The LTC4151-1/LTC4151-2 split the SDA line into SDAI (input) and SDAO (LTC4151-1 inverted output) or SDAO (LTC4151-2 output) for convenience of opto-coupling with a host controller that sits at a different ground level.When using opto-isolators with the LTC4151-1, connect the SDAI to the output of the incoming opto-coupler and connect the SDAO to the anode of the outgoing opto-coupler