How to draw a SIMATIC S7-1500 PLC function diagram (1)

How to draw a functional diagram

Function chart (SFC) is a graphical representation method for describing the control process, functions and characteristics of the control system. It is simple and intuitive, does not involve specific technologies of control functions, is a universal language, and is the preferred programming language of IEC (International Electrotechnical Commission). In recent years, it has been popularized and promoted in PLC programming. It is called sequential function chart in IEC60848 and function table chart in my country's national standard GB 6988-2008. Siemens calls it the graphical programming language S7-Graph.

The SFC is a tool for designing PLC sequential control programs. It is suitable for large-scale systems with complex program relationships, and is particularly suitable for the control of sequential operations. When writing complex sequential control programs, S7-Graph is more intuitive than ladder diagrams.

The basic idea of ​​the function diagram is that the designer divides a working cycle of the controlled equipment into several working stages (referred to as "steps") according to production requirements, and clearly indicates the output to be executed in each step. The "steps" are converted between each other through the established conditions. In the program, as long as the conversion between "steps" is carried out through correct connection, all the actions of the controlled equipment can be completed.

The basic process of PLC executing function chart program is: select the working "step" according to the conversion condition and perform the logical processing of the "step". The basic elements of function chart program are steps, conversion conditions and directed lines, as shown in Figure 1.

Figure 1

(1) Step

A sequential control process can be divided into several stages, also called steps or states. The step corresponding to the initial state of the system is called the initial step, which is generally represented by a double-line frame. The control system is controlled in each step. Certain "commands" need to be issued, and the controlled system needs to complete certain "actions". Both "commands" and "actions" are called actions. When the system is in a certain working stage, the step is in an active state, which is called an active step.

(2) Conversion conditions

The signal that causes the system to enter the next step from the current step is called a transition condition. The sequential control design method uses transition conditions to control the programming elements representing each step, allowing their states to change in a certain order, and then uses the programming elements representing each step to control the output. The "transition conditions" of different states can be different or the same. When the "transition conditions" are different, only one working state can be selected at a time in the function chart program (called a "selection branch"). When the "transition conditions" are the same, multiple working states can be selected at a time in the function chart program (called a "selection parallel branch"). Only when the conditional state is met can logical processing and output be performed. Therefore, the "transition condition" is the "switch" for the function chart program to select the working state (step).

(3) Directed links

The connection line between steps is called a "directed line". The "directed line" determines the direction and path of the state transition. There are short lines on the directed line, indicating the transition conditions. When the conditions are met, the transition is realized, that is, the action of the previous step ends and the action of the next step begins, so there will be no action overlap. There must be a transition condition between steps.

The double frame in Figure 1 is the initial step, M0.0 and M0.1 are step names, 10.0 and 10.1 are transition conditions, and Q0.0 and Q0.1 are actions. When M0.0 is valid, the output instruction drives Q0.0. The line between steps is called a directed line, and its arrow is omitted.