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Introduction to Logisim: Setup & Overview

Instructor: Martin Gibbs

Martin has 16 years experience in Human Resources Information Systems and has a PhD in Information Technology Management. He is an adjunct professor of computer science and computer programming.

In this lesson, you will learn about Logisim, and how you can use it to design and test digital logic circuits. A working example is provided to show the flexibility and ease-of-use of this free software.

What Is Logisim?

When learning computer architecture and logic circuits, you will need a real-world, graphical example of what you are studying. Text and diagrams only go so far. A helpful tool for designing and simulating logic circuits is Logisim.

Because the tool lets you create large circuits from smaller circuits, you can design entire CPUs using Logisim. Further, the tool will run on any computer!

The interface itself is very intuitive and the use of color-coding of wires and elements allows for easy analysis and testing of circuits. You can also save the completed file as an image, or as a .circ file (core to Logisim).

Download

Logisim is free to download. Although it isn't being updated anymore, it is still heavily used by teachers, students, and professionals. You can download the software here:

http://www.cburch.com/logisim/download.html

Logisim Overview

After you have installed Logisim and launched it, take a moment to familiarize yourself with the interface. All of your circuits will be arranged in the canvas as shown in Figure 1.

The explorer pane contains all of the elements needed to create circuits (gates, plexers, etc.).


Figure 1: Logisim Layout
Logisim Layout


In order to add an element from the explorer pane, you select the item in the pane. Next, you click on the canvas to drop the element onto your design. In Figure 2, we've dropped an AND gate onto the canvas.


Figure 2: Logisim Add AND Gate
Logisim add gate


You'll also notice the attribute pane has now filled up with some options. We can change the direction the gate faces (east, west, north, south), or the number of data bits (the default is 1). As you build circuits, you will be changing the attributes of your elements here.

Changing Values in Circuits

In the toolbar, you'll notice a little hand icon (shown in Figure 3). This icon is used to change values in circuits. This is how you test your circuits.


Figure 3: Logisim Testing Tool
Logisim Testing Tool


To continue moving/adding elements, click the arrow icon in the toolbar. You can click the A icon in the toolbar, then click on the canvas to add the text. You change the actual text and font in the attributes pane.

Common Tools

You'll also notice a set of common tools up in the tool bar as shown in Figure 4.


Figure 4: Logisim Common Tools
Logisim Common Tools


From left to right, there are two pins, a NOT gate, an AND gate, and an OR gate.

Let's Create a Circuit!

We haven't covered all possible options in Logisim, but you should have enough of an overview to start creating circuits. During this course, you will be using Logisim to create more complex circuits. But for this lesson, let's create a simple combinational circuit.

Our circuit will include the following:

  • Three input options, A, B, C
  • An OR gate
  • An AND gate
  • Output (Z)

The inputs C and B go to the OR gate, while A flows through the AND gate.

Our truth table is as follows:

A B C Z
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 1
1 1 0 1
1 1 1 1

Step 1: Add the Inputs

Select the first Pin from the toolbar (the square one). Click on the canvas to add three of them. It's good practice to also label them; click the A icon and add a couple of labels to your canvas.


Figure 5: Add Labels
Logisim Add labels


You can also select an element, such as a gate, and change the label in the attributes pane. This will place the label inside the object.

Step 2: Add the OR gate

Click the OR gate in the toolbar and drop it to the right of the C and B inputs, leaving room for us to add connectors.

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