A method for signal transmission between a first and a second system; wherein the first and the second system are connected with one another via a current loop. A direct current signal is transmitted in a first frequency range and an alternating current signal in a second frequency range via the current loop. The direct current signal and the alternating current signal are superimposed and the alternating current signal is used for digital signal transmission and the direct current signal is used for analog signal transmission. By means of a control unit, a first data stream is produced, in which the direct current signal and alternating current signal to be set in the current loop are digitally encrypted. The first data stream is transmitted to an electrical current output circuit and the transmitted first data stream is converted by the electrical current output circuit into an electrical current signal. The electrical current signal is composed of the superimposed direct current signal and alternating current signal, or the transmitted first data stream is converted by the electrical current output circuit into the direct current signal and the alternating current signal. The electrical current signal or the direct current signal and the alternating current signal are set in the current loop.