PLC is a kind of special computer for industrial automation control, which belongs to computer control mode in essence. Like ordinary computers, PLC uses CPU as word processor to realize word operation and data storage. In addition, there is a bit processor for point operation and control. PLC control is generally characterized by high reliability, easy operation, maintenance, simple programming and strong flexibility.1. Reliability: for Repairable Products, reliability includes product effectiveness and maintainability.(1) PLC does not need a large number of active components and wiring electronic components, and its wiring is greatly reduced. At the same time, the maintenance of the system is simple and the maintenance time is short.(2) PLC adopts a series of reliability design methods, such as power-off protection, fault diagnosis, information protection and recovery, so as to improve its reliability.(3) PLC has the characteristics of high operability, simple programming, convenient operation, easy maintenance and so on. Generally, it is not easy to make operation errors.(4) PLC is a control device specially designed for industrial production process control. It has simpler programming language and more reliable hardware than general computer control.(5) in terms of PLC hardware, a series of measures to improve reliability are adopted. For example, using reliable components: using advanced technology to manufacture assembly line; shielding, isolation and filtering of interference; power-off protection of power supply; protection of memory content, etc.(6) in terms of PLC software, a series of measures are also taken to improve the system reliability. For example, software filtering, software self diagnosis, simplified programming language, etc.1. The operability of PLC is shown in the following aspects:(1) the operation of PLC is convenient, including the operation of program input and program change.(2) programming convenience PLC has many programming languages to use.(3) maintenance convenience PLC has self diagnosis function, which reduces the requirements of maintenance skills for maintenance personnel.