1 Basic Principles of PLC Ladder Diagram Example 3: Latched circuit for SET and RST instructions See the diagram in the right hand side for the latched circuit Stop first consist of RST and SET instructions. X1 In the stop first diagram, RST is placed after SET. PLC X2 executes the program from up to down, so the On/Off of Y1 will be determined upon its status in the end of the program. Start first X2 Therefore, when X1 and X2 are enabled at the same time, Y1 will be Off. It is the reason why this is called “stop first”. X1 In the start first diagram, SET is placed after RST. When X1 and X2 are enabled at the same time, Y1 will be On. It is the reason why this is called “start first”.
Example 4: Power shutdown latched The auxiliary relay M512 is latched (see instruction sheets for X1 DVP series PLC MPU). The circuit can not only be latched when the power is on, but also keep the continuity of the X2 original control when the power is shut down and switched on Y1 M512 again. Example 5: Conditional control Frequently Used Control Circuit X3 Y1 X1 X1 Y1 X3 X4 Y2 X2 Y1 X2 Y2 X4 Y1 Y2 X1 and X3 enables and disables Y1; X2 and X4 enables and disables Y2, and all are latched. Due to that the normally open contact of Y1 is connected to the circuit of Y2 in series, Y1 becomes an AND condition for Y2. Therefore, only when Y1 is enabled can Y2 be enabled.
DVP-PLC Application Manual 1-18 SET Y1 RST Y1 RST Y1 SET Y1 SET M512 RST M512