Doepfer A-143-2 Quad ADSR

Each of the four envelopes is identical. They are attack-decay-sustain-release type envelopes with a gate input for fireing the envelope and a re-trigger input for restarting the envelope once it has reached the decay phase.
A switch serves as a time range selector - slow, medium or fast.

Each ADSR of the A-143-2 has three special digital outputs - digital in the sense that they emit on/off signals, or in modular language: gate signals. This happens for each output after having finished the attack phase, the decay phase and the release phase.
If you, for instance, connect the End-Of-Decay output with the envelope's gate input, the looping attack-decay stage turns into a LFO with adjustable waveform. You might as well use these outputs to start other envelopes with a delay ... and if you mix them with a polarizing mixer you can realize longer envelopes which work bidirectionally.