Provided is a data transmission system of providing preambles of different classes according to a characteristic of a terminal. The connectivity relationships may then be used to derive a logical relationship among the external connections. The input optical deflectors in each set are arranged in the incident direction of an optical signal that is input to one of the light input ports. The transmitter also has means for signal parameter adaptation of the communication signal on basis of the feedback signal for obtaining a desired character of the transmission signal. The second test contact is spaced from the first test contact and is preferably electrically isolated therefrom on the substrate. The lubricating agent film serves to reduce the friction between the threaded bore and the clamping screw.