IIC bus is simulated by C51 program according to its time sequence.Based on this simulated IIC bus,a voltage measurement system is designed by the single chip microcomputer and 8bit A/D and D/A converter PCF8591.The PCF8591 is a singlechip,singlesupply low power 8bit CMOS data acquisition device with four analog inputs,one analog output and a serial IIC bus interface.Three address pins A0,A1 and A2 are used for programming the hardware address,allowing the use of up to eight devices connected to the IIC bus without additional hardware.Address,control and data to and from the device are transferred serially via the twoline bidirectional IIC bus.The functions of the PCF8591 include analog input multiplexing,onchip track and hold function,8bit analogtodigital conversion and an 8bit digitaltoanalog conversion.The maximum conversion rate is given by the maximum speed of the IIC bus.The circuit and main program flow chart are proposed in this paper.The system has simple structure and strong expandability owes to the IIC bus.It provides reference for the application of IIC bus in acquisition system.