Chapter 16 PLC Function Applications│CP200016-1616-4 Basic principles of PLC ladder diagrams16-4-1 Schematic diagram of PLC ladder diagram program scanningOutput results arecalculated on the basis ofthe ladder diagramconfiguration(internal devices will havereal-time outputbefore results are sent toan external output point)Repeatedimplementation16-4-2 Introduction to ladder diagramsLadder diagrams comprise a graphic language widely applied in automatic control, andemploys common electrical control circuit symbols. After a ladder diagram editor has been used tocreate a ladder pattern, PLC program designed is completed. The use of a graphic format to controlprocesses is very intuitive, and is readily accepted by personnel who are familiar with electricalcontrol circuit technology. Many of the basic symbols and actions in a ladder diagram comprisecommonly seen electrical devices in conventional automatic control power distribution panels, suchas buttons, switches, relays, timers, and counters.Internal PLC devices: The types and quantities of internal PLC devices vary in differentbrands of products. Although these internal devices use the same names as conventional electricalcontrol circuit elements such as relays, coils, and contacts, a PLC does not actually contain thesephysical devices, and they instead correspond to basic elements in the PLC's internal memory(bits). For instance, if a bit is 1, this may indicate that a coil is electrified, and if that bit is 0, it willindicate that the coil is not electrified. An NO contact (Normal Open, or contact a) can be used todirectly read the value of the corresponding bit, and an NC contact (Normal Close, or contact b) can
