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I have found inexpensive (~$15) 3.5in floppy to USB adapter cables but have been unable to find a similar adapter for 5.25 floppy drives. Does anybody make such a thing?

Are the pinouts of a 3.5 the same as 5.25 so that I could just make a cable end converter for the 3.5 adapter?

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    Note that the USB-floppy standard officially only supports 3.5" drives, so while the cable pinout is the same, it might still nor work (timing differences etc.). If you get it to work, please add an answer of your own (which you can also accept), because that would be interesting to other people as well.
    – dirkt
    Mar 3, 2019 at 18:49
  • Wouldn't the 5.25 floppy use a standard IDE cable like a 5.25 DVD-ROM? Can you take a picture of the 5.25 drive's plugs?
    – Brythan
    Mar 4, 2019 at 15:54
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    Whatever happens, please do come back and give an update on what worked for you. I still have both my 5.25" and 3.5" HDD floppy drives from 1989.
    – Criggie
    Mar 4, 2019 at 18:45
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    @Brythan - The 3.5 and 5.25 were usually on the same cable with different connectors. When I wrote the question, I forgot I had a dual floppy cable in my parts box.
    – jwzumwalt
    Mar 4, 2019 at 21:10
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    I know it's not cheap, but Kryoflux already solved this for most vintage disk formats.
    – Brian H
    Apr 16, 2019 at 15:00

6 Answers 6

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The data pin-outs are the same, assuming a reasonably “new” 5.25″ drive, not an early ‘80s model.

It was a common upgrade to replace the second 5.25″ drive on an older machine with a 3.5″ one, so you could have a choice of format depending on your needs. They were interchangeable.

You might have an issue with the power though. I don’t remember seeing a 5.25″ drive using the compact power connector pictured there. There might be, and I just don’t remember. There was another older connector type that was wider, with four tubular thick pins in a line. I’m learning that it is commonly referred to as “Molex” but that’s a company name. Power supplies typically provided both sets for choice of device. You will probably need an adapter cable for power. The electric characteristics are the same, it’s just a pin converter.

Be careful to get the right “direction”: you need a male 3.5″ style to “molex 4 pin” female. The converter might be a bit hard to find because most people who needed a converter needed the opposite thing (plug a 3.5″ drive on a 5.25″-only power supply). Cabling is not my area, so others might provide you with better information.

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    Looking at the photos, those Molex connectors seem to be the "regular" Molex connectors that have been in use forever for IDE drives, they shouldn't be difficult to source (you can just take them from any broken ATX power supply).
    – Matteo Italia
    Mar 3, 2019 at 23:03
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    Those connectors are more generically known as "peripheral power connectors", though most people do just call them Molex.
    – Bob
    Mar 4, 2019 at 0:37
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    I also hate the "Molex" name. I use a lot of Molex connectors in my projects, neither is the 4-pin power connector.
    – pipe
    Mar 4, 2019 at 12:44
  • While the data pinout is the same, the connector is different (card edge connector on 5,25" and dual inline pin header on 3,5"), so an adapter will be needed (as alephzero's answer states). I never saw a 5,25" PC drive with the dual inline pin header, even 1993. Furthermore, to read 5,25" DD disks in a 5,25" HD drive, you would need to support a data rate of 300kbps (see the answer by user12037) and use double stepping. You don't need it for 3,5" drives, so 3,5" USB-to-floppy solutions are likely to omit this data rate and the double stepping feature.
    – Michael Karcher
    Apr 17, 2019 at 10:48
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The connections on the 34-pin ribbon cables are identical, but the size of the connectors are different.

You can get ready made cables with both 5.25 and 3.5 connectors, for example http://www.cablesonline.com/36unflopdriv.html (and of course from Ebay also). These are likely to be more reliable than trying to wire a 5.25 connector onto a 3.5-sized cable by hand. These cables used IDC (insulation displacement) connectors which are crimped, not soldered, and they only work as designed with the correct size ribbon cable.

Rewiring the 4-pin power cable is a much more feasible proposition than rewiring the ribbon cable, if you can't find what you need ready-made. It should be possible to release the individual wires from the connector by pressing down the spring clip that locks the wire in place (visible through the slots in one side of the connector) with a thin screwdriver blade or something similar, while pulling on the wire. The wire itself is soldered onto the metal "pin and spring clip" part which goes into the plastic housing.

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  • Actually, all floppy power connectors I have ever seen (since the early 1990s) had crimped connectors. I have never encountered a single one that was soldered.
    – user149408
    Mar 3, 2019 at 21:49
  • The pins on a typical 5.25 drive's power connection are nowhere near the ones on the 3.5 connector, so releasing the terminators isn't going to do much in that you'd have to cut them off anyway. It's generally much simpler to cut the wires behind the connector and then solder\shrink-tube the wires themselves.
    – Comintern
    Mar 4, 2019 at 4:26
  • Power is not a problem, my motherboard has the correct "Molex" 5pin plug.
    – jwzumwalt
    Mar 4, 2019 at 11:35
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I am somewhat sceptical that you'll get this to work for high density. The basics are that normal (single or double density) 5¼" floppies are turning at a rate of 300rpm while high density 5¼" floppies mimic 8" double density floppies with a rate of 360rpm. In contrast, 3½" floppies always use 300rpm. Double density on either have data rates of 250mbps in MFM (double density) mode while high density has 500mpbs. The slower rotational speed of 3½" HD disks is what results in the higher capacity.

It's probably safe to assume that only MFM will be supported and it would be rather surprising if both 250mbps and 500mbps data rate and/or 300rpm and 360rpm would be since those generally have to come from the drive. In theory, the "drive" could decide to vary those based on the size/kind of images stored on the USB stick but at least for double density, the size of 3½" and 5¼" images would be the same. Note that essentially it is the drive that "detects" the format of the floppies.

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  • Thanks for the additional info. I had forgotten about the rpm differences.
    – jwzumwalt
    Mar 4, 2019 at 11:36
  • This probably does not represent the floppy disk as a "USB stick" but rather as either USB mass storage Uniform Floppy Interface or something else. So it is not clear that the implementation details are going to be as hidden (and therefore fixed) as you assume. Consider that someone designing silicon for this role would have wanted to make it as universal as possible (and probably did so some time ago when there was still greater demand). So while boards might be specific to 3.5 inch drive connectors it is less likely that chips would be.
    – Chris Stratton
    Mar 4, 2019 at 15:22
  • OTOH if this is actually a USB enabled MCU with some firmware to operate the drive itself, then it could indeed be quite restricted in what it can do.
    – Chris Stratton
    Mar 4, 2019 at 15:23
  • 5.25" floppy drives for HD (1.2MB) and double density (360KB) are different. Double density drives are 40 track with wider tracks. HD drives are 80 tracks with thinner tracks, and can usually read double density floppies, but can't write them correctly since the write width is half the size of a double density track. I don't know if any "hybrid" 5.25" inch drives with both sizes of write heads were ever made. Although lower capacity, double density media lasts much longer. I was able to read a "library" of 25+ year old double density media a while back.
    – rcgldr
    Mar 4, 2019 at 19:08
  • @rcgldr: I think that with a few tweaks to the electronics, it would have been possible to make a high-density drive write 360K disks in a manner that could be reliably read by low-density drives, though doing so efficiently it would have required that DOS understand some special handling requirements and have a minimum of 4.5K of buffering available. The trick would be to have the high-density drives add a few extra bytes between the sector header and a sector, which would get ignored by older drives, but could be used to recognize what type of drive wrote a sector last.
    – supercat
    Oct 8, 2020 at 16:27
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The price tag is certainly higher (~100€), but AFAICT you can use a KryoFlux to read everything (even strange low-level software protections):

  • Works with all major 3.5" and 5.25" drives
  • Works well with selected 3" (e.g. Amstrad FDI-1) drives.
  • Also works with 8" (e.g. Shugart 851; might require additional adapter) drives; other types of drives and media currently under investigation.
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Instead of the board you suggested or a KryoFlux, maybe build a FluxEngine.

You just buy the specified FPGA development board, solder one row of pins from a floppy connector onto it (or, in a pinch, a row of pin header), flash it, and you're done.

You'll also need one of those wall-to-Molex power bricks that you see bundled with USB-PATA/SATA adapters, but the FluxEngine itself will cost around the same as that adapter you linked and a lot of 5.25" floppy formats are officially supported.

Another option is the Greaseweazle but be warned that, if you go for the "build it using a Blue Pill" option, it won't work with a counterfeit STM32.

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  • The FluxEngine sounds a lot like something I'm working on, which would be intended to facilitate reading single-sided disks of any sort using an Apple //c, producing Apple II disks from an image, or feeding software directly from a PC to an Apple //c without having to store it to disk first. My approach would require using an Apple //c rather than a stand-alone drive, but would be able to read the back side of a "flippy" disk without needing to see the index hole.
    – supercat
    Oct 9, 2020 at 22:27
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The firmware on all the 3.5" USB floppy drives of relatively modern make I've looked into seems to present the floppy as a LBA device with exactly 2880 sectors (of 512 bytes) to the host computer. That is the very same thing as an USB thumb drive (and in fact, most OSs seem to see such a thumb drive. You can even partition such a floppy).

Replacing the 3.5" drive with a 5.25" drive might work in principle, (however, most recent drives in such floppies don't even have standard floppy connectors anymore, there you couldn't) but there simply is no such drive that could store 2880 sectors.

A Kryoflux as proposed in another answer seems to be your best (and only, in 2020) bet to connect a 5.25" drive to USB.

The main problem with Kryoflux for your purpose is: It doesn't present itself as a floppy drive (not even as a mass storage device) to the host computer, but rather as a commandable streaming data source and sink and will only work with it's own software, that can mainly produce disk image files. Any other floppy disk software that will happily work with a "real" floppy disk controller is not gooing to work with Kryoflux.

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  • Most 8 inch floppies use/used 50 pin connectors, but there are/were 34 pin to 50 pin adapters. I don't know if MSDOS or Windows supports 8 inch floppies. I have an old CP/M system that can use them.
    – rcgldr
    Oct 8, 2020 at 20:02

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