1. PLC Concepts Example 5: Correct the circuit of reverse current. The pointed reverse current loops are modified on the right. X0 X1 X2 X0 X1 X2 X3 X4 X5 X3 X4 X5 X10 X6 X7 X10 LOOP1 X6 X7 X5 reverse current X10 LOOP1 Example 6: Correct the circuit of reverse current. The pointed reverse current loops are modified on the right. X0 X1 X2 X3 X4 X5 X0 X1 X2 X3 X4 X5 X6 X7 X10 LOOP1 X6 Reverse current reverse current X3 X7 X10 X0 X1 X2 X6 X3 X4 X0 X1 X7 X10 X4 LOOP1 X5 X6 X7 X10 LOOP2 LOOP 2 1.9 Basic Program Design Examples Example 1 - Stop First latched circuit When X1 (START) = ON and X2 (STOP) = OFF, Y1 will be ON. If X2 is turned on, Y1 will be OFF.
This is a Stop First circuit because STOP button has the control priority than START Y1 X2 Y1 X1 Example 2 - Start First latched circuit When X1 (START) = ON and X2 (STOP) = OFF, Y1 will be ON and latched. If X2 is turned ON, Y1 remains ON. This is a Start First circuit because START button has the control priority than STOP X2 Y1 X1 Y1 Example 3 - Latched circuit of SET and RST The diagram opposite are latched circuits consist of RST and SET instructions. In PLC processing principle, the instruction close to the end of the program determines the final output status of Y1. Therefore, if both X1 and X2 are ON, RST which is lower than SET forms a Stop first Y1 X1 X2 SET Y1 1-13 RST