1 Basic Principles of PLC Ladder Diagram without adopting this principle, some instructions can make the same ladder diagram. Example Program 1 See the ladder diagram below. There are 2 ways to indicate the ladder by instruction programs with the same result. X0 X2 X4 X5 The two instruction programs will be converted into the same ladder diagram. The difference between the ideal one and less ideal one is the operation done by the MPU. For the ideal way, the combination is done block by block whereas the less idea way combines all the blocks combine with one another in the last step.
Though the length of the program codes of the two ways are equal, the combination done in the last step (by ANB instruction, but ANB cannot be used continuously for more than 8 times) will have to store up the previous calculation results in advance. In our case, there are only two blocks combined and the MPU allows such kind of combination. However, once the number of blocks exceeds the range that the MPU allows, problems will occur. Therefore, the best way is to execute the block combination instruction after a block is made, which will also make the logic sequence planned by the programmer more in order. Example Program 2 See the ladder diagram below.
There are 2 ways to indicate the ladder by instruction programs with the same result. X0 X1 X2 X3 In this example, the program codes and the operation memory in the MPU increase in the less ideal way. Therefore, it is better that you edit the program following the defined sequence. Incorrect Ladder Diagram PLC processes the diagram program from up to down and left to right.
Though we can use all kinds of ladder symbols to combine into various ladder diagrams, when we draw a ladder diagram, we will have to start the diagram from the left power line and end it at the right power line (In WPLSoft ladder diagram editing area, the right power line is DVP-PLC Application Manual 1-13 Ideal way Less ideal way LD X0 OR X1 LD X2 OR X3 ANB LD X4 OR X5 ANB LD X0 OR X1 LD X2 OR X3 LD X4 OR X5 ANB ANB 1 3 Ideal way Less ideal way LD X0 OR X1 OR X2 OR X3 LD X0 LD X1 LD X2 LD X3 ORB ORB ORB