The Basics of Ladder Logic

What is Ladder Logic?

Ladder logic stems from the history of relays. At one time, relays were the primary control for most automatic systems. These electromechanical devices consisted of coils and contacts that they moved. Energized coils moved their contacts from their resting position to their active position (either closed to open or open to closed.)

In practical application, these relays were wired together using a ladder diagram, which when viewed looks similar to the ladder we are all familiar with. The conventions of these instructions have been passed down to modern-day ladder logic. Relays did (and still do) their job well, but can be cumbersome due to the sheer size of multiple relays wired together. This is where programmable logic controllers using ladder logic can be advantageous: able to do the same kind of job but in significantly less space.

Like these early ladder diagrams, ladder logic still places power information at the top of the drawing and has the power (or hot) rail going down its left side. Neutral rail information is placed on the right side. Additionally, contacts are typically placed on the left side of the drawing, and outputs are placed on the right. Each output is given a separate rung on the ladder, represented by a horizontal line.

Is Ladder Logic Still Used? 

Yes. According to one 2016 study by Technavio, ladder logic continues as the dominant language for PLC programming--accounting for more than 81% of the global market--although some PLCs use instruction list programming, function blocks, structured text, or sequential function blocks. However, ladder logic remains popular and common.

Global market shares of PLC programming languages, 2015.  Source: Technavio

One caveat for this may be the slow transition from PLCs to PACs within industrial settings. PLCs (programmable logic controllers) have been around for decades. Newer PACs, or programmable automation controllers, offer the same essential functions and are used for generally the same things, but are typically programmed using C or C++. However, the jury is still out on whether PACs will be fully embraced by the manufacturing community as a replacement for reliable PLCs.

How to Read Ladder Logic Diagrams

Ladder logic is typically read as you would text in a book: left to right and top to bottom. Power in a ladder logic diagram is always flowing left to right, and instructions always descend from the top of the ladder to the bottom. Originally based upon electrical diagrams of relay logic, it's easy to see why these diagrams were called ladders since they so closely resemble an upright ladder with two vertical rails and several rungs between.

Image by Hugh Jack at English Wikibooks Used Under CC by-SA 3.0

Image by Hugh Jack at English Wikibooks, Used under CC By-SA 3.0. 

What Can Ladder Logic Actually Do? 

Ladder logic is most often used in the development of software for industrial programmable logic controllers (PLCs). It can be used as the central architecture that connects to function blocks that hold more complex instructions. This type of structure allows for ease of programming while instructions become more complex. By using ladder logic, PLCs can:

  • Have counting functions (count down/count up) to decrease or increase values on each transition of the input
  • Make comparisons to determine if values are greater than, less than, or equal
  • Have timing instructions for on- or off-delayed events.
  • Include special functions like PID loops, shift registers, communication instructions, ramp generators, etc.
  • Use math instructions for everything from addition and subtraction to finding square roots and changing the sign of a piece of data.
  • Use Boolean expressions like "AND", "NOT", and "OR."

How Do I Learn Ladder Logic? 

Nowadays there are literally thousands of free resources on the web for learning ladder logic. Start by checking out videos on YouTube or on PLC supplier sites (Siemens, Allen Bradley, Omron) for training courses. You can also try this search string in Google:

site:edu "ladder logic" mooc

which should result in an additional resource list of previous Massive Online Open Courses on ladder logic. Many of these are now closed but may still have class notes, videos, or other resources available.


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