345 Part 2 Bit-Level Programming Part Objectives After completing this part, you will be able to: •• Know what happens during the program scan •• Demonstrate an understanding of input, output, and internal relay addressing format for a tag-based Logix controller •• Develop ladder logic programs with input instructions and output coil combinations •• Develop ladder logic programs with latched outputs Program Scan When a CLX controller 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 de-energizes the outputs according to the user program. This process is known as the program scan. Figure 15-26 illustrates the signal flow into and out of a Logix controller during a controller’s operating cycle when ladder logic is executing. During the program scan, the controller reads rungs and branches from left to right and top to bottom as follows: • Only one rung at a time is scanned. • As the program is scanned, the status of inputs are checked for True (1 or ON) or False (0 or OFF) conditions. Input tag data base PLC processor PLC output module Figure 15-26 Logix controller operating cycle. • The status signals from the inputs are sent to the input tags where they are stored. • As the program is scanned by the processor, inputs are checked for True or False conditions and the ladder logic is evaluated based on these values. • The resulting ON or OFF action, as a result of evaluating each rung, is then sent to the output tags for storage. • During the output update portion of the scan, corresponding output values are sent to the process or machine by way of the output module. Ladder logic program Field output devices Field input devices Output tag data base PLC input module Field power supply Field power supply M
Petruzella_ProgrammableLogicControllers__5e
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