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I/1 I/2 I/3 I/4 I/5 I/6 I/7 I/8 I/9 I/10 I/11 I/12 I/13 I/14 I/15 I/16 I/17 I/18 I/19 O/4 O/2 O/3 O/4 O/5 O/6 O/7 VDC O/8 O/9 O/10 O/11 VAC VDC O/0 Figure 5-7  Typical micro PLC with predefined addresses. Source: Image Courtesy of Rockwell Automation, Inc. Internal checks on memory, speed, and operation. Service any communication requests. The output image date is transferred to the external output circuits, turning the output device ON or OFF. 78 Chapter 5  Basics of PLC Programming 5.2  Program Scan When a PLC executes a program, it must know—in real time—when external devices controlling a process are changing. During each operating cycle, the processor reads all the inputs, takes these values, and energizes or deenergizes the outputs according to the user program. This process is known as a program scan cycle. Figure 5-8 illustrates a single PLC operating cycle consisting of the input scan, program scan, output scan, and housekeeping duties. Because the inputs can change at any time, it constantly repeats this cycle as long as the PLC is in the RUN mode. The time it takes to complete a scan cycle is called the scan cycle time and indicates how fast the controller can react to changes in inputs. The time required to make a • If the program calls for a specific output to be ON, its corresponding bit in the table is set to 1. • If the program calls for the output to be OFF, its corresponding bit in the table is set to 0. • The processor continually activates or deactivates the output status according to the output table file status. Typically, micro PLCs have a fixed number of inputs and outputs. Figure 5-7 shows the MicroLogix controller from the Allen-Bradley MicroLogix 1000 family of controllers. The controller has 20 discrete inputs with predefined addresses I/0 through I/19 and 12 discrete outputs with predefined addresses O/0 through O/11. Some units also contain analog inputs and outputs embedded into the base unit or available through add-on modules. Figure 5-8  PLC program scan cycle. The status of external inputs is written to the input image table (file or register). Each ladder rung is scanned and solved using the date in the input file. The resulting logic is written to the output image table (file or register). HOUSEKEEPING START INPUT SCAN PROGRAM SCAN OUTPUT SCAN AC COM I/0 AC COM VAC VDC VAC VDC VAC VDC VAC L2 L2 L1 L1 VAC 2 VDC 1 VDC 2 VAC 2 COM VDC 1 COM VDC 2 COM VDC 3 COM VDC 3 L2 Discrete Inputs Discrete Outputs L1 CR CR CR CR CR CR CR CR CR CR


Petruzella_ProgrammableLogicControllers__5e
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