A device for unidirectional transmission of data from first to second networks includes a network TAP reading data from a first subscriber of the first network to the first network and outputting read data to a second subscriber of the second network. A testing device for freedom from errors of read data includes a processor testing the read data for freedom from errors and a first digital output for a digital signal. The processor actuates the first digital output depending on the test of the read data such that the digital output outputs a first digital signal depending on the result of the test of the read data to form a feedback channel to the first subscriber by the output first digital signal to indicate the result of the test of the read data to the first subscriber. A corresponding method, network system and computer program are also provided.