With the development of office automation, more and more companies build up the local area calling network of itself, in order to increase efficiency and decrease the communication cost. This calling system is generally composed of a master telephone and several extension telephone numbers. The master telephone has a fixed number, and the several extension telephones have their own numbers in the network. When the outside phone dials, the master telephone firstly receive the call and decide the department which the call is to, then the call is transferred to the department. Because the outside call is not pay by the company, the company need not limit the call. When the workers of the company call the outside, www.751com.cn , the company should set limits of authority to discriminate the white numbers from the white numbers. This project mainly use the microcontroller to control the inside call which is to the outside. The microcontroller discriminate the white numbers from the black numbers . AT89C51 is CPU of this system, and it is core of this system. This system uses 74LS373, 74LS138, 7404, DAC0832 to discriminate the white numbers from the black numbers. Software of this system is programmed by assembler language and it is mainly composed by main program, subprogram of generation of the inside phone number , subprogram of discriminating the white numbers from the black numbers, subprogram of digital to analog converting and outputting of the phone number. After designing hardware of the system and programming the software, the system is simulating and debugging online. Well-going of the system verifies correctness in the design. In the end, circuit schematic diagram is drawn by Protel 99SE.Keywords intelligent phone； microcontroller；phone number controlling
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