Introduction to Ladder Logic With Codesys 3.5

Updated on July 17, 2018
Example Of A Ladder Logic Rung/Network
Example Of A Ladder Logic Rung/Network

About Ladder Logic

Ladder Logic, usually referred to as simply "Ladder," is the most common language used in programming a PLC (Programmable Logic Controller). It has been in use for decades, and the fundamentals haven't changed all that much over time.

The reason it is so popular is because it reads like a schematic, from left to right, top to bottom. It was and still is, relatively easy for electrical engineers to get a copy of the software and read the logic.

In this introduction article, we will be looking at Ladder using CoDeSys 3.5 (which is free, you can find a download link at the CoDeSys 3.5 download center). If you need support with CoDeSys 3.5, there is plenty of help online. I may even write an article or two on getting started with CoDeSys in the future, let me know in the comments what's missing, and I'll update the article

Programming Environment

Every editor is different (Allen Bradley, Mitsubishi, CoDeSys, Siemens for example); however, they all follow the same basic guidelines.

The Ladder environment is made up of Networks (or Rungs) and logic elements that sit on the network from left to right. The logic "flows" from left to right until the rung is complete and then moves downward to the next rung.

Consider this example:

The first network consists of a contact, followed by a negated contact and finally a Coil (or Flag/Output - Not to be confused with a physical output)

The second network consists of a contact and another coil.

In Ladder, the name of the contact, coil or other elements is the assigned variable or "Tag" that is being referenced. So, "Coil" appears twice, once as a Coil and once as a Contact. If a Coil is set to True, a contact with the same name will also be true.

In order for a Coil to be True, everything to the left of the coil must also be in a True state. Think of this as an electrical circuit, where your V-Supply is the vertical rail on the left and on the right is your ground, a coil is a device that requires power and contacts are relays. Some environments actually show a right side rail too.

! IMPORTANT !

Remember that the logic flow is across and then down. If a coil on network 5 is set to True, and a contact with the same name is on network 3 and 7, only 7 will be True until the program loops around to the top again.

Flow Example

The above image shows the "logic flow" when the PLC is in run mode. Contacts / Coils that are blue are TRUE, if they are not blue, they are FALSE.

(All variables are set to false in the above image, note that the negated contact is blue)

To set a coil to TRUE (blue), all contacts and conditions to the left of the coil must be TRUE also, allowing the blue path to reach the coil:

In the above logic, "Contact" (the first element on network 1) was set to TRUE. Because "Negated Contact" is a negated contact, FALSE = TRUE. As both conditions are satisfied to the left of "Coil," the Coil is now set to TRUE.

The second network's contact "Coil" is now true. As all conditions to the left of Coil_2 are now TRUE, Coil_2 is set to TRUE.

Function Blocks & Functions

Ladder Logic makes use of Function Blocks and Functions extensively, for the purpose of this introduction we will assume that all Function Blocks & Functions are built using Ladder.

So what's the difference between a Function and a Function Block? It comes down to "Instances." A Function is simply a function, inputs are passed, and an output is given, nothing is held in memory. A Function Block, however, is defined in the program's logic, it's given an instance, and memory is allocated for that block.

Consider you want to do the following mathematical calculation:

X + Y = Z

A Function would provide the means to pass value X and Y and return value Z.

Suppose you wanted to do the following mathematical calculation:

X + Y + Z = Z

A Function Block would allow Z to be defined as an Output, but because the block is given an instance, the value of Z is stored inside the block, so it does NOT need to be passed in again to be used in the calculation. A function block is given an instance by naming it in the same way that a contact or coil is named:

In the above example, a Function Block has been placed on the network. The Function Block has been defined as a TON (Timer On Delay) and has been given the name Timer_1. This has now been allocated memory, Timer_1 is an instance of TON. This allows Timer_1 to use the Functions that TON provide, while keeping track of Timer_1 in its own instance, irrespective of any other TON function blocks.

This is just a brief introduction to function blocks, they are very powerful and need further explanation to understand their full use (look out for an article dedicated to them!)

Conclusion

Ladder Logic is simple, clean and an effective language to program simple and complex control systems with. It is also easy to read, print and follow. It's downfall, in my opinion, is that its very tempting to simply keep adding to the end of a POU or ladder file, creating long and difficult to follow code.

The best use of ladder is for straight logic, basic mapping and small function blocks.

© 2018 Liam

Comments

    0 of 8192 characters used
    Post Comment

    No comments yet.

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, turbofuture.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://turbofuture.com/privacy-policy#gdpr

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)