第十四章 PLC内存.ppt

14.1 INTRODUCTION 14.2 MEMORY ADDRESSES 14.3 PROGRAM FILES 14.4 DATA FILES 14.5 SUMMARY 14.6 PRACTICE PROBLEM 14.1 INTRODUCTION Advanced ladder logic functions allow controllers to perform calculations, make decisions and do other complex tasks. Timers and counters are examples of ladder logic functions. They are more complex than basic input contacts and output coils and they rely upon data stored in the memory of the PLC. The memory of the PLC is organized to hold different types of programs and data. 14.2 MEMORY ADDRESSES There are two fundamental types of memory used in Allen-Bradley PLCs - Program and Data memory. Memory is organized into blocks of up to 1000 elements in an array called a file. The Program file holds programs, such as ladder logic. There are eight Data files defined by default, but additional data files can be added if they are needed. These are a collection of up to 1000 slots to store up to 1000 programs. The main program will be stored in program file 2. SFC programs must be in file 1, and file 0 is used for program and password information. All other program files from 3 to 999 canbe used for subroutines. This is where the variable data is stored that the PLC programs operate on. This is quite complicated, so a detailed explanation follows. 14.3 PROGRAM FILES In a PLC-5 the first three program files, from 0 to 2, are defined by default. File 0 contains system information and should not be changed, and file 1 is reserved for SFCs. File 2 is available for user programs and the PLC will run the program in file 2 by default. Other program files can be added from file 3 to 999. Typical reasons for creating otherprograms are for subroutines. When a user creates a ladder logic program with programming software, it is converted to a mnemonic-like form, and then transferred to the PLC, where it is stored in a program file. The contents of the program memory cannot be changed while the PLC is running.