（ 5 ）对在线和离线编程的挑选
离线编程示指主机和编程器共用一个 CPU ，经过编程器的办法挑选开关来挑选 PLC 的编程、监控和运转作业状况。编程状况时， CPU 只为编程器服务，而不对现场进行操控。专用编程器编程归于这种状况。在线编程是指主机和编程器各有一个 CPU ，主机的 CPU 完成对现场的操控，在每一个扫描周期结尾与编程器通讯，编程器把修正的程序发给主机，在下一个扫描周期主机将按新的程序对现场进行操控。核算机辅佐编程既能实现离线编程，也能实现在线编程。在线编程需置办核算机，并配置编程软件。选用哪种编程办法应依据需求决议。
Different loads have corresponding requirements for the output method of PLC. For example, for inductive loads that are frequently switched on and off, transistor or thyristor output type should be selected instead of relay output type. However, the relay output PLC has many advantages, such as small conduction voltage drop, barrier effect, relatively cheap price, strong ability to withstand instantaneous overvoltage and overcurrent, flexible load voltage (AC and DC) and large voltage level. Therefore, AC and DC loads with infrequent actions can choose relay output type PLC.
(5) Selection of online and offline programming
Offline programming indicates that the host and programmer share a CPU, and the programming, monitoring and operation conditions of PLC are selected by selecting the switch through the programmer. When programming, the CPU only serves the programmer and does not operate on site. Special programmer programming is attributed to this condition. Online programming refers to that the host and programmer have a CPU respectively. The CPU of the host completes the control of the site, communicates with the programmer at the end of each scanning cycle, and the programmer sends the modified program to the host. In the next scanning cycle, the host will control the site according to the new program. Computer aided programming can realize both offline programming and online programming. Online programming requires an accounting machine and programming software. Which programming method should be selected should be determined according to the requirements.
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