70+ PLC Programming Instructions | Classification and Types

Ever want to learn Programmable Logic Controller (PLC) programming? especially, ladder diagram programming.

Along with basic digital electronic logic gates concept, programming rules and you have to learn programming instructions.

In the earlier tutorial, I have already explained the different types of logic gates and programming rules for writing PLC programming.

If you have not gone through it, you can check here.

• Logic Gates using PLC Programming
• 6 Rules for PLC Ladder Diagram Programming

Now in this article, I am listing the topmost useful PLC programming instructions. These instructions are useful for ladder diagram programming language.

[Explained with Ladder Diagram(LD)]

Let’s dive in from the basics.

What is the Instruction?

Instruction is nothing but the command that we give to the machine to perform certain tasks.

We can club multiple instructions to perform one logical operation. The sets of multiple operations are called as ‘Program‘.

In PLC, the different instructions are used to control and maintain the operations. Every instruction is having different specifications like – working principle, function, use, advantage, etc.

According to the PLC program or project requirement, different instructions are needed.

According to different PLC software brands, there are different instructions sets.

Every instruction has input and output.

Input/Output Basics:

In PLC programming, inputs and outputs are very basic terms.

• Input is shown by two parallel lines- Normally Open (NO) or Normally Closed (NC)
• Output is shown by Parentheses or round brackets.

This is easily understood by the below diagrams.

As shown in given LD program, NO & NC input are connected with the output.

PLC Programming Instructions Types and Classifications

What are different instructions of PLC?

Let’s see here, the different types of instructions set which are used for the ladder diagram PLC programming.

1. Basic PLC Programming Instructions

1. Input (I or X) Instruction
2. Output (O or Q ) Instruction
3. Set (S) and Reset (R) Instruction
4. Latch (L) and Unlatch (U) Instruction

The Set/Reset and Latch/Unlatch instructions are similar to each other.

The only difference is- Latch/Unlatch instructions are used for the cyclic process. I have already explained in the PLC programming rules tutorial.

2. Time-Based PLC Programming Instructions

The timer instructions follow task or command for/at a particular duration of time.

Sometimes, timers are used for the ON/OFF condition for the PLC.

Here are different timer instructions that fall into this category.

5. Single Pulse (Monoflop) Timer Instruction
6. Flashing Timer Instruction
7. On Delay Timer (TON) Instruction
8. Off Delay Timer (TOFF) Instruction
9. Retentive Timer (RTO) Instruction
10. Pulse Timer (S-Pulse) Instruction
11. Pulse Extended Timer (S-PEXT) Instruction
12. On Delay Timer (S-ODT) Instruction
13. Off Delay Timer (S-OFFDT) Instruction
14. Extended On Delay Timer (S-ODTS) Instruction
15. On Delay with Random Time Timer Instruction

In an earlier tutorial, I have already explained the types of timer instructions with an example.

Which PLC brands use which Timer Instructions?

• On Delay timer, Single Pulse (Monoflop) timer, On Delay with Random Time timer and Flashing timer are used in the ABB PLC.
• On Delay timer (TON),  Off Delay timer (TOFF) and Retentive timer (RTO) are used in the AB PLC.
• Pulse timer (S-Pulse), Pulse Extended timer (S-PEXT), On Delay timer (S-ODT), Extended On Delay timer (S-ODTS) and Off Delay timer (S-OFFDT) in the Siemens PLC.

2. Counting-Based PLC Programming Instruction

Counter instructions are used for counting pulse in the PLC program. You can count the pulse or digit by increasing or decreasing.

The classification of different Counter instructions are-

16. Up Counter (CTU) Instruction
17. Down Counter (CTD) Instruction
18. Up-Down Counter Instruction

These three different counters are widely used in AB PLC and Siemens PLC programming.

3. Comparison Based PLC Programming Instruction

These instructions are used to compare inputs.

Here are some of the basic instructions used for comparison.

19. Greater than (GET) Instruction
20. Lesser than (LET) Instruction
21. Equal to (EQU) Instruction
22. Not Equal to (NEQ) Instruction
23. Greater than or equal to (GEQ) Instruction
24. Lesser than or equal to (LEQ) Instruction
25. Increment and Decrement Instruction
26. Limit (LIM) Instruction

Most of the instructions are self-explanatory. And you can easily understand their purpose.

4. Mathematical PLC Programming Instruction

These PLC instructions are used to perform different mathematical operations like arithmetic, trigonometric and logarithmic operations.

27. Addition (ADD) Instruction
28. Subtraction (SUB) Instruction
29. Multiplication (MUL) Instruction
30. Division (DIV) Instruction
31. Square Root (SQRT) Instruction
32. Absolute (ABS) Instruction
33. Sine (SIN) Instruction
34. Cosine (COS) Instruction
35. Tangent (TAN) Instruction
36. Arc Sine (ASN) Instruction
37. Arc Cosine (ACS) Instruction
38. Arc Tangent (ATN) Instruction
39. Natural Log (LN) Instruction
40. Log to Base 10 (LOG) Instruction

Note: These mathematical instructions, comparative instructions, and counter instructions are used for the counting pulse as per project requirement.

5. Data Transfer Based PLC Programming Instruction

On the basis of programming data transfer, multiple instructions are applicable for multiple purposes.

Here, I am shortlisting the topmost instruction used for data transfer.

41. Fill File (FLL) Instruction
42. Move (MOV) Instruction
43. Copy (COP) Instruction
44. Jump (JMP) Instruction
45. Jump Not (JMPN) Instruction
46. Conditional Jump Instruction
47. Jump to Subroutine (JSR) Instruction
48. Subroutine (SBR) Instruction
49. Return (RST) Instruction
50. Suspend (SUS) Instruction
51. Lable (LBL) Instruction
52. Jump and Lable (JMP & LBL) Instruction
53. Master Control Set (MCS) instruction
54. Master Control Reset (MCR) instruction
55. One-Shot Rising (OSR) Instruction
56. One-Shot Falling (OSF) Instruction
57. Convert from Integer to BCD (TOD) Instruction
58. Convert from BCD to Integer (FRD) Instruction
59. Temporary End (TND) Instruction
60. Convert Radians to Degrees (DEG) Instruction
61. Convert Degrees to Radians (RAD) Instruction

7. Logical or Bitwise PLC Programming Instruction

Many times, we need to perform logical operations on input data. The logical instruction basically depends on the logic gate concept.

62. Bitwise NOT (NOT) Instruction
63. Bitwise AND (AND) Instruction
64. Bitwise OR (OR) Instruction
65. Bitwise Exclusive OR (XOR) Instruction
66. Flip-Flop (RS or SR Flip-Flop) Instruction
67. Positive H Trigger  (P) Instruction
68. Negative H Trigger (N) Instruction

To understand the logical operations, check logic gates using PLC programming.

8. Sequence-Based PLC Programming Instruction

To implement certain logic, we need to follow a particular sequencing. Here are some of the sequencing operations you can perform using PLC instructions.

69. Sequencer Input (SQI)
70. Sequencer Output (SQO)
71. Bit Shift Left (BSL)
72. Bit Shift Right (BSR)
73. Sequencer load (SQL)
74. Sequencer compare (SQC)

What’s Next?

I have listed the most common, important and widely used instructions for the ladder diagram PLC programming.

For the industrial environment, AB PLC and Siemens PLC software are used. One of the advantages of using these PLC softwares is that they have a lot of instructions set. This makes your job of writing PLC programming logic easy.

I have listed different PLC instructions. If you want to know detail about any instruction, let me know in the comment.

Ready for PLC Test:

If you are ready for online test, click on is a PLC Automation Quiz.

Happy PLC Learning!

Dipali Chaudhari

I have completed master in Electrical Power System. I work and write technical tutorials on the PLC, MATLAB programming, and Electrical on DipsLab.com portal.

Sharing my knowledge on this blog makes me happy.  And sometimes I delve in Python programming.