This is the IS-AGB-MIDI. A music development cartridge for the Gameboy Advance.

Designed and built by Intelligent Systems.

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The cartridge was not actually used with a standard GBA handheld directly, as one would think. Developers would insert the cartridge into a special slot located on the IS-AGB-EMULTOR development kit. A midi device such as a Keyboard or sequencer could then be interfaced to the cartridges 5-PIN DIN connector using a standard midi cable. Midi tracks could then be previewed on the dev kits GBA handheld in real-time.

You can watch the IS-AGB-EMULATOR Dev Kit demo below for further details on this AGB dev kit: