As a programmable synth, it enables users to create their own unique synthesized tones and sound effects by using the algorithms and oscillators.

The DX21 has MIDI in, out and thru jacks, which enable the instrument to be connected to a computer, drum machine, sequencer, or other MIDI-compatible device.

The CPU is a Hitachi HD63B03XP, a derivative of the Motorola 6803, and the sound generation chip is the YM2164 (OPP), a variant of the YM2151 (OPM).

The 128 ROM preset voices can be grouped into 16 categories: piano; electric piano; organ; strings; brass; plucked strings; comping (accompanying instruments); percussion 1; percussion 2; lead synth; other keyboards; wind reeds; bass; and three sound effect categories, which include sounds such as "racing car", "helicopter", "whistling", and basic sounds such as "LFO noise".

Users can save their own newly-created synth tones to an external cassette recorder.

The low frequency oscillator (LFO) has modulators for amplitude and pitch, using saw, square, triangle wave shapes.