Serial communication testing

Shown is a coupling unit (10) for providing at least one signal path between at least two of: a first device under test - DUT- (85), a first coupler (60), and a second coupler (70). Each signal path is adapted to provide a unidirectional serial communication. The first coupler (60) is adapted to be coupled to by a signal analyzer (65) adapted to analyze signals received, to receive signals from the coupling unit (10) at one or more inputs (65i), and to provide signals to the coupling unit (10) from one or more outputs (650). The second coupler (70) is adapted to be coupled to by a second DUT (75), to receive signals from the coupling unit (10) at an input (75i), and to provide signals to the coupling unit (10) from an output (75o). The coupling unit (10) comprises a first (20) and a second (30) multiplexer, each adapted for selecting as its output one of its plural inputs. An output (85o) of the first DUT (85) is coupled (100) to a first input of the first multiplexer (20) and to a first input (120) of the first coupler. A first output (140) of the first coupler (60) is coupled to a second input of the first multiplexer (20). The output of the first multiplexer (20) is coupled (160) to the input of the second coupler (70). The output of the second coupler (70) is coupled to a first input of the second multiplexer (30) and to a second input of the first coupler (60). A second output (150) of the first coupler (60) is coupled to a second input of the second multiplexer (30). The output of the second multiplexer (30) is coupled (110) to an input (85i) of the first DUT (85).