Motor vehicle having two vehicle electrical systems with different vehicle-electrical-system voltages

Motor vehicle with a first onboard power supply with a first voltage source which provides a first onboard power supply voltage and with a second onboard power supply with a second voltage source which provides a second onboard power supply voltage different from the first onboard power supply voltage, wherein the first onboard power supply voltage and the second onboard power supply voltage are direct-current voltages, wherein at least one power supply isolating device connects a connection point of the first onboard power supply and a connection point of the second onboard power supply, wherein the power supply isolating device comprises at least one semiconductor device, wherein the power supply isolating device has a reverse direction in which substantially no current transport occurs below a breakdown voltage, and a forward direction in which current transport occurs above a minimum voltage, wherein the forward direction of the power supply isolating device is directed from the first onboard power supply into the second onboard power supply,

• wherein the first onboard power supply and the second onboard power supply are designed in order to transmit a first communication signal from the first onboard power supply with the first onboard power supply voltage into the second onboard power supply with the second onboard power supply voltage, wherein the value of the second onboard power supply voltage is higher than the value of the first onboard power supply voltage, and wherein the first onboard power supply and the second onboard power supply are designed to transmit the first communication signal by controlling a voltage at the connection point of the first onboard power supply and in order to transmit a second communication signal from the second onboard power supply with the second onboard power supply voltage into the first onboard power supply with the first onboard power supply voltage by controlling a controllable impedance between the connection point of the second onboard power supply and a reference potential which constitutes a common reference potential for the first onboard power supply and the second onboard power supply.