Design and programming method of PLC ladder diagram

Ladder diagram is a common programming language for various PLCs. Although the instruction symbols used by PLCs of different manufacturers are not consistent, the design and programming methods of ladder diagrams are basically the same.

When programming with ladder diagrams, you must first determine the programming component number used. PLCs distinguish operating components by number. The address numbers of the internal components of the FX2N PLC we selected are shown in the following table. Be sure to clarify when using it. Each component must not play several roles at the same time. Generally speaking, the number of input points of a configured PLC always corresponds to the number of input signals of the control object, and the number of output points corresponds to the number of output control loops (if there is an analog quantity, the number of analog quantity channels must be equivalent to the actual one). Therefore, the allocation of I/O is actually to assign the input and output point numbers of the PLC to the actual I/O circuits. When programming, establish the logic or control relationship according to the point number, and when wiring, "match the number" according to the point number. The I/O address allocation of the FX2N series and some other memory allocations have been introduced before. Students can also refer to the programming manual of the FX series.

2. Ladder diagram programming rules

1. The coil and contacts of each relay are numbered the same, and there is no limit on the number of contacts of each component that can be used.

2. Each row of the ladder diagram starts from the left, and the coil is connected to the rightmost side (no more contact points are allowed on the right side of the coil).

3. The coil cannot be directly connected to the left busbar.

4. In a program, if the coil with the same number is used twice, it is called double coil output. It can easily cause misoperation and should be avoided as much as possible.

5. There is no real current flow in the ladder diagram. In order to facilitate the analysis of the PLC's periodic scanning principle and logical cause and effect, it is assumed that there is "current" flowing in the ladder diagram. This "current" can only flow in one direction in the ladder diagram - that is, from left to right, and the level can only change from top to bottom.