What do you mean part 3? I thought it was finished. You promised it was over.
You had the big bumper bank holiday post last week. This is just one more wafer thin post.
Something I skipped over before is the Samsung SFD-321B is a dual density drive. Not double density. Dual density. That means it supports both double density and high density disks, as did a lot of standard PC floppy drives. There were double density only in the early days, but I am not aware of any drives being high density only, except perhaps the thin ones used in laptops and USB floppy drives.
In 3.5" disk terms, double density is the blue ones, the ones with one hole for write protect. They are 1MB unformatted, 720K PC formatted. 80 tracks, 2 side, 9 sectors per track (or 10 in the case of the 1581 and 11 for the Amiga).
High density is the black disks, the ones with the extra hole on the other side. 2MB unformatted, 1.44MB PC formatted. 80 tracks, 2 sides, 18 sectors per track (22 on the later Amigas that supported HD disks).
My first experiences with floppy disks was the BBC micro. On there, it was important to get the right match of disk and drive. The 80 track heads were thinner than the 40 track ones, so if you had a 40 track disk from a 40 track drive and you wrote to it in an 80 track drive, it might not read back right in the 40 track drive as you had only written to half of the track width, the old data was still present in the rest of the track.
It seems the same issues does not affect the 3.5" drives, the number of tracks and crucially the head width are the same, only the number of sectors. Behind the scenes, there are some physical differences in the way the drive writes to the disk, the coating used, the write current etc.
However, you shouldn't need to know anything about that. If you have a drive that supports high density, it will detect the disk type using the hole and write to the disk appropriately.
(as long as you don't start drilling holes in DD disks or covering the holes in HD disks, both of those might work short term but both are misusing the disk and would not be reliable long term)
But wait, the SFD-321B in my 1581 supports high density. Does that mean I can use high density disks in it?
Well, yes. Yes it does.
But, you won't get any more capacity out of it, because the 1581 does not know about it, so it will be formatted 80/2/10 and you will get the same capacity as if it were a normal disk.
There does not seem to be a way the drive can find out the type of the disk, other than reading the formatted disk and the geometry information stored in there.
On a PC, if you have a HD drive and you put an DD disk in there and try to format it, it will try to format 1.44MB, unless you select 720K from the drop down (when there was one) or use /f720 in DOS.
Both the hardware and the operating system need to support for 720K disks, and it was dropped from the format menu from Windows XP onwards, and is not supported by the USB floppy drive I was using.
It may be possible to modify the 1581 firmware to support high density and get double the capacity. I don't see the point, I have not filled up one of the double density ones yet. But that might make an interesting project for someone so inclined. It should also be possible to reduce the timing of the 1581 for the more modern drive and disks and speed it up, should they wish.
I am just using HD disks because I have more good ones of those, they were produced more recently, so are more likely to be in good condition. It means they will not be compatible with a real 1581, but I'm not bothered, I am unlikely to see one of those (and even if I do, I can still use DD disks, nothing has actually changed).
Unless anyone can tell me otherwise, I can just use those and each track will only be half used. They work fine, just as well as DD disks. It's not like anything is being misused, simply underused.
OK, that's the last 1581 post for now.
Promise.
Well, maybe.
You see, I've got this idea for a track display.......
And I want to revist the disk change signal....
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Minstrels
Have you eaten too much chocolate already? You need a different type of Minstrels.
There are Minstrel 2 and Minstrel 3 kits and accessories available.
You can now get a Minstrel 2 kit for $200. Same as the 1980 pricing for the ZX80 kit on which it is based.
Or if you want ZX81 compatibility, you can get a Minstrel 3 kit for $200
There are a selection of other repair and upgrade parts for various machines, many of which can be seen on this PET 2001 board.
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