PLC programming has five types of languages – ladder logic, instruction list, structured text, functional block diagram, and sequential flow chart. Each type of PLC language has its own merits and demerits. While some of the languages look good visually and are easy to troubleshoot, other languages have lower memory consumption and faster processing speed.
One of the most basic PLC languages used is the instruction list. It is not as famous as other languages and is used only by a few PLC programmers due to outdated technology, but still available in almost all the software of PLC manufacturers. In this post, we will see the concept of instruction list language in PLC.
A PLC program written in Instruction List language consists of a series of instructions that are executed sequentially by the logic controller. Each instruction is represented by a single program line and consists of the following components:
Basically, if you have seen traditional assembly language used in microprocessors, then you would easily relate to this language. It can also be termed as a mixture of ladder logic and structured text. Ladder logic in the sense that instructions must be written in a linear way, and structured text in the sense that mnemonics are used in words.
Refer to the below image for more understanding. The first image shows a PLC logic written in ladder language. The logic is – %M3 will turn on if %M0 is on and %M1 is on or %M2 is on.
Now, refer to the below logic for the instruction list. You can see that each line has only one component – the first line has %M0, and the second line has %M1 doing AND logic with the next line.
The third line has %M2 doing OR logic with the previous line, the fourth line closes the commands and the fifth line turns on the output %M3. It is thus a representation of both the ladder logic and structured text.
The main components of a PLC instruction list are mentioned below.
Some of the instructions list of PLC are mentioned below.
Apart from these, it has other instructions too like jump, subroutine, end, AND with, OR with, etc. depending on the PLC manufacturer.
In this way, we saw the concept of the instruction list in PLC programming.
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