Bare Bones Basic Digital Logic

Introduction

Digital logic is the way computers process inputs from a binary system. Binary simply means there are two possible values. The values are determined by the presence of either high voltage or low voltage. In most logic and control circuitry, high voltage can be 3.3V, 5V, 12V, or 24V being common as the high value. Low voltage is 0V to usually but not always less than 2V. The value range is dependent on the part. 

The working value ranges are typically written on the side of the component for industrial components, or a data sheet from the manufacturer may be needed to confirm. The range between high and low state values is an indeterminant state. In this state, there is no way to logically predict the behavior since the system is only designed to work with very specific voltages on the high and low.

If you are getting unexpected behavior, check your voltage. Your parts may be working fine, but your supply voltage could be compromised. Confirm not only the supply voltage but the operating range of the equipment in question that is displaying the odd behavior. This could save you from throwing parts at a problem, before properly diagnosing the underlying issues.

How Computers Talk

The way computers talk is binary. Binary is a number system with only 2 digits. Our day-to-day is decimal which has 10 digits. Binary digits can be expressed as 0 or 1, NO or YES, HIGH or LOW, ON or OFF. For practical purposes when evaluating a circuit on paper, we use 0 and 1. 

Using a lighting circuit as an example can help you visualize a basic logic circuit. When the switch is off, its logic level is 0. Since 0 is no voltage, the light will not turn on. When the switch is turned on its logic state changes to 1, allowing power to flow into the light bulb.

3 Basic Gates

Gates are the building blocks of logic circuits. They each contain a basic function and can be combined to create more advanced gates and functions. For now, we will address the primary 3 which can be used to make any machine perform some type of logical operation.

NOT gates - also called flip flops or inverters, convert a high to a low and vice versa. It's the most basic of all gates and when combined with other gates makes it very versatile. On its own, it acts as a switch. Turning the voltage on and off. Later when combined with other gates its use will become clear

AND gates - these gates take multiple signals, a minimum of 2, and send an output based on the input. All inputs need a high signal for the output to read high. The most common example is safety switches. On bigger machines, all the doors need to be closed before it can start. If all the doors are closed it allows the signal to go through to allow the machine to turn on. If just one is open it stops and/or prevents operation.

OR gates - similar to the and gate, these take at least 2 signals. Their output is based on if only one of the signals is a high value. The most common example of this is a machine with multiple control panels that can start the machine. If you press start on the first panel, second panel, OR any other panel, it will turn on.

In the past, plants used relays instead of electronics to make these same types of logic gates. The problem was they were very large and took up a ton of space. Troubleshooting a panel with hundreds of relays in it must have been a nightmare for techs of the past. Now it's primarily done with electronics, which have a much smaller footprint.

Example and Closing thoughts.

Now I'll address a way in which these can be combined. To start a dishwasher you need doors 1, 2, AND 3 to be closed. The start buttons are currently off so they are NOT active. Pressing one OR more of the start buttons will start the machine at this point. It made sure all doors were closed, and either button was pressed. The signal from the button had to be inverted, but it would not have worked if any door was open. This is just a simple example. These same 3 gates make up all logical operations in machinery and computer chips. In the future, I will address special gates made from these 3 and their uses.

Look around your factory floor, you'll see some products move or stay in place. Sometimes one line is backed up and needs to be cleared out. If that's normal, try and identify at what point it changes and why. Do the same if it's abnormal. It could be something blocking a sensor and thinking a product is somewhere when it is not. 

Understanding how the basic operation of these gates work and identifying how they work together in your system will help you troubleshoot issues and how they are all interconnected. If the machines aren't acting how they are supposed to, it's very likely something is interfering with these operations, causing a false signal. Sometimes it's as simple as some tape is stuck and causing the light from a photosensor to refract in a strange way, causing a mess of signals. Other times they are misaligned. It's your job to figure it out at this point.

Keep these 3 gates in mind in the future and you'll have a deeper understanding of all things electromechanical.