Ladder diagram is a conventional form of programming the programmable logic controllers (PLCs). Ladder logic was originally developed as replacement for electrical relays in control circuits and motor starter circuits. PLCs were used to implement control circuits similar to wired-control circuits with electrical relays. Over time, many advanced control functions capabilities have been incorporated in the PLCs. Ladder diagrams are best suited for discrete control and interlocking.
Ladder , better known as ladder logic, is a programming language used to program PLCs (programmable logic controllers). This article will briefly describe what ladder logic is and go over some examples of how it functions.
It uses long rungs laid out between two vertical bars representing system power. Along the rungs are contacts and coils, modeled after the contacts and coils found on mechanical relays. The contacts act as inputs and often represent switches or push-buttons; the coils behave as outputs such as a light or a motor.
Outputs don't have to be physical, though, and can represent a single bit in the PLC's memory. This bit can then be used later on in the code as another input. Contacts are placed in series to represent AND logic and in parallel when using OR logic. As with real relays, there are normally open contacts and normally closed contacts.
The LAD Programming Package is an integral part of the STEP 7 Standard Software. This means that following the installation of your STEP 7 software, all the editor functions, compiler functions, and test/debug functions for LAD are available to you.
Using LAD, you can create your own user program with the Incremental Editor. The input of the local block data structure is made easier with the help of table editors.
There are three programming languages in the standard software, STL, FBD, and LAD. You can switch from one language to the other almost without restriction and choose the most suitable language for the particular block you are programming.
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