to say Ive written it all down for future reference!!!

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Old 23rd Aug 2011, 7:51 am   #7
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Default Re: questions about DD and HD floppies

Great result! I'm glad to hear a real-world example of replacing an old 720K drive with a 1.44M one. It's nice to know what the modifications were and that they really worked
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