PLC learning series 8: Instruction List programming

Basics of Instruction list programming:

Instruction List (IL) is a low level textual language used in PLC system, which has a structure similar to a simple machine assembler.

A instructions list provides programs with each instruction on a new line as a sequence of instructions. Each instruction is made up of an operator followed by one or more operands, that is, the operator’s subjects.


LD A (Load A)

AND B (And B)

ST Q (Store result in Q, i.e. output to Q)

In the first line of the program, LD is the operator, A the operand, and the words at the ends of program lines.

LD A is thus the instruction to load A into the memory register. It can then later be called on for further operations. The next line of the program has the Boolean operation AND performed with A and B. The last line has the result stored in Q, that is,output to Q.

Mnemonic codes for instruction list:

For operators, mnemonic codes are used, each code corresponding to an element operator / ladder. The codes used vary somewhat from manufacturer to manufacturer, although a standard was suggested and commonly accepted under IEC 1131-3.

Operands differ for some users which are listed below:

Rules for understanding IL:

  • There is an instant spell check when entering keywords, separators and remarks.
  • If a keyword, a separator or a comment is detected, it is identified with a color shading
  • If unauthorized keywords (instructions or operators) are entered, color shading will also identify this.
  • There is no syntax impact on spaces and tabs, they can be used anywhere.

PLC Learning series:

PLC learning Series I: What is PLC? Functions of PLC
PLC learning series 2: PLC programming languages
PLC learning series 3: PLC Architecture and wiring of PLC
PLC learning series 5: 8 Rules for ladder diagram programming
PLC learning series 6: PLC process Scan basics
PLC learning series 8: Instruction List programming
PLC learning series 9: PLC selection criteria.
PLC learning Series 10: PLC timers
PLC learning series 11 : How to interface PLC with SCADA?
PLC learning series 12: Troubleshooting PLC
PLC learning Series 13: Counters in PLC


Instrumentation Engineer

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