An interface device which is provided with a DC-DC converter and a power output terminal so that the internal circuit of the interface device and the external equipment may share a common power source in spite of their different power supply requirements, and the wiring connection between the interface device and the external equipment may be simplified. In particular, if the interface device is configured as a terminal base and coupled to the controller via serial transmission line, a significant simplification of the wiring between different parts of the controller system can be accomplished. If the power terminals include positive and negative power output terminals arranged in an alternating fashion, and the power terminals and the signal lead terminals are arranged in two different rows, preferably, in staggered relationship, the leads from different pieces of external equipment may be clustered close to each other, and the external wiring work is extremely simplified. This contributes to the elimination of wiring errors and the reduction in the cost of installing and controller system.