The process of identifying and corresponding with the inputs and outputs linked to the PLC is known as PLC I/O addressing. I/O modules, which are physical objects with interfaces to external I/O devices and CPUs, are frequently used to accomplish this.
In PLCs, various I/O addressing techniques are employed, such as:
1. Direct addressing
In a Programmable Logic Controller (PLC) software, direct addressing refers to a technique for explicitly specifying the memory location in the instruction. Without using a symbolic name or variable, the PLC software reads or writes data to a specified memory location via direct addressing.
An address, such as a numeric or hexadecimal integer, is used to specify the memory location in direct addressing. For instance, LD 100 would be the command to load data from memory position 100. The command to write data to memory location 200 would be worded similarly as OUT 200.
When the PLC program wants to quickly access a particular memory region, direct addressing can be used to avoid the requirement to declare and utilize a variable or symbolic name. However, it can make the PLC program less understandable and more difficult to maintain, particularly when the program uses a lot of memory.
2. Indirect addressing
By employing a symbolic name or variable to represent the memory location, indirect addressing is a way for a Programmable Logic Controller (PLC) application to access data memory. Indirect addressing allows a memory location to be indirectly indicated by the use of a symbolic name or variable, which allows a PLC program to read from or write to that location.
In indirect addressing, the symbolic name or variable designates the location in memory where the requested data is stored. For instance, if memory location 100 is defined as the symbolic name “Temp” points to, the instruction to load data from that memory location would be represented as LD Temp. A similar instruction would be expressed as OUT Temp and would instruct the computer to write data to the memory location indicated to by “Temp”.
When a PLC program has to access a dynamic memory location that might vary while the program is running or when there are too many memory locations to address separately, indirect addressing is helpful. By representing memory locations with symbolic names or variables, the PLC program is also made easier to comprehend and maintain.
Usually, sophisticated PLC programs or parts of systems whose memory locations are flexible or expandable employ indirect addressing. Moreover, it is utilized with indexing, which enables a variable to point sequentially to several memory regions.
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3. Symbolic addressing
In a Programmable Logic Controller (PLC) software, symbolic addressing is a way to access data memory by referring to a specific memory location by a symbolic name or label. By referencing a memory place by its name or label rather than by its physical address, the PLC software can read from or write to that location.
A name or label is given to a memory location, such as a variable, timer, or counter, in symbolic addressing. The Rockwell Automation PLC program then reads or writes data to the memory region using the symbolic name or label rather than the physical memory address. For instance, the instruction LD Speed might be used to load data from a memory location with the name “Speed”. OUT Temperature would be used to represent the instruction to write data to the memory location “Temperature.”
By employing names or labels that are descriptive rather than numbers or hexadecimal values to represent memory locations, symbolic addressing makes PLC programs easier to comprehend and maintain. Also, because memory regions may now be referred to by name rather than by physical address, the PLC program can be modified more quickly.
In PLC programs where readability and maintainability are crucial, such as in big or complex programs or in systems that need regular modification, symbolic addressing is frequently employed. The PLC must first look up the physical memory address of the symbolic name or label before it can access the memory location, hence symbolic addressing can be slower than direct addressing.