1 Basic Principles of PLC Ladder Diagram cyclic process of sending the results to the output interface re-reading of the input status calculation output. The time spent in the cyclic process is called the “scan time” and the time can be longer with the expansion of the program. The scan time can cause delay from the input detection to output response of the PLC. The longer the delay, the bigger the error is to the control. The control may even be out of control. In this case, you have to choose a PLC with faster scan speed. Therefore, the scan speed is an important specification requirement in a PLC.
Owing to the advancement in microcomputer and ASIC (IC for special purpose), there has been great improvement in the scan speed of PLC nowadays. See the figure below for the scan of the PLC ladder diagram program. Read input status from outside X0 X1 Y0 Start The output result is calculated based on the ladder diagram. Y0 (The result has not yet sent to the M100 X3 X10 Y1 Executing in cycles external output point, but the internal device will perform an : : immediate output.) X100 M505 Y126 End Send the result to the output point Besides the difference in the scan time, PLC ladder and traditional ladder diagram also differ in “reverse current”.
For example, in the traditional ladder diagram illustrated below, when X0, X1, X4 and X6 are On and others are Off, Y0 output on the circuit will be On as the dotted line goes. However, the PLC ladder diagram program is scanned from up to down and left to right. Under the same input circumstances, the PLC ladder diagram editing tool WPLSoft will be able to detect the errors occurring in the ladder diagram. Reverse current of traditional ladder diagram Reverse current of PLC ladder diagram X0 X1 X2 Y0 X0 X1 X2 Y0 Y0 X3 X4 X5 a b X6 Error detected in the third row 1.3 How to Edit Ladder Diagram Ladder diagram is a diagram language frequently applied in automation.
The ladder diagram is composed of the symbols of electric control circuit. The completion of the ladder diagram by the ladder diagram editor is the completion DVP-PLC Application Manual 1-3 X3 X4 a X5 b X6