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Here's some random stuff i've drawn (hover for a brief description):

Disclaimer 0: This assumes you're familiar with the NEC PC-98 family of computers. For information on this, I would recommand you to read this post on Wyatt's Blog.

Disclaimer 1: This is only useful if you don't have a flux writer (like a greaseweasle for example).

Disclaimer 2: You can also write a floppy by using your PC-98 itself, if you can easily transfer data to it.

crash course

The designers of the PC-9801 architecture (that is, the people at NEC) initially chose a simple approach for dealing with Sony's then new 3.5 inch (90 millimeters) diskette format: they didn't.

That is, in the PC-98 world, a 3.5 inch drive can behave the same way as a 5.25 inch one. This had the main consequence of most games and software being distributed on what is commonly called the "1.2 MB format" (in reality, 1232 KiB).

This is different from the one used in the IBM PC world, which is exactly 1200 KiB (more on that later).

Since the interfacing was initially identical, the other consequence from this is that the spindle motor of those early 3.5 inch PC-98 drives rotated at a speed of 360 RPM, to match the speed of the 5.25 inch drive.

When the people at NEC later decided on adding support for the concurrent DOS/V format (which rotates at a speed of 300 RPM), they used a way to allow the drives to change speed on the fly to allow both formats to be read (it is unclear if this originated from NEC, as others Japanese computers of the era used the same approach).

3-mode floppy drives

The industry standard way of changing the speed of the spindle motor of a 3.5 inch floppy drive is by using pin 2 on the 34-pin interface (this pin is usually called "density").

In this regard, most floppy drives are not created equal, as only a fraction of those can actually change the rotation speed of the spindle motor.
Here is an exaustive list of drives (made by testing units that I have) with the various states of support for that feature:

Full support without any modifications:
- NEC FD1231T
- NEC FD1231M
- Mitsumi D353M3D-5000 (probably other variants)
- Teac FD-235HG (A103-U5 variant, probably others)

Support with hardware modifications on some exemplaries:
- Sony MPF920 (put solder on pad marked SH1 on the PCB)
- Alps DF354 (put solder on pad marked either MS2 or SW2 on the PCB)

Unsupported (may be possible with extensive modifications:
- Panasonic JU-257A606P
- Newtronics (Mitsumi) D359T7 (this one is hell to disassemble, so i didin't)

aside: 3-mode IBM compatible motherboards ?

A lot of people who messed with the BIOS setup of their computer in the 2000s will remember an option that was named "3-mode floppy support" in it.

Well, this option was meant for allowing Japanese users of Windows to have a way of reading and writing (but not formatting) their PC-98 floppies in the era when both types of machines were common.

The way it worked was by a custom floppy drive interface driver (some were supplied in the japanese releases of windows, some others were pre-installed by the OEM) that changes the way the operating system talks to the Super I/O chip on the motherboard.

Since it's very limited in its functionality and it's very hard to setup properly (tried on different motherboards, it does not want to work), I decided against using that, and preferred another method.

an imperfect but easy way of writing disks

"if you force 360RPM via hardware, couldn't you make it treat the drive like a 5.25" drive and have it work?"

This idea from Wyatt was the missing piece in the puzzle to solve this (very specific) problem.

The ingredients you will need for this recipe are:

- A 3-mode compatible 3.5 inch floppy drive
- A IBM compatible machine with a floppy interface
- A Windows 2000 or later installation that works on this machine
- Some images in the D88 format (if not, you can convert them with the tools here)
- The SAMdisk utility and the fdrawcmd.sys floppy driver

hardware preparation

Short pin 2 of your floppy drive to ground. Using a soldered wire is preferred, but you can also use a jumper (or switch) and extend the others floppy pins by soldering a male header with a female one like this:

Before doing modifications, double-check that the ground point you're using is actually connected to ground. Some drives for example does not connect pin 1 to ground, which makes the jumper method useless.

software usage

In the BIOS setup of your computer, modify the drive type of the one you installed to "5.25 inch, 1.2MB".

Install the fdrawcmd.sys driver using the instructions provided on the website, and copy the SAMDisk executable to a known location.

Then, you can test if your drive modification was sucessful by using the following command (with a floppy in the drive):

    SAMdisk rpm a: (or b: if the drive is the second one)

If everything worked fine, the software should report a speed of approximatively 360RPM.

To write a disk, use the following command:

    SAMdisk [disk_image.d88] a: (or b: if the drive is the second one)

You can also image a disk with the following command:

    SAMdisk a: [disk_image.d88] (or b: if the drive is the second one)

See the documentation on the website for additional options.

additional thoughts

Since the fdrawcmd.sys part of this software is open-source, it should be possible to write a piece of software that would handle the many formats that the PC-98 floppy images uses.

Here are some (most) of the books that I've read, ordered by reading date.
Electronics books and mangas are excluded from the list, I have too many of them to list and I don't remember when I first read them.
I've written the English titles when available.

childhood:
- Microsoft Windows 95, 1998 New Edition, Warren Bates
- Building a PC for Dummies, Mark L. Chambers
- UNIX for Dummies, 4th edition, John R. Levine, Margaret Levine Young
- Linux for Dummies, 4th edition, Dee-Ann LeBlanc
- MS-DOS 5 utile, Daniel Rougé
adulthood:
- The Lord of the Rings, J.R.R. Tolkien (vol. 1 only)
- Introduction à Ada, Pierre Le Beux, Second Edition
- Sword Art Online, Reki Kawahara (vol. 5 and 6 only)
- The C Programming Language, Brian Kernighan, Dennis Ritchie
- Haruhi Suzumiya, Nagaru Tanigawa (12/12 vol.)
- Curiosities of Lotus Asia, Jun'ya Ōta "ZUN"
- Shakugan No Shana, Yashichiro Takahashi (1/22 vol.)
- Kino's Journey, Keiichi Sigsawa (16/23 vol.)
- Higehiro, Shimesaba (2/5 vol.)

currently on the reading stack:
- Structure and Interpretation of Computer Programs, Second Edition, Harold Abelson, Gerald Jay Sussman, Julie Sussman
- The Apothecary Diaries vol. 1, Natsu Hyūga

Some notes:
All of my computer science books came from thrift stores.
Availability of light novels in a language other that Japanese can be abysmal. Kino and Shana only have the firsts volumes officialy translated in English, have long been out of print, and will probably ever be.

I'm currently looking for a copy of the following software. If you have any information on it, please get in touch.

- NEC PC-UX or PC-UX/V for PC-98
- Any complete copy of any version (CD and floppies) of Debian(98), the PC-98 version of Debian.

Here is a curated list of useful weblinks. More will be added later.

Miscellaneous:
archive.org, endless source of documentation and software.
J-Card Template, for audio casettes.

Hardware:
Bitsavers, lots of databooks here.
Retronik, databooks from French companies.
List of 7400-series integrated circuits
List of 4000-series integrated circuits
List of AMD Am2900 and Am29000 families
Caps Wiki
XDevs, manuals for instrumentation.
Circuit Simulator Applet
The GIICM, ASCII pinouts of chips.

PC:
minuszerodegrees, info, software on XT, AT, and ISA cards.
IBM AT 80286 BIOS, source and docs.
Ultimate Retro, info on motherboards, including BIOS database.

Macintosh:
Apple Macintosh before System 7, manuals and software.
Resurrecting my Macintosh Plus · tomek's blog, includes a PS/2 to Mac Plus adapter.
Macintosh SE Hardware Designs
a gcc-based cross-compiler for classic 68K and PPC Macintoshes
A 1:1 Recreation of the Macintosh Classic logic board
A collection of old world Macintosh ROM files.

TI99/4A:
The TI-99/4A Tech Pages
Tool for decoding TI-99/4a Cassette Tape recordings
Action game for the TI-99/4A home computer

I had this really dumb idea of creating a webpage that would be contained in only one file. Here are some random thoughts and obstacles I ran through while doing this:

This will absolutely backfire one day.

There is no menu to design.

Images. They are encoded in base64, and I had to make compromises on quality to keep the file sizes small and thus the html file somewhat readable:

-Mostly JPEGs in low quality (favicon for example is so small PNG is more efficient)
-Monochrome images
-Smallest resolution acceptable

Here is a 4KB JPEG example.
One compromise I didn't make was to use arhitmetic-coded JPEGs. stb-image.h and old browsers hates them, and the size loss wasn't that interesting.
As a side-effect, this webpage lacks colour. Except links.