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Beyond that, anything else is VERY optional that most people don't bother with, like the audio input (which is included on some Analog I/O board now built in) that is just for using your original cassettes with the PC cores, or for some spinner controllers. Or an RTC module that just keeps track of the time while the system is off (it still gets the time automatically from the internet on boot, and is mostly just to keep logs/saves organized, and some RTC-enabled games).
Some have basically taken all of these components and put them into a single all-in-one board such as the Multisystem 2. Others have created boards you plug the DE-10 into that let you do things like mount it in a computer case or an arcade cabinet.
As for controllers, if you don't care about original controllers you can use any standard USB (Or Bluetooth if you get a WiFi/BT adapter) controller. You can also use original controllers though the user port using devices called SNAC. They are very very simple and basic devices that let you plug your original controllers into the user port. Note that SNAC adapters are NOT USB ADAPTERS and the user port is NOT A USB PORT despite looking like one. You can cause damage if you plug standard USB devices into it or a SNAC adapter into a USB port.
The user port connects directly to the FPGA chip on the DE-10, it basically plugs the controllers (or other accessories like a Gameboy link port) electronically into the simulated system on the FPGA in the same way they would have been wired on the original hardware. This gives the benefit of introducing zero additional latency (Standard USB would add latency) and work with just about any original accessory (Many USB adapters tend to have problems with 3rd party controllers or other accessories that plugged into the controller port, like light guns or steering wheels).
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