【正文】 elatively mon. In asynchronous munication, data is transmitted frame by frame, each frame of serial data format by a start bit, 5 to 8 data bits, a parity bit (optional) and a Stop bit of four parts. Before the serial munication, the sender and receiver to agree the specific data format and baud rate (munication protocol). PCprogrammable serial asynchronous munication controller 8250 to implement asynchronous serial munication. By 8250 the initial programming, you can control serial data transmission format and speed. In the PC, in general there are two standard RS232C serial interface COM1 and COM21MCS 51 series microcontroller chip includes a fullduplex serial interface, can also be realized by programming the serial munications. CMOS smart sensor IC has received wide application for low cost, miniaturization, intelligence and standardization. In smart sensor IC designs, the munication interface circuits also play an important role other than sensors and signal processing circuits. There are more serial munication standard interfaces, such as RS232, RS485, USB Interface, IEEE1394, which has itself good features and also has focused on areas of application. RS485 has a antinoise capacity transmitting data, so it is monly used in industrial production. USB interface and IEEE1394 have fast speed, but some puters and operating systems do not support such interfaces. RS232 is most widely used as a serial interface in a PC puter and munications industries. RS232C standard is a protocol of asynchronous serial munication and has been widely used in PC and munication industries. Many short distance municational peripherals of PC such as printers, disks and terminals all municate with PC via RS232 serial interface. RS232C standard RS232C is the American Electronics Industry Association (EIA) formally announced, in the asynchronous serial munication, the most widely used standard bus. The standard for DCE and DTE serial binary munication between the maximum data transfer rate of up to 19. 2kbps, the longest transmission cables up to 15 meters 1RS 232C standard defines the 25lead, twoway munication in general, just Serial input RXD, serial output TXD and ground GND1RS 232C standard levels using negative logic, the provisions of + 3V ~ + 15V at any level between a logic 0level, 3V ~ 15V of Any level between a logic 1 level, with TTL and CMOS levels are different. In the interface circuit and puter interface chip in the mostly level TTL or CMOS, so the munication, the need for level conversion, so with the RS 232C standard level matching 1MAX232 chip level conversion can be done in this work. 4 RS232 is defined as the singleended standards, which has much characteristic, such as more munication distance in a lowspeed serial munications, moderate price and good practicality of the system, so it is most appropriate to be a munication port. RS232 is an essential modern puter interface, it includes ports COM1 and COM2, the port COM1 has 9pin connector. The new generation puter has a 9pin connector (DB9) so as to connect with the RS232 munication port. In order to achieve reliable and real time transmission, the system adopt a threewire connection method, that is to say that the GND pin, RXD pin and TXD pin of RS232 port is connected with the external port, it is shown in Figure 1. Figure 1. The RS232 port III. THE HARDWARE DESIGN OF SERIAL PORT COMMUNICATION The inout level of single chip puter is TTL level, while the RS232 serial interface of a PC puter is standard serial interface, so both the electrical specifications are inconsistent. In order to municate with the upper and lower puter, it is needed that the out level of single chip puter is changed to the TTL level. The mon level converter chip between TTL and RS232 level is MCI1488 and MCI1489. The MCI1488 converts TTL level to RS232 level, it needs the supply voltage of 177。 12V, and that the MCI1489 converts the RS232 standard level TTL level, it needs power supply voltage of +5V. Due to three power supply voltage, the circuit will be plicated, so this circuit adopts the standard RS232 chip MAX232. MAX232 is the product of the MAXIM Company。 it is the road receiver and driver IC chip. About MAX232 chip MAXIM MAX232 chip is produced by lowpower, singlesupply dual RS232 transmit / receiver 1 for all EIA232E and V. 28 / V. 24 munication interfaces 1MAX232 a chip 5 supply voltage converter, can+ 5V input power converted into RS 232C 177。 10V output voltage level required, so using this chip serial munications interface system as long as a single +5 V power supply can. MAX232 requires four external electrolytic capacitors C1, C2, C3, and C4, is the internal capacitance of a power conversion requirements are 1μF/25V1 should choose its value and should be as close as possible tantalum chip capacitor 1C5 is 0. 1μF decoupling capacitor. MAX232 pin T1IN, T2IN, R1OUT, R2OUT to pick TTL / CMOS level pin 1 pin T1OUT, T2OUT,