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Instruction List PLC Programming
People who have experience programming microprocessors or experience with Assembler language programming will see similarities with Instruction List programming. This language contains of many lines of code, with line representing exactly one operation. It is very step by step in format and layout, which make the entry of a series of simple mathematical function easy. In other words, if the programmer uses only instructions that defined by IEC, a program written in this language can be moved easily between platforms of hardware. These advantages to make with this language vary popular in Europe, a fact that is surprising to many US programmers who prefer the ease of maintenance in the graphical languages, and place a lower premium in the program transferability. Instruction List language is a low language level and as such, will execute much faster in the PLC than a graphical language, like Ladder. This language will consume less space in memory and is also much more compact.
The simple one line text key in method supported by this language also allows for very fast program entry, no tab click, no mouse required. Program written in this language are easier to edit and display on a handheld programming unit in legacy systems, with no software or laptop required.
Despite this advantages this language provides to a programmer, it seems that service engineers and maintenance do not prefer Instruction List. Perhaps because it is less visual than Ladder and therefore more difficult to get a sense of what errors it is experiencing and what the program is doing.
It can be struggle to key in complex functions such as PID in Instruction List PLC programming similar to the issues with Ladder Diagram and increasing PLC program complexity. This also applies to complex computation of mathematical. Instruction List does not lend itself well to any structured of form programming, such as step ladder or step programming.