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avatar_khaz

Closing the HD hole of a 3.5 disk

Started by khaz, 19:29, 19 September 17

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khaz

Does it even matter on a CPC? What happens at the hardware level?

Use scenario: a 3.5 HD drive connected to a CPC, the CPC formats an HD disk with its HD hole uncovered. The drive is unmodified (except for RDY) and the DD/HD detection pin works properly.

Same question but for PC/Windows, writing .dsk files on floppies with CPCdiskXP or SAMdisk, or any other proper tool.

I read online that DD drives, like on the Amigas, prefer real DD disks. Non-factory formatted HD disks formatted as DD by the user are cool too. But I can't find information about HD drives and their preference. Does the hole trigger something in the drive and make it read/write differently? Or is it entirely managed by the controller? Does the controller, HD-able on the PC but DD-only on CPC and Amiga, even matter?

I've been using this page as my main resource on DD and HD disks and drives: http://www.retrotechnology.com/herbs_stuff/guzis.html

GeoffB17

I think that link you show contains significant info about this.


Simple visual inspection can confirm that the magnetic disk material inside the 3.5" floppies is different, between the DD and the HD types.   Aspects of the read/write processes are different too.


So, what you describe IS possible, and MAY work, but it will VERY likely be NOT reliable.   In the short term, it may be OK, but don't expect a disk to still work months later.   It MIGHT, but you'll be lucky.


If you're going to write DD, then try to find DD disks.   FAR more reliable.


If you find some older mfg HD disks, they may be better than more recent ones.   Prob depend on the visual check, the more transparent they seem to be, the more likely they're HD _ONLY_!


Geoff

khaz

I'm not sure I get you.

Shouldn't an HD disk formatted as DD on an HD drive be just as reliable as a DD disk formatted on a DD drive? The difference in reliability seems to stem from the HD coating not being designed for a DD head, and vice versa.

GeoffB17

I think that the article referred to above explains most of this.


When the drive swaps between the two disk types (DD and HD) it changes some settings as to how it records, flux level, and something else?  I'm not sure if this is the right way around, but the HD disk is designed to take a smaller, but more intense, signal (or something like that).   If you use a HD disk but with DD settings, then the signal being recorded is NOT ideal for the characteristics of the magnetic surface on the HD disk.   As noted, it MAY be OK, and it may be OK initially, but it may well not be reliable.


For example, if you wipe the disk, save something on the disk, and then store it, this may be OK.   If you use the disk for regular read/write operations, then this will most likely go wrong, with problems like incomplete erasure of previous signal, or new data causing damage to adjacent tracks that should NOT be touched.


As I suggest, depending on the specific disks you try, you may be lucky.   Later variants of HD disks are more likely to be a problem.


By all means, try it and see.   There are a number of variables, so again, you might be lucky!


Geoff

khaz

Quote from: GeoffB17 on 23:34, 19 September 17If you use a HD disk but with DD settings, then the signal being recorded is NOT ideal for the characteristics of the magnetic surface on the HD disk.   As noted, it MAY be OK, and it may be OK initially, but it may well not be reliable.

Yeah, I get that. What I'm asking is using a HD disk with HD settings: not fooling the drive by covering the hole and pretending the HD disk is a DD disk, have the HD drive write the HD disk as normal, not swap to the DD settings. What you are saying, "use a HD disk with DD settings" is what we all do currently with our 3.5 drives on CPC when covering the holes of our floppies. Or you need to clarify what you mean by "DD settings".

My question was about whether a HD drive have actual HD and DD settings triggered by the sensor. My understanding is that it does not, HD drives and DD drives have different heads that read and write data slightly differently (calibrated to HD and DD disks).


[edit]sorry, not trying to be combative, but I feel we're talking about different things. Or that I'm really stupid and I just don't get what you're saying.

GeoffB17

As far as I know, a HD drive is capable of writing to a DD disk using DD settings, and can also write to a HD disk using HD settings.   The switch is implemented via detection of the extra HD hole.


Some drives may have jumpers, or other means, of turning OFF this switch, or of selecting the write method some other way.


If you write to a HD disk using HD settings (however this is implemented) then you should be OK.   Ditto regarding DD settings and DD disks.   Outside of that, you risk problems.


By the way, all this has nothing to do with the head.   I'm not sure that there is any difference between the head in a DD (only) drive and that in a DD/HD drive.   Both formats use 80 tracks, the HD drive just packs more sectors/data into each of the 80 tracks.   The difference between the DD and the HD is in the magnetic/recording characteristics of the signal.   This difference also requires different characteristics of the magnetic surface being written to for optimum performance.   Less than optimum performance can (will) put your data at risk.


Geoff


khaz

#6
So don't mess with that hole, let the drive decides how to write data on the disk by checking whether it's a DD or HD one. That regardless of how you want to format it, be it FAT12 1.44M, FAT12 720K, DATA, ROMDOS or whatever image you're trying to write with SAMdisk.

?

That means pages like these shouldn't advise modifying the DD/HD sensor or modifying the floppies:
http://www.cpcwiki.eu/index.php/Modify_PC_floppy_drives
http://www.cpcwiki.eu/index.php/Drive_Compatibility
http://www.cpcwiki.eu/index.php/Guide_on_how_to_connect_a_3.5%22_drive_to_a_CPC6128/664
http://www.cpcwiki.eu/index.php/Guide_on_how_to_connect_a_3.5

[edit] My mention of the heads is because in the link I first quoted, they talk about "720k drives" and "1.44M drives" as if they were only able to read and write at these densities. While it's true for the 720k drive, it's isn't for the HD drive, hence me being confused. They don't really address the 1.44M drive writing DD tracks.

Cholo

#7
Always assumed its a speed/handling issue why you "cant" use 1,44 floppies. Even old manuals (like parados) suggest using 700-800k formats with a "bit of a caution". 5,25" drives also could have issues with speed but that was usually fixed with a jumper setting on the drive.


First time i hooked up a 3,5" to my amstrad 6128 i totally tried to use a 1,44 floppy straight up and it resulted in one giant mess of errors. Asked around & got the "use 720k DD or tape over the hole"-hint everywhere. And it worked (normal amsdos data format).


Of cause on top of "the technical reasons" there is quite clearly also quality reason today. I recall back in the late amstrad days (early amiga & pc days) that HD/DD floppies had a really good quality as you could buy a 10x pack and only if you was unlucky one of the floppies had a bad track/error on format. However with time floppies got cheaper and cheaper .. so in the late amiga days i could buy even "good quality named floppies" & would have to trash half of them just right away. Donno if it was the zip disc or cd-rom .. but every floppy producer just seem to completely give up back then on quality.


Modern floppy drives also seem to have decreased a fair bit in quality as well even though it is possible to get drives with reliable access to more than 80 tracks (aka 81-84). USB drives is also quite a mixed bunch where many only support 1,44 floppies, so if you want to use a USB floppy with CPCDiscXP pseudo format for easy transfer pc<>cpc then you need one that supports 720k DD as well.

1024MAK

#8
So if I understand you correctly, the question is, can a CPC use a HD disk in a HD drive? Assuming that is your question...

Well, a HD drive when using a HD disk communicates data to and from the computer at a higher speed (twice the speed in fact) than a DD drive does. So the disk controller needs to be able to cope with this. On Atari ST computers, there are modifications that can be done so that the clock signal fed to the floppy disk controller chip can be doubled when an extra circuit detects that a HD disk is in the drive. Disks under this system are (subject to filing system differences) compatible with PC HD floppy drives, so data can be exchanged.

But most 8 bit CPU based computers are not fast enough to cope with the data speed needed by a HD drive using a HD disk. Also the floppy controller chip is often not suitable for the higher speed needed.

However, it may be possible to use a HD drive with HD disks with some hardware modifications. But the disks would not be formatted to any of the standard formats, so NOT useable on any other computer. Not even DD drives. So it's not very practical.

Mark
Looking forward to summer in Somerset :-)

khaz

Quote from: 1024MAK on 17:30, 22 September 17
So if I understand you correctly, the question is, can a CPC use a HD disk in a HD drive? Assuming that is your question...

No, the question was what happens when you do use an HD drive on a CPC, and how to make the best of it.

When someone wants to add a 3.5" drive to their CPC, their only choice nowadays is to use a 3.5 HD drive that was designed for the PC. Mods can be done to make it behave more properly (RDY signal, switches to swap sides and drive number...), but the question is what to use for the media itself. Can one use an HD disk, and if so, should they cover the HD detection hole?

What I've learned is that HD and DD disks have different coatings, so that the HD disk can hold more data. An HD drive can read and write at both HD and DD speed, read and write at both HD and DD currents (needed for HD and DD coatings), read and write tracks at both high and double density.

The speed and track density is managed by the computer via its disk controller. The read and write currents are determined by the drive itself according to whether it has a DD coated disk in or an HD coated one. It is done so by detecting the absence of a density hole on the floppy.

You can write and read double density tracks at double density speed on an HD disk, but it has to be done at HD currents. Otherwise the writing and reading can be unreliable, especially if there are severe rewrites at the incorrect current. This is why one should be using DD disks to save data on an unmodified Amiga and its DD drive, as it is not equipped to write on HD coatings. However, replacing the original drive with an HD drive makes it ok to use HD disks, as long as their density hole isn't covered.

Now this is a conclusion I drew from reading on tangential subjects. I have yet to come to a floppy expert that clearly says "don't cover that density hole with tape, whatever the formatting you want or the drive you put it in!" It's a conclusion that could very well be entirely incorrect based on wrong premises, which is why I was asking here then.

HD and DD are an ambiguous nomenclature. It can indicate the format of the disc, the track density, the data quantity, the read and write speed, the read and write electrical currents, the magnetic coating, and probably more.

khaz

fuck me I guess. When I try to copy a dsk to a floppy with cpcdiskxp or samdisk in windows xp, I have plenty of write errors: I often have to retry several times before getting a good copy with cpcdiskxp, samdisk fails every tie. Taping the hole makes for a flawless copy process. Of course I can't vouch for the quality of the magnetic data, but I just don't know what to think any more. It's stupid, it may be anecdotal evidence, maybe both software have a bug when presented with an HD disk to write DD data, I don't know. I feel terrible right now.

1024MAK

With a HD disk, it is possible to store approximately twice the amount of data per track (in the same space) compared to a DD disk . Hence when a HD disk that has been formatted to HD (because most HD disks come preformatted) is put in a HD drive and connected to a computer, the data sent to the computer is sent twice as fast as that from a DD drive. So a disk controller chip in a computer designed only for use with DD drives will not be able to cope.

With most modern PC 3.5" drives, it is the sensor (normally a switch) that detects the hole (or lack of) which decides which type of disk has been inserted. This switch sends a signal to the drive's control chip. It is this chip which then changes the magnetic signal strength and the data rate. 

If you cover the hole on a HD disk, then the drive will use the DD magnetic levels, but these will be incorrect for writing and reading a HD disk, as a different magnetic coating is used. How reliable the disk is, then depends on a number of factors.
If a HD drive is used with a DD disk, this should work with most computers that can use DD drives (assuming the RDY signal is provided where needed).

Mark

Looking forward to summer in Somerset :-)

robcfg

As 1024MAK said, it's the drive itself the one that detects the hole and changes the recording parameters.


Just put some tape covering the hole and you're good to go.


Obviously it's not ideal to write DD data to a HD disk, but it's not that we have any other chance.

khaz

Quote from: 1024MAK on 21:26, 19 October 17If you cover the hole on a HD disk, then the drive will use the DD magnetic levels, but these will be incorrect for writing and reading a HD disk, as a different magnetic coating is used. How reliable the disk is, then depends on a number of factors.
If a HD drive is used with a DD disk, this should work with most computers that can use DD drives (assuming the RDY signal is provided where needed).

This is the conclusion I came to. However, Windows XP is screwing with my expectations, and not covering the hole to write with samdisk and cpcdiskxp means tons of error. I believe the OS has the upper hand in which head to use and XP simplified its driver so much that the head choice is made solely on the track density to be written, and not the actual physical floppy to receive it.

I tried with an old MSDOS 6.22 boot disc I have, and unless I specify /720, FORMAT.EXE will format the floppy as HD regardless of its hole status. So the problem isn't recent. I have a format720k.exe somewhere that formats floppies to 720k (for Windows versions that don't have the interface to), but I can't know how it treats magnetic levels. I don't have access to other OSes either.

Quote from: robcfg on 21:34, 19 October 17
As 1024MAK said, it's the drive itself the one that detects the hole and changes the recording parameters.


Just put some tape covering the hole and you're good to go.


Obviously it's not ideal to write DD data to a HD disk, but it's not that we have any other chance.

That's the thing though, you're contradicting yourself. If the drives detects the hole and adjusts its parameters, you should not want to mess with it and cover it. The CPC drive controller sends DD data, the HD drive switches depending on the hole and write to the HD or DD disc properly with the settings each need. I suppose it's PCs with Windows that needs the hole covering, possibly because of Windows itself.

robcfg

I think you're mixing the physical layout and the logical layout of the disk.


You can format a HD disk to a different capacity than 1.44MB, but the drive will use HD disk parameters so that a DD drive may read it or not.


The idea behind taping the hole is to force the drive to write the information using the DD parameters.

1024MAK

The disk drive does not care what file format the computer tries to use. Most computers don't know what type of disk is in a 3.5" floppy drive.
If the computer writes too much data to a track, the data at the start of the track will just get overwritten.

I have not tried with Windows 7 or Windows 10, but XP (and I think Vista) can format 720k using the command line format command.

Mark
Looking forward to summer in Somerset :-)

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