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The MSX was an 8 bit computer standard created by Microsoft and ASCII in the early 1980s in Japan. It did find its way to other shores, but it was most popular in Japan. Many games were released, some on cartridge, others on cassette tape, or floppy disk later on.
8 and 16 bit computers in Japan in the 1980s and 90s was a mishmash of platforms that weren't compatible with each other. NEC reigned supreme with their PC88 (8 bit) and PC98 (16 bit) computers. Sharp and Fujitsu staked their claim and found their niche as well.
When the MSX came along it did two things. It allowed consumers to be able to go in a store and buy a game or piece of software for MSX, and it would work in their MSX at home no matter who made it. It also gave companies a way to compete in the 8 bit computer market against the giant that was NEC.
While the MSX standard laid out some baselines for certain specifications, there was still a whole lot of freedom to create unique machines that stood out against others. Some had built in floppy drives, some built in data recorders (Cassette tapes), and some built in printers. The amount of ram and vram varied model to model, as well as a/v out methods. One model released in the Middle East even had a built in Sega Mega Drive, good luck finding and affording one. The point is, there's a huge variety to choose from.
The MSX standard evolved over time creating 4 generations of hardware.
The original and most limited units. There were some really interesting models released, but you're limited to MSX software only.
The second generation and best compromise, imo. All generations are backwards compatible with the generations prior. So you get access to the full MSX and MSX2 catalog of software, which is the overwhelming majority of it. Some later models included built in floppy drives. If you're after real software, then you want a model with a floppy drive. Add on drives were released, but it's easier and cheaper to find a model with one built in. The floppy drive can be emulated via flashcart, so if you like to use roms don't feel left out.
Another upgrade in capabilities. There are only a handful of software titles that need the MSX2+. At the time of this writing, 2023, these models aren't too expensive. Some MSX2 models can be upgraded to 2+ is you're handy.
MSX Turbo R
A small dribble software requires one of these models. These are also less common and expensive.
What to look out for
The MSX uses 5V, +12V, and -12V internally. Many MSX versions had a fully internal power supply with an attached cord. Some companies did silly things though. Panasonic was a big player in the MSX game, and some of their MSX2 models like the FS-A1 used an AC adapter that attaches to the unit with a 3 prong connector. 1 pin is ground, 1 is 9V, and 1 is 18V AC. There isn't a modern equivalent to this. If it's missing you're screwed, or you have to modify your internal power supply. As of now, they sell for $70+ alone. It's a shame because they're my favorite looking machine, and they released some really cool peripherals that match the aesthetic nicely. Be sure to research the unit you're interested in so you don't end up with a paperweight.
Most models have at least 2 slots. Some might only have 1 slot and an expansion slot. You would need an adapter to use the expansion slot as a standard cartridge slot, and those can be hard to find. Why does having more than 1 matter? There were a lot of expansion options in cartridge form, extra memory, extra sound chips, etc. These would go in 1 slot, a game in the other. There were even slot expansions available that would increase the number even more. Another reason is that Konami had fun with this and some of their games had easter eggs if you inserted certain games together.
Many models had RGB out, some only composite, and/or RF. The most common connector used was an 8 pin DIN, but others were used including JP21 (Japanese Scart, same connector but different pinout from Euro Scart). Pin assignment could vary from model to model. Some models only had composite out and might send it through RCAs 5 pin DIN, or others.
Flashcart or Expansion Compatibility
If you're planning on getting a flashcart or other expansion cart, make sure it's compatible with the model you're interested in. Compatibility is very high, but I believe a few models are troublesome.
This is only a tiny fraction of information about this platform. There are many great sites with loads of information about everything you can think of.