I see these ROM’s on the internet with old Atari and NES games. How do they get the game data off of the cartridge?
How does anything do anything?
You know how a cd-rom drive reads the cd? It doesn’t distinguish between a commercial cd copier, and a Cd-rom drive. Same with a game copier. The carriage can’t tell the difference between a game system, and a well designed copier.
Well, the copier simply reads the cartilage, and takes down all the info. It then saves the info to memory, exactly as it was before. Then, you can take the memory, and have it read by the device of your choice.
For more info, see http://www.howstuffworks.com/
I want more information. What should I search for on howstuffworks.com? I tried “video game” and didn’t see what you’re talking about.
You don’t. You get data off a goose.
Did I win?
What? 
The reason why you see nothing specific is that the answer is not specific to video games. You might as well be asking “How does Flash (Or any other kind of semi permanent Memory Works?”, then throw in some rudimentary info on how a devices receives, information, and feeds it back out.
You can also search the pages which sell such devices, but they are not going to tell you all that much, since you can make them yourself. There are home made pages documenting how to build coppiers, but as they often link to roms, I will not link to them.
Whoo whoo! I was going to make the same statement, except I was going to say you get data off a duck. You must be a vert smart, interesting person, big_dogs!
Here’s an example:
Super Nintendo Game Cartridges (there are thousands to choose from and SNES emulators are at every corner of the emulation world)
The cartridges are just a pcb(printed circuit board) with memory modules soldered on. Remove the plastic and you’ll see. The output is (i believe) a 16 or 24 pin interface (not actually pins, but flat connectors, not too dissimilar from your ordinary PCI card). It the yellowish part of the pcb with the copper leads . For those who want to read from this can get PCI adapter from Radioshak that can read a 24 interface)
It’s that simple.
They copy it over the contents to a disk image or ROM format to be read by the emulator.
To write back to the card would pose certain illegal(law) procedures. As if the entire process isn’t illegal enough.
That’s not the whole story, I don’t think.
Usually the information is retrieved from those old Atari/NES/SNES cartridges using developper’s kits connected via serial port with drivers that are written from scratch. They’re not exactly flash based memory, and they’re more complicated than you’re making them out to be.
You cannot write a different game onto a cartridge. Those old game cartridges have certain amounts of data “hard coded” into the chips on the cartridge itself. There’s legends of “empty” carts for developpers that can be loaded with any information at all, but I’ve never actually seen one or heard from someone who had.
To go a bit lower on the hardware heirarchy, each pin on the cartridge can carry either a high voltage or a low one (1 or 0). The pins will be designated for certain tasks. Some of the pins will be address pins. They will hold the address of the memory location to return. Some of the pins will probably be a control code that controls what operation to perform (for a read-only game cart, this might be unnecessary, since all you ever do is read memory). There will be some output pins. There’ll be a ground pin (or several), and a clock pin. Depending on the internal logic, some of the pins may have multiple uses at different times.
All of this is either documented somewhere or could be reverse engineered by hooking up a logic analyzer to the pins while something was going on. One of my coworkers has been involved in a Super Nintendo emulation project because he has access to expensive hardware debugging tools.
Some quick googling found me a developers site with various hardware and software emulation projects, including text files listing the pinouts for dozens of systems and games. I also found a guy who had built his own reflashable nintendo cartridge. Since I didn’t have time to look closely at the content of those sites, and they may well contain illegal information, I’m not going to link to them here, but they’re not hard to find.