Serial communication testing

A coupling unit provides a signal path between at least two of: a first device under test (DUT), and first and second couplers. The first coupler is coupled to a signal analyzer. The second coupler is coupled to another DUT. Both couplers provide and receive signals to and from the coupling unit which includes first and second multiplexers. An output of the first DUT is coupled to a first input of the first multiplexer and to a first input of the first coupler. A first output of the first coupler is coupled to a second input of the first multiplexer. The output of the first multiplexer is coupled to the input of the second coupler. The output of the second coupler is coupled to a first input of the second multiplexer and to a second input of the first coupler. A second output of the first coupler is coupled to a second input of the second multiplexer. The output of the second multiplexer is coupled to an input of the first DUT.