PLC I/O addressing is the process of locating and correlating with the inputs and outputs connected to the PLC. This is commonly done via I/O modules, which are actual physical objects having interfaces to other CPUs and I/O devices.
In PLCs, various I/O addressing techniques are employed such as:
1. Direct addressing
Direct addressing is a method for explicitly stating the memory location in the instruction in Programmable Logic Controller (PLC) software. The PLC program reads or writes data to a specific memory area using direct addressing without requiring a symbolic name or variable.
Direct addressing uses an address, such as a numeric or hexadecimal integer, to identify the memory location. For instance, the instruction to load data from memory location 100 would be LD 100. Similar instructions would be sent as OUT 200 to write data to memory address 200.
Direct addressing allows the PLC program to easily access a specific memory location without the need to define and use a variable or symbolic name. However, especially if the program utilise a lot of memory, it might make the PLC program harder to comprehend and maintain.
2. Indirect addressing
A Programmable Logic Controller (PLC) program can access data memory by using indirect addressing, which uses a symbolic name or variable to represent the memory location. By using a symbolic name or variable to imply a memory location, indirect addressing enables a PLC program to read from or write to that place.
The symbolic name or variable specifies the place in memory where the requested data is stored via indirect addressing. For instance, the instruction to load data from memory position 100 would be represented as LD Temp if memory location 100 is specified as the symbolic name “Temp” points to. Similar instructions might be written as OUT Temp, which would tell the computer to write data to the “Temp”-designated region in memory.
Indirect addressing is useful when a PLC programme has to access a dynamic memory location that could change while the programme is running or when there are too many memory locations to address individually. The PLC programme is also made simpler to understand and maintain by symbolically naming or defining memory locations in place of literal names or variables.
Indirect addressing is typically used in complex PLC program or in areas of systems with expandable or flexible memory locations. Additionally, it is used in conjunction with indexing, allowing a variable to point sequentially to several memory areas.
3. Symbolic addressing
Symbolic addressing is a technique used in Programmable Logic Controller (PLC) software to access data memory by designating a particular memory region by a symbolic name or label. The PLC programme can read from or write to a memory region by referring it by its name or label instead of by its physical address.
In symbolic addressing, a memory location, such as a variable, timer, or counter, is given a name or label. The symbolic name or label, rather than the actual memory location, is then used by the PLC programme to read or write data to the memory region. For instance, data from a memory address named “Speed” may be loaded with the instruction LD Speed. The command to write data to the memory address “Temperature” would be represented as OUT Temperature.
Symbolic addressing makes PLC programmes simpler to understand and maintain by using descriptive names or labels rather than numbers or hexadecimal values to represent memory locations. Additionally, the PLC programme may be changed more quickly since memory regions can now be referred to by name rather than by physical address.
Symbolic addressing is widely used in PLC programmes when readability and maintainability are essential, such as in large or complicated programmes or in systems that need frequent change. Symbolic addressing can be slower than direct addressing because the PLC must first search up the physical memory address of the symbolic name or label before it can access the memory location.
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