Graphic method is to design PLC program by drawing. The common methods are ladder diagram method, logic flow chart method, sequence flow chart method and step sequence control method.
A ladder diagram method: ladder diagram method uses ladder diagram language to compile PLC program. This is a programming method that imitates the relay control system. Its graphics and even component names are very similar to relay control circuits. This method can easily transplant the original relay control circuit into the ladder diagram language of PLC. This is the most convenient programming method for those familiar with relay control.
B logic flow chart method: logic flow chart method uses logic block diagram to represent the execution process of PLC program and reflect the relationship between input and output. Logic flow diagram method is to express the process flow of the system with logic block diagram to form the logic flow diagram of the system. The logic of PLC control program compiled by this method is clear, and the causal relationship between input and output and interlocking conditions are clear. The logic flow chart will make the context of the whole program clear, facilitate the analysis of control programs, find fault points, debug programs and maintenance programs. Sometimes for a complex program, it may be difficult to program directly with statement table and ladder diagram, so you can draw the logic flow chart first, and then compile the PLC application program with statement table and ladder diagram for each part of the logic flow chart.
C. sequence flow chart method: the sequence flow chart method enables to first draw the sequence diagram of the control system (that is, the control sequence diagram of which control should be carried out at a certain time), then draw the program block diagram of the corresponding control task according to the sequence relationship, and finally write the program block diagram into PLC program. The sequential flow chart method is very suitable for the programming method of time-based control system.
D. step sequence control method: step sequence control method is to design complex control program with the cooperation of sequence control instructions. Generally, complex programs can be divided into several program segments with simple functions. One program segment can be regarded as a step in the whole control process. From the whole point of view, the control process of a complex system is composed of such steps. The task of system control can actually be considered to complete the control of each step at different times or in different processes. Therefore, many PLC manufacturers have added step sequence control instructions to their PLC. After drawing the state flow chart of each step, the control program can be easily written by using the step sequence control command.