Normally open (NO) contacts or inputs in the PLC ladder logic program do not mean the same as a wired normally open switch. NO contacts on the ladder logic provide the logic condition for the rung to turn on the output. We will be looking at three things about the normally open (NO) contact on our ladder logic program. The normally closed NC ladder logic contact will also be discussed at the end. Let’s get started.
All programmable logic controllers (PLC) have the same numbering system at their core. This is the same with all computers as well. The status of any memory location can be either on or off. A one (1) will represent an on or “True” status and zero (0) will represent an off or “False” status. This is called binary.
Understanding PLC Numbering Systems
We will be looking at how numbering systems work and some of the common systems that programmable logic controllers use. Let’s get started.
PLC inputs are one component of our PLC block diagram. The output actions of the PLC will be controlled based on the inputs. We will be looking at digital and analog inputs that can be wired to the programmable logic controller.
We will be looking at wiring of a normally open (NO) push button, normally closed (NC) push button, 3 wire PNP sensor, and an analog sensor to the PLC. These will all be sinking inputs. Let’s get started.
I have been writing PLC programs for over 20 years. I often get asked what the best way to learn PLC programming is. Programming in the way I was taught in college was with the Motorola 6809. (Yes, I know that I am dating myself) This was microprocessor programming, but it was the best way to explain the methods behind PLC programming. Manufacturers of PLCs had proprietary software unrelated to their appearance and programming methods. Today we have a few standards that have changed the look and feel of the programming software packages, so each manufacturer is similar. The following is the best recommendation for beginners to learn PLC programming today.
The first place to start learning PLC programming is a free publication by Kevin Collins. This PDF will teach you PLC programming without telling you what a PLC is and how it functions. He also includes some test questions along the way for you to retain and understand the essential points that he is making.
PLC Programming for Industrial Automation
by Kevin Collins (Note: This book is now for sale on Amazon.)
After learning the basics from the above manual, practice, create programs yourself, and test what you have learned. You can accomplish this by using simulators. A lot of the programming software will have simulators. The simulator will mimic the PLC hardware so you can test your programs before installing them in the field. Traditionally I have not been a fan of simulators, but recently Automation Direct has introduced a simulator with their Do-More PLC. It is the Do-More Designer Software. This software simulator includes the instruction set (Not Just a Bit of Logic) and communication protocols. It can be downloaded and installed for free from the above link.
Advanced PLC Programming Instructions
The next step I recommend is then to advance into some of the advanced instructions. Math, PID, register manipulation, and conversion instructions are just a few of the advanced programming you can learn. An understanding of the numbering systems in the PLC will be a benefit. This and more instruction information can be obtained from reviewing the documentation from the PLC manual you are programming. Once again, all of these instructions are included in the Do-More Designer Software.
PLC Programming Structure
The program structure is the next topic. Many programmers would stop here and can do well with developing software; however, there is much more you can learn. Sequencers give programmers the methods to change the logic on the fly and allow troubleshooting the system more accessible. This programming methodcan significantly benefit you and reduce your logic’s development time.
PLC Programming – Information Sharing
The last step that I recommend learning is the sharing of information. I mean the information that you program through an HMI and SCADA package. This refers to the understanding of how data can be gathered from the PLC and displayed in different ways. Here are a couple of previous articles that have been written on this subject:
As you can see, there is a lot of information available to you to begin and learn PLC programming without spending a dime! Remember that PLCs are similar to computers (Moore’s Law). They increase in size and ability. Systems are expanding and changing every day. Happy programming.
Do you know of additional tips or methods to share? PLC Beginner’s Guide – There are many different PLC manufacturers with other hardware and software. All of the programmable logic controllers have similar basic features. Here is how I would approach learning about basic PLCs.
If you have any questions or need further information, please contact me.
If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLCs are not challenging to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, decimals, Hexadecimal, ASCII, and Floating points.
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The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.