Apparatus for testing data communications on two wire combined power and data network systems for automation, comprising a control means and connection means adapted to connect said apparatus to a subject network segment, in which said control means comprises data signal output means adapted to impart a test data signal to said subject segment via said connection means, and a data signal disruption means adapted to vary one or more physical layer attributes of said test data signal which affect its functional quality and/or to vary one or more physical layer attributes of said subject segment which are capable of affecting the functional quality of data signals carried thereon.