When operating a first unit in a daisy chain of units allowing bidirectional communication, each unit is capable to transmit and to receive signals. A plurality of units including the said first unit transmits a respective signal to a preceding neighbor unit preceding in said daisy chain and to a following neighbor unit following in the daisy chain. In the daisy chain, the first unit determines whether or not it receives a signal from both of these neighbor units or not and if so, said first unit operates so as to put the at least one subunit into a first state. If the at least one subunit is not put into a first state, it operates so as to put the at least one subunit into a second state different from said first state.