What are the most useful PLC ladder logic functions and instructions for automation and control?

You should master contacts / coils and their interactions (ladder structure) before moving on to anything else. The reality of most industrial automation processes is that most I/O is digital, thus using these instructions to a great extent.

Programmer like me doing a programm for Process industry we used to call pre programmed blocks but you go inside check the block they used coils. Thats the instruction we used from small machine to high level process Automation.

Timers and counters are the necessary evil of every automation process. Ideally, you'd want to automate based on feedback instead of timers. For example - Fill the tank until the "tank full" sensor is set instead of filling the tank for 12 minutes. However, it's not always possible to avoid the use of times / counters. In most programming languages they're set by a single bit and will increment as follows: A timer will continue to run while the bit is set and while it hasn't reached the preset value. A counter will increment once if it hasn't reached the preset value.

Mathematical instructions are key in most PLC programs - They allow you to manipulate data, scale signals, calculate setpoints, adjust variables, etc. Logic also plays a big role depending on how the program is structured. In some IDEs, the logic instructions can be implemented branches of a rung.

Subroutines, or Emerson’s User Defined Function Blocks (UDFBs), are an excellent tool for organizing an automation application into manageable logic segments. UDFBs may be used to create Application Building Blocks - Libraries of re-usable Functions and Function Block to standardized and accelerate application development. Most programming languages, including Ladder Diagram and Structured Text, have built in instructions or operators to handle looping and branching, without the need for Jumping. From a “software purist” perspective, we are taught that Jumps are poor programming practices and should be avoided. I tend to agree with that perspective and propose that perhaps the logic has not been structured as well as it could be.

Subroutines are an excellent way to stay organized and split a larger program into easy-to-understand chunks. Jumps, on the other hand, are very easy to lose track of as a program scales. They can be used to skip sections of the program, leading to unexpected behavior as the program is modified later.

A friend of mine in the Automation Business often said “It’s the artist not the paint brush”. Having a real understanding of what the system needs to do first then keeping it simple with lots of comments is the mark of a true artist.

After over 30 years of “growing up” on PLC’s I’ve come to the conclusion like any programming languages, It’s an art form with numerous ways to do everything. I started with simple coils, timers, counters and math instructions and graduated doing add on instructions, user defined data types, virtual layouts, object oriented programming of objects that are reused over and over, and writing code that writes code to building giant libraries of my own “objects” of code used over and over. The key takeaway is documentation is key. Chances are you will be trying to figure out what YOU were thinking at the time. So comment everything you write. My “systems” at the end were much better than I began writing. I could expound forever on this topic.