用IIC芯片PFC8574实现IIC扩展，8574可以实现8位IO口和IIC的双向转换，本程序实现了用89C52模拟IIC检测4位IO口的状态并由4位IO输出检测的状态-Achieve IIC IIC chip PFC8574 for extension 8574 can achieve 8 IO port and two-way conversion of the IIC, the program four IO ports with 89C52 Analog IIC detects state by four IO output detection state