Copyright Goodheart-Willcox Co., Inc. 100 Programmable Logic Controllers: Hardware and Programming 6.5 PLC Program and Data Files A PLC project consists of the ladder logic diagram file and its associated data files. The project tree area on the left side of the screen in the RSLogix 500 software program displays the files in the PLC project. See Figure 6-20. Notice that the files are organized into two subfolders: Program Files and Data Files. There are always at least three program file icons in the Program Files area. The PLC processor uses the SYS0 program file to keep track of its internal register settings. The SYS1 program file is reserved. Therefore, these two system files are not available to the RSLogix 500 software user. However, PLC programmers have access to at least one program file called the main program file, which holds the main PLC ladder logic diagram. In Allen-Bradley SLC 500 series PLC systems, the main program file is in file two (LAD2). A PLC project may have several subroutine program files in addition to the main program file. Subroutine program files are additional program files that are called on or accessed from the main program file. These files are also listed in the project tree area. Subroutine files and how they are accessed through the main program file will be studied in later chapters. When you select a main program file or a subroutine file in the project tree area, its ladder logic diagram is displayed in the ladder view area. Ladder view area Project tree area Goodheart-Willcox Publisher Figure 6-20. The project tree area contains the program files and data files. The ladder view area displays the ladder diagram from the main program file (LAD2) or subroutine files. In the Data Files area, there are icons for nine data files. The following describes each file: • Input (I)—The file that holds the input bit status of the input modules. There are 12 input ports in module zero of the Allen-Bradley fixed SLC 500 PLC. These inputs can be examined if open (XIO) or examined if closed (XIC) instructions. • Output (O)—The file that holds the output bit status of the output modules. In the Allen-Bradley fixed SLC 500 PLCs, there are eight output ports in module zero. • Status (S2)—The status file holds the flag or status bits of the ladder logic diagram. If a central processing unit fault is generated in the PLC, you should click this icon. Then, find the fault indicator status bit and read the reason that the message was generated. Finally, correct the program to solve the fault condition. Figure 6-21 displays a typical Status dialog box.