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Figure 5-46  RSLogix SLC 500 main window. Source: Image Courtesy of Rockwell Automation, Inc. OSR L U User Bit Timer/Counter Input/Output Compare Figure 5-47  Typical instruction toolbar with bit instructions selected. Figure 5-47 shows a typical instruction toolbar with bit instructions selected. To place an instruction on a rung, click its icon on the toolbar and simply drag the instruction straight off the toolbar onto the rung of the ladder. Drop points are shown on the ladder to help position the instruction. In addition, instructions can also be dragged from other rungs in the project. There are several different methods that you can use to address instructions. You can enter an address by manually typing it in or by dragging the address from data files or other instructions. Some of the windows you will need to use when working with RSLogix 500 software include: • Main Window—This window opens each time you create a new project or open an existing one. Some of the features associated with this window include the following: -- Window Title Bar—The title bar is located at the topmost strip of the window and displays the name of the program as well as that of the opened file. -- Menu Bar—The menu bar is located below the title bar. The menu contains key words associated with menus that are opened by clicking on the key word. -- Windows Toolbar—The Windows toolbar buttons execute standard Windows commands when you click on them. -- Program/Processor Status Toolbar—This toolbar contains four drop-down lists that identify the current processor operating mode, current online the rung must be true when the external pushbutton is open and false when the pushbutton is closed. Using an Examine If Open instruction to represent the pushbutton satisfies these requirements. The NO or NC mechanical action of the pushbutton is not a consideration. It is important to remember that the user program is not an electrical circuit but a logic circuit. In effect, we are interested in logic continuity when establishing an output. Figure  5-45 shows a simple program using both the XIC and XIO instructions. The logic states (0 or 1) indicate whether an instruction is true or false and is the basis of controller operation. The figure summarizes the on/off state of the output as determined by the changing states of the inputs in the rung. The time aspect relates to the repeated scans of the program, wherein the input table is updated with the most current status bits. 5.9  Entering the Ladder Diagram Most of today’s PLC programming packages operate in the Windows environment. For example, Allen-Bradley’s RSLogix software packages are Windows programming packages used to develop ladder logic programs. This software, in various versions, can be used to program the SLC 500, ControlLogix, and MicroLogic family of processors. Entering the ladder diagram, or actual programming, is usually accomplished with a computer keyboard or hand-held programming device. Because hardware and programming techniques vary with each manufacturer, it is necessary to refer to the programming manual for a specific PLC to determine how the instructions are entered. One method of entering a program is through a handheld keyboard. Keyboards usually have relay symbol and special function keys along with numeric keys for addressing. Some also have alphanumeric keys (letters and numbers) for other special programming functions. In hand-held units, the keyboard is small and the keys have multiple functions. Multiple-function keys work like second function keys on calculators. A personal computer is most often used today as the programmer. The computer is adapted to the particular PLC model through the use of the relevant programmable controller software. Figure  5-46 shows the RSLogix SLC 500 main window. Different screens, toolbars, and dialog boxes are used to navigate through the Windows environment. It is important that you understand the purpose of the various screens, toolbars, and windows to make the most effective use of the software. This information is available from the software reference manual for the particular PLC family and will become more familiar to you as you develop programs using the software. Basics of PLC Programming  Chapter 5 91


Petruzella_ProgrammableLogicControllers__5e
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