Abstract: To permit ready matching of a Diesel engine operating circuit to a glow plug system, an interface-and-control unit (1) is provided which includes a microprocessor (MP) which decodes data words received from an engine control unit (MS). The data words are in the form of serially transmitted bits which define, by time duration of the bits. respectively logic "0" and logic "1" values to provide, inherently, simultaneously data information and synchronizing information. For example, the logic "0" bits may be 1/8 of a predetermined clock period (T) whereas the logic "1" bits may be 1/2 of the clock period (T). The microprocessor, thus, can be easily matched to engines of different types and can carry out, on its own, diagnostic routines and indicator functions under command of respective bits being transmitted thereto. Preferably, glow plug current is provided in pulsed form, the duty cycle of which is controlled by four data bits (D0-D4) transmitted to the microprocessor (MP).