PLC Programming | How to Read Ladder Logic

Community How-to-Tutorials
#1

Ladder Logic is one of the top 5 most popular types of PLC programming languages used in manufacturing environments. Before Programmable Logic Controllers, manufacturing plants employed relay-based circuitry to energize different loads based on how the relays were wired together. Relays were costly, required constant maintenance, and could not be easily reconfigured. As PLCs took over this process, it was essential to keep a similarity of the old system; thus, ladder logic was created as the first PLC programming language.

This is a companion discussion topic for the original entry at https://www.solisplc.com/tutorials/how-to-read-ladder-logic
#2

Hi Karim, Great explanation about the ladder logic. I have one question about reading the ladder diagram which has ONS instruction in it, Can you please help me how to read the PLC scan cycle if only One Bit Rise in one PLC cycle.
Screenshot 2022-03-30 003359

#3

When you push I:0.0/0, the B3:0/1 will turn on one-scan (one shot) of PLC scanning. During that scan, the O:0.0/0 will turn on. During next scan of PLC, at first B3:0/1 will turn off. It does not matter how long you hold the button. then when PLC scan the second rung, the lower part of rung will run through power while (O:0.0/0 was ON last scan). It will hold the output.

By the way, normally we used Normally Closed Contact for a physical stop button. Therefore, the stop button in the logic should be normally open contact (XIO).