Replace the relay control is the trend of PLC control, if the relay control system with PLC transformation, under the original relay diagram to design of ladder diagram is clearly a shortcut. This is due to original of following electrical control system after long-term of using and test, has was proved can completed system requirements of control function, and following electrical circuit diagram and and trapezoidal figure has many similar of at, so can will following electrical circuit diagram after appropriate of “translation”, to design out has same function of PLC trapezoidal figure program, so will this design method called “transplant design method” or “translation method”.
Analysis of PLC control system in function, you can control systems PLC as a relay in the control box. PLC control box for external wiring diagram describes the external wiring, PLC ladder program is the control box inside the “route map“, PLC input relays and the output relay is the control box and external links “relay“, so that you can use to analyze relay diagram methods for analysis of PLC control system.
Can enter the relay contacts as contacts of the corresponding external input devices, output relay coil coil as the corresponding external output devices. External coil output devices in addition to the PLC control, the control may also be affected by external contacts. Relay diagram can these ideas be converted to function the same PLC external wiring diagram and the ladder diagram.
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