Rules to be followed in PLC ladder programming - Database & Sql Blog Articles

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Beginner plc ladder programming, should follow certain rules and develop good habits. The following is a brief introduction to the rules to be followed when programming PLC ladder diagrams.

First, the trapezoidal ladder starts from the left busbar and finally the right busbar (usually it can be omitted, only the left busbar is drawn). On the left side of each line is a combination of contacts, indicating the conditions for driving the logic coil, and the logic coil representing the result can only be connected to the busbar on the right. The contact cannot appear on the right side of the coil.

Second, the joints should be drawn on the horizontal line and should not be drawn on the vertical line.

Third, it is not advisable to use a double coil output. If the coil of the same component is used twice or more in the same ladder diagram, it is called double coil output or coil reuse. The double coil output is one of the most common problems that beginners are likely to make. In the case of a double coil output, only the last coil is valid, while the front coil is inactive. This is determined by the scanning characteristics of the PLC.

4. When the parallel blocks are connected in series, the more contacts should be placed on the left side of the ladder diagram (the principle of left and right light); when the series blocks are connected in parallel, the parallel connection of multiple contacts should be placed above the ladder diagram (up and down light) the rules). In doing so, the program is simple, which reduces the scan time of the instructions, which is especially important for some large programs.