1 Basic Principles of PLC Ladder Diagram Example 6: Interlock control X3 Y1 X1 Y2 Y1 X1 X3 X2 X4 Y2 X2 Y1 X4 Y2 Y1 Y2 Which of the X1 and X2 is first enabled decides either the corresponding output Y1 or Y2 will be enabled first. Either X2 are enabled at the same time, Y1 and Y2 will not be enabled at the same time due to that the ladder diagram Y1 or Y2 will be enabled at a time, i.e. Y1 and Y2 will not be enabled at the same time (the interlock). Even X1 and program is scanned from up to down. In this ladder diagram, Y1 will be enabled first. Example 7:
Sequential control X3 Y1 X1 Y2 If we serially connect the normally closed contact of Y2 Y1 in example 5 to the circuit of Y1 as an AND condition for X4 Y2 X2 Y1 Y1 (as the diagram in the left hand side), the circuit can not only make Y1 as the condition for Y2, but also allow Y2 the stop of Y1 after Y2 is enabled. Therefore, we can make Y1 and Y2 execute exactly the sequential control. Example 8: Oscillating circuit An oscillating circuit with cycle ΔT+ΔT Y1 Y1 Y1 T T The ladder diagram above is a very simple one. When the program starts to scan the normally closed contact Y1, Y1 will be closed because coil Y1 is Off.
When the program then scan to coil Y1 and make it On, the output will be 1. When the program scans to the normally closed contact Y1 again in the next scan cycle, because coil Y1 is On, Y1 will be open and make coil Y1 Off and output 0. The repeated scans will result in coil Y1 outputs oscillating pulses by the cycle ΔT(On)+ΔT(Off). DVP-PLC Application Manual 1-19