Patent ID: 7289615

Claim:
A private branch exchange (PBX) comprising: (a) a main control unit for working as a central processing unit (CPU); and (b) a plurality of line cards, and each one of said line cards including: (b-1) a sub-control unit; (b-2) a first module group for regularly working responsive to an instruction from said sub-control unit; (b-3) a second module group for working responsive to an instruction from one of said main control unit and said sub-control unit; (b-4) an internal bus arbitrator for arbitrating between said main control unit and said sub-control unit about a bus right of passing through a bus; and (b-5) an internal module bus selecting unit having a bus arbitration circuit and a bypass selector for bypassing said bus arbitration circuit, wherein when one of said main control unit and said sub-control unit accesses said second module group, said internal module bus selecting unit carries out a routing of the access, wherein said main control unit recognizes types and numbers of said line cards at startup of the PBX for predicting a load applied to a system, and the prediction prompts said main control unit to determine which one of said main control unit and said sub-control unit controls which individual module of the first and the second module groups on each one of said line cards, wherein when a module to be accessed is directly controlled by at least one of said main control unit and said sub-control unit, the access bypasses said bus arbitration circuit, and wherein when the module to be accessed is controlled by both of said main control unit and said sub-control unit, said internal module bus selecting unit carries out a process using said bus arbitration circuit.