The KSU is the systems control center. It houses the Power Supply, has nine PCB slots and provides for connection to trunks and extensions. The first slot in the KSU is dedicated to the NTCPU. The next slot is a universal slot which should be reserved for a Digital Station Card. The remaining seven slots are also universal. They can be used for any combination of Common, Trunk or Station PCBs. The KSU can be floor, wall or rack mounted.You should plug a Digital Station Card into the first universal slot.