What does the PLC sequential function diagram consist of?

The SFC is a graphical representation method that describes the process, function and characteristics of a sequential control system. It is mainly composed of steps, directed segments, transitions (including transition conditions) and actions. To draw a good SFC, it is essential to understand and master each of its components.

Composition of Sequential Function Chart

1. Step

A working cycle in a sequential control system is divided into several stages, which are called steps. When a step is in an active state, the corresponding action is executed; when a step is in a static state, the corresponding non-storage action is stopped.

2. Directed line segments

A line segment (with direction) connecting steps.

3. Transfer

Transfer is the switching condition when transitioning from one step to another. The switch between two steps can be represented by a directed line segment, and the arrow of the directed line segment representing the downward transfer can be ignored.

The implementation of the transfer must meet two conditions at the same time:

1. All previous steps of this transfer are active steps.

2. The corresponding transfer conditions are met.

4. Action

The action is the task to be performed by this step, which can be output, timing or counting.

Precautions

1. Two steps must never be directly connected, they must be separated by a conversion.

2. Two conversions must never be directly connected, they must be separated by a step.

3. The initial step is essential, otherwise the initial state cannot be represented and the system cannot return to the stopped state.

4. The automatic control system should be able to repeat the same process many times and should form a closed loop, that is, the last step returns to the initial step (single cycle) or the first step of the next cycle (continuous cycle).

5. This step can only become an active step if the previous step is an active step. Generally, when programming elements without power-off retention function are used to represent steps, they are all in the disconnected state when entering the RUN working mode, and the system cannot work. The normally open initialization pulse N M8002 must be used as the conversion condition to preset the initial step as an active step.