Expansion modules add functions to the basic Solar Monitor unit. In case you install SM2-MU without any other modules, skip this stage.
Connect HBUS to SM2-MU. Then connect all modules to the SM2-MU as in the picture bellow.
SM2 - AD SM2 - BE SM2 - DI SM2 - GSM SM2 - PC
In order to connect expansion module to the basic Solar Monitor unit you need to buy HBUS. But each expansion module has its own HBUS. In order to connect two expansion modules, you don't need an extra HBUS.