【正文】 the chain and interlock control, self diagnostic circuit and module type structure and other measures, in order to improve the reliability of hardware and modular interchangeability。 in the software uses fault detection, diagnosis and other measures. Other practical advantages in terms of software is simple and easy to learn. 2 PLC convenient use。 set filter links, to eliminate the external interference and the interference effects between each module。 has the advantages of small volume, light weight, convenient installation. Because PLC has such a series of advantages, in the industrial control is widely used in the following three kinds of control: ( 1) switch quantity sequence, logic control. In the place of the relay control system, such as the metallurgical industry in the charging system of blast furnace, rolling mill, continuous casting machine, fee shear control system。 the shielding measures to prevent space electromagic interference。 machinery industry a variety of automated production lines, automatic processing tools, mechanical hand, Longmen milling machine control。s own knack in hardware used in photoelectric isolation device to prevent the output to the input of the feedback interference。 light industry in the injection molding machines, packaging machines, food processing machinery. ( 2) analog control. Such as temperature, pressure, flow rate。s automation level remains to rise, only the equivalent of the world advanced technology in the level of eighty time. With the development of industrial modernization, robot technology has also increased, the development trend is the working strength is high, flexibility, accurate and reliable, can automatically detect and motor mand, the integration of advanced artificial intelligence, makes people only as usual simple maintenance, it is also the development trend of modern factory. The mechanical hand is mainly used for machining, casting and fing, heat treatment and other aspects, quantity, variety, performance can not meet the need of the development of industrial production. So, in the domestic is gradually expand the scope of application, focusing on the development of casting, heat treatment mechanical hand, in order to reduce labor intensity, improve working conditions, in the application of special mechanical hand at the same time, the corresponding development of u