Page 110 - Programmable Logic Controllers, Fifth Edition

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 im-
portant to remember that the user program is not an elec-
trical 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) indi-
cate 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 Figure 5-46 RSLogix SLC 500 main window.
updated with the most current status bits. Source: Image Courtesy of Rockwell Automation, Inc.

5.9 Entering the Ladder Diagram L U OSR

Most of today’s PLC programming packages operate in User Bit Timer/Counter Input/Output Compare
the Windows environment. For example, Allen-Bradley’s
RSLogix software packages are Windows programming Figure 5-47 Typical instruction toolbar with bit
packages used to develop ladder logic programs. This soft- instructions selected.
ware, in various versions, can be used to program the SLC
500, ControlLogix, and MicroLogic family of processors. Figure 5-47 shows a typical instruction toolbar with bit
Entering the ladder diagram, or actual programming, instructions selected. To place an instruction on a rung,
is usually accomplished with a computer keyboard or click its icon on the toolbar and simply drag the instruc-
hand-held programming device. Because hardware and tion straight off the toolbar onto the rung of the ladder.
programming techniques vary with each manufacturer, it Drop points are shown on the ladder to help position the
is necessary to refer to the programming manual for a spe- instruction. In addition, instructions can also be dragged
cific PLC to determine how the instructions are entered. from other rungs in the project. There are several different
One method of entering a program is through a hand- methods that you can use to address instructions. You can
held keyboard. Keyboards usually have relay symbol enter an address by manually typing it in or by dragging
and special function keys along with numeric keys for the address from data files or other instructions.
addressing. Some also have alphanumeric keys (letters Some of the windows you will need to use when work-
and numbers) for other special programming functions. ing with RSLogix 500 software include:
In hand-held units, the keyboard is small and the keys • Main Window—This window opens each time you
have multiple functions. Multiple-function keys work like create a new project or open an existing one. Some
second-function keys on calculators. of the features associated with this window include
A personal computer is most often used today as the the following:
programmer. The computer is adapted to the particular - Window Title Bar—The title bar is located at the
PLC model through the use of the relevant programmable topmost strip of the window and displays the name
controller software. of the program as well as that of the opened file.
Figure 5-46 shows the RSLogix SLC 500 main win- - Menu Bar—The menu bar is located below the title
dow. Different screens, toolbars, and dialog boxes are bar. The menu contains key words associated with
used to navigate through the Windows environment. It is menus that are opened by clicking on the key word.
important that you understand the purpose of the various - Windows Toolbar—The Windows toolbar buttons
screens, toolbars, and windows to make the most effective execute standard Windows commands when you
use of the software. This information is available from the click on them.
software reference manual for the particular PLC family - Program/Processor Status Toolbar—This toolbar
and will become more familiar to you as you develop pro- contains four drop-down lists that identify the
grams using the software. current processor operating mode, current online

Basics of PLC Programming Chapter 5 91

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