Abstract: In a self-propelled vehicle such as a shoulder-fired missile, a bidirectional data interface between a missile processor and an external apparatus includes a pair of function signal paths which extend partly outside the missile. A first component of the interface uses one of the signal paths to couple reprogramming information to the missile processor in response to a prompt from the processor. A second component of the interface responds to a periodic signal on the second signal path to receive missile processor data which the missile processor connects to the first signal path in synchronism with the periodic signal.