The Power Supply Unit (PSU) Adapter for S9 Machines for ResMed Power Station (RPS) II is required to charge the RPS II. The PSU connects the RPS II to the S9 External Power supply and fits between the two components. The PSU is NOT used to operate the RPS II. During use, the S9 Series DC Cable for ResMed Power Station (RPS) II connects directly between the RPS II and the S9 machine. This adapter is for use ONLY with ResMed S9 machines. It is not compatible with any other machines made by ResMed or any other manufacturer.