MiniBus™ Digital Databus System Operation: Heavy Duty vehicles The above schematic shows 8 rocker switches connecting to the SIM which is mounted behind the panel. The SIM connects via a twisted pair to a PDM which controls some of the functions. A second PDM controls other functions. More SIMs can be added to the system to give a second control location. Similarly more PDMs can be added. The PDMs are mounted next to the heavy equipment which they control. This means that the heavy battery cables do not need to be routed to the panel.