1 Basic Principles of PLC Ladder Diagram An oscillating circuit with cycle nT+ΔT X0 Y1 X0 T0 Y1 Y1 T T n The ladder diagram program controls the On time of coil Y1 by timer T0 and disable timer T0 in the next scan cycle, resulting in the oscillating pulses in the output of Y1. n refers to the decimal set value in the timer and T is the cycle of the clock. Example 9: Flashing circuit X0 * Y1 T n1 * The ladder diagram is an oscillating circuit which makes the indicator flash or enables the buzzer alarms. It uses two timers to control the On/Off time of coil Y1. n1 and n2 refer to the set values in T1 and T2 and T is the cycle of the clock. Example 10:
Trigger circuit M0 X0 X0 Y1 Y1 M0 M0 T M0 Y1 Y1 The rising-edge differential instruction of X0 makes coil M0 generate a single pulse of ΔT (one scan cycle). Coil Y1 will be On during this scan period. In the next scan period, coil M0 will be Off and the normally closed contact M0 and Y1 will all be closed, making coil Y1 continue to be On until another rising-edge arrives in input X0, making coil M0 On for another scan period and Y1 Off. Such kind of circuit relies on an input to make two actions execute interchangeably. Also from the timing diagram on the last page, we can see that input X0 are square pulse signals of the cycle T and coil Y1 output are square pulse signals of the cycle 2T.
DVP-PLC Application Manual 1-20 TMR T0 Kn TMR T1 Kn1 T X TMR T2 Kn2