A lighting system includes various ballasts or LED drivers having respective dimming interface circuits coupled in parallel to an external dimming device. Each device includes a controller regulating output to a lighting load based on desired dimming output. Dimming interface circuits coupled are between the controller and the external dimming device, each configured to dynamically adapt a level of constant current sourced from the dimming interface circuit to the external dimming device via first and second interface terminals, based on a determined analog mode or digital mode associated with the external device. The dimming interface circuit generates dimming control signals to the controller based on dimming input signals received via the first and second interface terminals, and in a digital mode, the interface circuit generates digital pulse signals to the external dimming device via the interface terminals for bidirectional communications therewith.