Preserving a floppy disk with a logic analyzer and a serial cable | Hacker News Reader

Preserving a floppy disk with a logic analyzer and a serial cable

chzsoft.de

102 pointsnynyny743 comments

I read "Notes on Floppy Disks" <https://extrapages.de/archives/20190102-Floppy-notes.html> last night. It, along with this, really makes the technology seem much more accessible to a non-electronics professional like me. These help make the various floppy emulator devices I've got for my retro-computing habits seem more understandable. (It also really makes me want to build some electronics skills, too!)

Magnetic recording is kinda fascinating because it is conceptually simple but very hard to do well. Conceptually all you need for a tape machine is two small write coils (erase+write) and the bias oscillator. In practice the "small coils" are complex precision parts. The circuitry of a Nagra is fairly straightforward to replicate, but building comparable tape heads in your workshop will take years.

A related project that has been going through an amazing amount of progress is the Applesauce Floppy Disk Controller.

HN Thread here: https://news.ycombinator.com/item?id=17256709

IMO the coolest part is the widget they wrote to visualize the raw magnetic flux on the floppy.

Example image: https://twitter.com/a2_4am/status/1026110390491643904

While I am sure they released plenty of images on the project's site, as an aside if you change the end of a twitter image url to ":orig", you'll get the original unresized image.

In this case: https://pbs.twimg.com/media/Dj153frUUAEWAgx.png:orig

Really helps you visualize the details of the data, it is really amazing, along the lines of those who look at nand gates and similar things to read decapped roms and silicon circuits. https://i.ytimg.com/vi/RqmN7_KgbFc/maxresdefault.jpg

I love seeing two dimensional images of disk data. One thing it makes clear is how far ahead today's computers are, where we can easily display at many pixels per bit an image containing all of the data of a multiple-loading LARGE program from the 1980's.

For those interested, the actual specifications of floppy disks are freely available from the ECMA:

http://www.ecma-international.org/publications/standards/Sta...

The standardese term is "Flexible Disk Cartridges", and the units are in Metric, but those documents basically provide almost all the information needed to build a disk drive or interpret the data on one.

The CD and DVD specs are there too.

> and the units are in Metric

As they should be. Why is this worth mentioning?

Floppy disks are typically measured in inches, common sizes being 8", 5.25", 3.5" and 3". Like any distance, these may also be measured in metric units. However, for whatever reason, when it comes to disks, they pretty much never are.

A good rule of thumb is that if it says somewhere "x inch" in a common kind of measurement, then the device never has any length of x inches.

Eaxmples:

- 3.5 in floppy disks have no 3.5 dimension (the disk is 86 mm, the cartridge is 94x90 mm, the drive is 100 mm). - 3.5 in hard drives have neither 3.5 in platters nor are any of their external dimensions 3.5 in - Ditto for 2.5 in drives, likely also true for 5.5 and 8 in drives. - Any kind of inch-sensor size (e.g. 1/2, 2/3) is not the size of the sensor measured in any way. - ...

5.25" disks are 5.25"/133mm along each side, making 5.25" a perfectly good description, in my view, for an object at this sort of scale. The disc inside must be 130mm in diameter, the case is presumably 3mm larger, and the whole thing is named after the size of the case. 5.25" = 133.35mm. I expect you could actually make a perfectly usable 5.25" disk that was exactly 5.25" wide...

Apparently 8" disks are 8"/203mm wide, no doubt for exactly the same reason, and with a rather similar result.

Huh, is this true even outside the US? As in, did people in the UK call them 3.5 inch floppy disks, just like us Americans?

Yes. Only a few things proliferate in inches, but products that are defined in inches always do. Subway sandwiches are a mixture, in the netherlands they are 30cm (an extra 30mm), in other metric countries they remain as a footlong.

30cm leaves you 4.8mm short of a foot. This is how we lose spacecraft.

In germany it was called "3 1/2 Zoll Diskette/Floppy" so yeah we didn't use the word inch, but I can confirm that this was true at least for us.

"Zoll" is a direct translation from "inch". Inches were used when measuring wood.

The actual size was different from region to region, because there was no agreement on whether it is 1/10 or 1/12 of a feet, and 1 foot itself would correspond to different lengths: https://en.wikipedia.org/wiki/Obsolete_German_units_of_measu...

You are correct that a document from ECMA is expected to be in metric. Floppy disks are tech from the US though, and everything about them is hence in inches, from physical specs to tech specs, so the metric docs translate the units.

It's like when flat screen TVs came out. When they first appeared in Europe, they sold in inch sizes. You'd go to a store in France and buy a 50 inch TV. Years later they started labeling things as "87.3cm" - and only much later started making sizes in even metric units - which are still uncommon to this day.

If you were reading a spec doc for the US 50" TV, and it was in metric, it absolutely is not something that "should be" - and is a very notable change. Just a friendly question - are you one of the scientists that worked on that mars probe that crashed?

> Floppy disks are tech from the US though, and everything about them is hence in inches, from physical specs to tech specs, so the metric docs translate the units.

Do read the specs someone else linked in this thread. All major dimensions are even metric measurements and thus are not even imperial measurements.

yes, because the document is from Europe for a device created in California. A European document is going to be in metric. That is what the OP was pointing out - that the document is European. The US document, the original one with the specs, would be in inches. I would recommend looking up the acronym ECMA.

while impressive, I wonder where people take the time to explore ancient tech in-depth. work, family, more work. how much time do you guys have?

besides, writing a n64 emulator, yes! fixing a decades old Sun, Xerox or VT terminal, yes! emulating old 8bit cpus in JS yes!

but who wants to understand/explore a floppy drive?? no offense, curious what's so interesting about floppy drives compared to typical retro tech stuff I mentioned above?

I think it's helpful to know where our technology came from for multiple reasons. (It's also fun learning to me, too.)

Knowing what came before makes us better at building new (or, even better, recognizing that we might not need to build new). Our creations would frequently benefit from application of knowledge about the analogous systems that came before. There's nothing new under the sun. There is valuable analogy hiding in so many past technologies.

I want to know about trade-offs made during design, how that influenced implementation, and the lessons that were learned when the technology actually made it (or didn't make it) into use. Decisions made so many years ago shaped the technology we use today (von Neumann vs. Harvard architecture, the minutiae of compatibility in x86 and the IBM PC that dates back all the way to the early Intel 8-bit CPUs the IBM 5150, etc).

I think software, hardware, and protocol "archaeology" are going to become more necessary as more of the people who implemented early systems that are still in use today die. So, besides being interesting, I think there's a marketable skill in the act of being able to build understanding of past technology.

I often wonder the same thing.

Except that I'm envious of both their wealth of time, and ability; rather than dismissive of their desire to understand technology at its fundamental level.

His rationale is right there at the beginning:

> "Being involved with retro computers, I have a few floppy disks (of the 3.5-inch variety) that I would like to preserve as faithfully as possible. Of course, I know there are dedicated devices for doing that, such as the Kryoflux or the SuperCard Pro. But it occurred to me that I already own the required hardware to capture the low-level data from a floppy disk: my Saleae Logic 8 logic analyzer."

It seems he is single, which provides a lot of time.

It's a hobby.

Other people care about collecting small colored pieces of paper, or tweak their cars, or go on hikes, or paint.

No offense to OP (of this thread) but a lot of people who have an obscure but intellectually stimulating hobby wonder why other people spend so much time watching Netflix yet passing judgement on others as having 'way too much spare time'. Is it really that alien to you to find this use of time justified? Have you considered that they might be able to do this efficiently in balance with the rest of their life?

I understand your sentiment,and I doubt I could make the time to do this kind of exploration myself....but, and I don't think this is why they do it- I can imagine this skill could be quite valuable when someone had some data they absolutely have to get, and it is on an old set of floppies.

Different people like different things. It’s exactly how different people like to eat different foods.

For example, I like Cheerios breakfast cereal, but my brother prefers Wheaties.

> work, family, more work. how much time do you guys have?

To stick with the food analogy, someone can eat a different breakfast than another person - but spend the same amount of time doing it!

For example, it takes my brother and I the same amount of time to eat breakfast, even though I eat Cheerios and he eats Wheaties.

Also, people with different interests than you may gain extra time to pursue them by not spending their time on the web criticizing other people’s personal preferences.

Sometimes through a confluence of previous experience and knowledge, ideas that seem hard or impractical to some (even if just as technically inclined) seem straightforward and fun to others, and make for little more than a weekend project. Once and a while I have such an idea and will work on it, but other times I see projects other people do and have similar sentiments as you do.

curious what's so interesting about floppy drives compared to typical retro tech stuff I mentioned above?

It's one of the most widely accessible forms of magnetic storage media that you can control at such a low level.

It’s the data that is on them.

Some people are fortunate enough to have that. I have created a successful company and I practically don't have to work anymore. I spend my time with hobbies and traveling.

What's the difference between understanding a 40-year-old CPU and understanding a floppy drive? The second is no less impractical than the first, in the modern world.

Oh I don't know. The floppy seems impractical, but the CPU does not.

Those 8 bit CPUs are still being used. And that level of compute can solve a lot of problems.

So it seems it still needs an actual floppy drive, but controls it in a better way to read it more reliably?

I wonder how hard it is to build something that reads the magnetic signal, or what's left of it, directly from the floppy platter

It controls it in the same way PC controller does, the trick is in grabbing raw data instead of relying on primitive hardwired decoders in descendants of uPD765.

>hard it is to build something that reads the magnetic signal, or what's left of it, directly from the floppy platter

Thats exactly what floppy drive does :)

> Thats exactly what floppy drive does :)

I meant as a viable home project, doesn't have to be a fully functional drive as long as it somehow allows to view the magnetic values, manually turn their 2D structure into the valid 1.44MB filesystem if necessary :)

Floppy drive does not decode disk data on its own, it merely amplifies analog signal from magnetic head, filters and quantizes it (zero crossing).

https://patents.google.com/patent/US4656533A/en

What you get on a READ pin of floppy drive is the same information you would probing read head (or rather preamp) using Oscilloscope. All you are interested in is magnetic field flipping, zero crossing, encoded on READ pin as an impulse.

Sure, you could precision machine turntable like contraption to do it manually, but floppy drives are plentiful and practically free.

Yes. You could make your own magnetics value reader which would also be a functional drive but perhaps not 'fully' in that it might be slow etc. For example you could make a high resolution x,y table with a read head that made a flux map and the you could post process this into the data. Using an existing floppy drive seems so much more practical for the job of data recovery.

Take a look at the kryoflux, https://www.kryoflux.com/

Doesn't Kryoflux still require an actual floppy drive to be connected? I think the parent poster was looking for a way to do this without using an actual full floppy drive

Do you really think the article is about someone connecting a logic analyzer directly to a floppy sitting on the lab bench? No, he's connecting it to a floppy drive. That's only logical and also described in the article that you didn't read. Also mentioned in the article in the first paragraph is Kyroflux... Most people only read the headline before commenting?

Thanks, I read the article and I understand all of this. I was just explaining how a kryoflux would not be appropriate for reading a floppy drive "sitting on a lab bench", which I assume is the question that the grand-parent comment was asking.

Which is not an unreasonable question to ask. People have done similar things with vinyl records - reconstructing the audio from a photograph/scan of the record with no "vinyl drive". https://mediapreservation.wordpress.com/2012/06/20/extractin...

Without floppy drive you would have to build a..floppy drive, kind of redundant.

Anyone able to recover 8" disks?

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