Saturday, March 3, 2012

Program VS Variable Memory of PLC

Advanced functions of ladder logic for instance timers and counters let controllers to carry out computations, make judgments and do other multipart jobs. They are more complicated than fundamental output coils and input contacts and they rely ahead data saved in the PLC memory. The PLC memory is controlled to hold diverse kinds of programs and data. This article will discuss these types of memory. Functions that utilize them will be discussed as following.

The memory in a PLC is separated into variable and program memory. The memory of program includes the instructions to be performed and can not be adjusted while the PLC is working. Some PLCs enable online editing to create minor program amends while a program is working. The memory of variable is amended while the PLC is working. In the memory of ControlLogix is an identified using name of variable, it also called aliases and tags.

The PLC has a Main Jobs list that includes the main program(s) work each PLC scan. Added programs can be made that are named as subroutines. Official program kinds contain Structured Text, Function Block Diagrams, Ladder Logic, and Sequential Function Charts. The files of program can also be made for Controller Faults and Power-Up Handling. The powers up programs are utilized to initialize the controller on the initial scan. Fault programs are utilized to react to particular errors or issues that may direct to the control system failure. Usually these programs are utilized to improve from small failures, or safely shut down a system.

The Allen Bradley PLC utilizes the terminology tags to explain status, variables, and values of input/output for the controller. The Controller Tags comprise I/O definitions and status values. These are reached, meaning that they can be used by all programs or global on the PLC. These can also be limiting their use to a program that owns it or go local. The variable tags can be an stage name for another tags, or be specified a type of data. Several common types of tag are listed as following.
TIMER: memory of Timer
STRING: A string of ASCII
BOOL: Holds values of TRUE or FALSE
SINT: 8 bit 2 tribute integers -128 to 127
CONTROL: common reason memory for multipart instructions
REAL: 32 bit hanging value of point +/-1.1754944e-38 to +/-3.4028237e38
PID: utilized for control functions of PID
COUNTER: memory of counter
MESSAGE: utilized for message with remote devices
INT: 16 bit 2s tribute integers -32,768 to 32,767
DINT: 32 bit 2s tribute integers -2,147,483,648 to 2,147,483,647

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