A bus interface bus is described. A first logical state is conveyed over the bus by a higher voltage level and a second logical state is conveyed by a lower voltage level. An output stage of the interface includes a power transistor configured to drive the lower voltage level onto the bus to convey the second logical state, and a protective device between the power transistor and the bus. The protective device couples the power transistor to the bus when turned on and limits negative voltage excursions at the power transistor when turned off. A control circuit of the interface is configured to turn on the protective device when the bus voltage is above the lower voltage level and to turn off the protective device when the bus voltage is at or below the lower voltage level.