The input scan normally records the inputs before the program scan, and the output scan normally updates the outputs after the program scan, as shown in Figure 15.1 Input, Program and Output Scan. Immediate input and output instructions can be used to update some of the inputs or outputs during the program scan.
Figure 15.1 Input, Program and Output Scan
Figure 15.1 Immediate Inputs and Outputs shows a segment within a program that will update the input word input_value, determine a new value for output_value.1, and update the output word output_value immediately. The process can be repeated many times during the program scan allowing faster than normal response times. These instructions are less useful on newer PLCs with networked hardware and software, so Allen Bradley does not support IIN for newer PLCs such as ControlLogix, even though the IOT is supported.
