 The processor scan cycle. Throughout a hardwired ladder relay circuit, the condition of the inputs causes an immediate response by the outputs. In a PLC, the response by the output to the input conditions is not immediate. Instead, a three-step process takes place to perform the same operation as that performed by a hardwired relay logic circuit. This process is called the processor scan cycle. In this learning object, we will use three illustrations to depict the three steps of a processor scan cycle, a loop that represents the cycle, a ladder diagram, and a graphical illustration of the data files. The first step of the scan cycle is to update the input image table by sensing the voltage of the input terminals. Based on the absence or presence of a voltage, a zero or a one is stored into the bit of the memory location designated for a particular input terminal. The illustration shows that because the push button switch is closed, a one is stored in bit five of word two of the input data file. The second step of the scan cycle is for the CPU to scan program instructions located in file two of the program file in the user memory. The process for each instruction involves three operations for each rung in a sequential order. The first operation, determine the status of the bits in the input image table. The second operation, the CPU makes decisions based on the input conditions and the logic function performed by the type of circuit programmed into the PLC. The program for the circuit in this example would read, if a one is present at memory bit five of word two of the input data file, then write a one at bit location two of word four of the output data file. The third operation, according to the logic decisions made by the CPU, the CPU writes a zero or a one in the output data file at the relevant bit location in memory. The processor executes the entire list of instructions rung by rung in ascending rung order. The third step of the scan cycle is to update the output terminals. The CPU takes data from the output data file and sends it out to the field devices through the terminals of the output module. The illustration shows how the lamp connected to terminal two of module four turns on. The signals produced at the output are based on the conditions of step one and step two during that processor scan cycle. If the status of any input devices changes when the processor is in step two or three, the output condition will not react to them until the next processor scan cycle. The three step scanning process is continuous and repeated many times each second. The time it takes to complete one scan cycle depends on the size of the program and the clock speed of the PLC's microprocessor. Let's try a couple of practice problems. Indicate the order to use when performing the processor scan cycle. The correct order is B, update the input image table, C, interpret logic of the program, and A, update output image table. And finally, if the input devices change while the processor is in steps two or three of the scan cycle, the output condition will be react during the next scan cycle. You have completed this activity, the processor scan cycle.