Data Files The data file portion of the processor’s memory stores input and output status, processor status, the status of various bits, and numerical data.
#A SIMPLE OSR LADDER LOGIC PROGRAM CODE#
Number or code information is represented by groups of bits that are stored in unique byte or word locations. Status is ON/OFF type of information represented by 1s and 0s, stored in unique bit locations. Contents of the data table can be divided into two categories: status data and numbers or codes. This includes information such as the status of input and output devices, timer and counter values, data storage, and so on. The data files store the information needed to carry out the user program. Most instructions require one word of memory. This logic consists of instructions that are programmed in a ladder logic format. It contains the ladder logic that controls the machine operation. The program accounts for most of the total memory of a given PLC system. Program files are the part of the processor memory that stores the user ladder logic program. Individual sections, their order, and the sections’ length will vary and may be fixed or variable, depending on the manufacturer and model. The memory space can be divided into two broad categories: program files and data files. Memory organization takes into account the way a PLC divides the available memory into different sections.
The memory organization for rack-based systems will be covered in this chapter and that for tagbased systems in a later chapter. Allen-Bradley PLCs have two different memory structures identified by the terms rack-based systems and tagbased systems. The memory map or structure for a PLC processor consists of several areas, some of these having specific roles.
Even within the same product line of a single manufacturer, different models may not be directly compatible. While the fundamental concepts of PLC programming are common to all manufacturers, differences in memory organization, I/O addressing, and instruction set mean that PLC programs are never perfectly interchangeable among different makers. You will also learn more about the program scan cycle and the scan time of a PLC. This chapter covers only the basic set of instructions that perform functions similar to relay functions. This chapter shows how these contact/coil functions are used to program a PLC for circuit operation. Similarly, in most cases, the internal PLC symbol for all outputs is a coil. In PLCs, the internal symbol for any input is a contact. Image Used with Permission of Rockwell Automation, Inc.Ĭhapter Objectives After completing this chapter, you will be able to: 5.1 5.2 5.3 5.4ĭefine and identify the functions of a PLC memory map Describe input and output image table files and types of data files Describe the PLC program scan sequence Understand how ladder diagram language, Boolean language, and function chart programming language are used to communicate information to the PLC Define and identify the function of internal relay instructions Identify the common operating modes found in PLCs Write and enter ladder logic programsĮach input and output PLC module terminal is identified by a unique address.