Basics of PLC timer- PLC timers are instructions that provide the same functions as on-delay and off-delay mechanical and electronic timing relays. A PLC timer provides a preset delay to the control actions. In general, there are three types of PLC timer delays, ON-delay timer, OFF-delay timer and retentive timer on. The terms represented in the timer block in the PLC are a Preset value which means the delay period of the timer, an Accumulated value which is the current delay of the timer. A timer begins the counting on time-based intervals and continues until the accumulated value equals the preset value. When the accumulated value equals the preset time the output will be energized. Then the timer sets the output. Basic PLC Ladder Programming Training Example- Implement controlling of Traffic Lights in PLC using Ladder Diagram programming language. Problem Solution There are two methods to solve this problem. One is by using stack operation and the other one is by using sequencer output method. Sequencer output method is best suited for this problem since very less configuration is needed and program length is also reduced. In this method, we need to assign SQO instruction by configuring all the parameters given in the instruction. File, Mask, Dest, Control, Length and Positions are parameters which we need to configure. File: It is the starting address for the registers in the sequencer file. Mask: Mask is the bit pattern through which data flow happens from source to the destination address. If there is 1 in the masking, it passes values and if 0, it blocks the data flow. Dest : It is the address of the input to which the Sequencer Output instruction moves the data. Control: Is the address that contains the parameters with control information for the instruction. EN, DN and ER are bit which sets according to the status of sequencer output. EN and DN bits are set just as in timers. ER bit stands for Error bit, it is set when a negative position/length value is detected by the processor, or zero length value. Length: It is the number of steps of the sequencer file starting at position 1. Position 0 is the start-up position. Position: It indicated the steps that is desired to start the sequencer instruction. The start position is all zeros, this is represented as the neutral position; so no outputs will be turned ON in position 0. So to start the actual function of output sequence, Position 1 is determined as starting sequence while programming.
