This is an old post, preserved for reference.
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The standard SD2IEC connected to the IEC port on a range of Commodore computers and replaces a traditional IEC disk drive such as a 1541. Sometimes it can be more convenient to have this integrated into the computer.
SD2IEC Classic from The Future Was 8 Bit, which is all the functionality on a single PCB. I have previous described fitting one inside a Commodore VIC20 - VIC20 Internal SD2IEC.
the TFW8B website. The are pins for power and data, and connecting buttons and LEDs etc.. For most applications, if you're not disk swapping, you can get away without the buttons, so we only need the 5 wires indicated below.