The purpose of the port is to tether 2 controllers together, either 2 7090's and one can be run in slave mode, or a more advanced controller and bypassing the 7090. I have had people say they were able to run another pump from it, but that was not the intended purpose and the factory cautions that the older pump lack a protection circuitry to prevent a back feed of current that could ruin the controller and pump so they advise against it.