Description

BUILDING CIRCUITS
In Lab 1, you design and build combinational circuits of logic functions. Before the lab, you prepare by drawing schematics of logic functions using Logisim Evolution. During the lab, you build the circuits using chips and wires. Although circuits are no longer built this way in industry, it is useful to show how discrete gates are connected together to form a logic function.
This document describes what you need to prepare and demonstrate for Lab 1. Section 1.3 introduces the digital lab space and describes the different physical parts and tools you need for your lab demonstration. Section 1.4 describes the tasks you must complete before your lab session. Section 1.5 describes the tasks you complete during your lab session. Pay attention as the next section describes lab logistics in more detail.
1.1 Logistics
You must upload every required file for your pre-lab submission to be complete. But you do not need to include images that are not on the list of required files (even if those images are in your lab report). If you have questions about the submission process, please ask ahead of time. The required files for Lab 1’s pre-lab (Section 1.4) are:
• Your lab report: lab1_report.tex, lab1_report.pdf (as generated from the tex file)
• Your digital designs: lab1_part1.circ, lab1_part2.circ, lab1_part3.circ
Introduction to the digital lab
1.2 Marking Scheme
Each lab is worth 4% of your final grade, where you will be graded out of 4 marks for this lab, as follows.
• Prelab: 1 mark
• Part I (in-lab): 1.5 mark
• Part II (in-lab): 1.5 mark
1.3 Introduction to the digital lab
This section describes the different pieces of equipment you use in this lab:
• The protoboard
• The digital switch/light board
• The logic probe
• The wire strippers
• The chip puller
• The 7400-series chip packages
1.3.1 The boards
The protoboard (also called the breadboard) is for holding and connecting chips. As illustrated in Figure 1.1, chips are inserted across the middle valley in the protoboard. The set of holes in a vertical line above the valley are connected electrically, as are the vertically aligned holes below the valley. Therefore, each pin of the chip in the board is connected to the holes above (or below) the pin as directed by the location of the pin with respect to the valley. To make a connection to a specific pin, you need only make connections between the holes, by plugging the bare end of a wire into any of the holes above (or below) that pin.

Figure 1.1: Protoboard
In Figure 1.1, the horizontal lines at the top and bottom of the board delineate holes that are connected horizontally; note that the space in the middle indicates a disconnection. The horizontally-connected Introduction to the digital lab
holes at the top and the vertically connected holes at the side are usually connected to the power and ground provided by the external connector. The power and ground of the chips are then connected to these strips of holes. The first thing you should do in the lab is connect power and ground to these horizontal and vertical strips. However, do not turn the power supply on until you have verified that all the connections in your circuit are as intended.
The digital switch/light board provides switches that have digital output (5V = logic 1, 0V = logic 0) and lights that can be driven by logic signals (i.e., logic 1 turns a light on, logic 0 turns it off). The pin out connections for the header on the digital switch board can be seen in Figure 1.3. Test the board by connecting the switches to the lights. The board also provides a clock, which can have its frequency varied by inserting different capacitors into the holes next to it, and a seven-segment display.
1.3.2 The tools
The logic probe is used for measuring the logic values of signals on the board. Ensure that it has power attached to the correct terminals. To test the probe, touch it to the +5V and ground on the protoboard, to verify that it correctly indicates the values high (1) and low (0) respectively.
A wire stripper is used to (more) easily remove the electrical insulation from electric wires. You strip a wire so that you can connect it to the breadboard. The wire strippers are attached to each workstation to make sure they don’t get lost. If you have never stripped a wire before, try it!
The chip puller should always be used to remove chips from the protoboard. Doing it with your fingers bends the pins and ultimately breaks them, so only remove chips with chip puller!
1.3.3 The chips

Figure 1.2: Pin Numbering of TTL Chips
All of the chips you will use are Dual In-line Packages or DIPs. Most of the packages have 14 pins, and the pins are numbered from looking at the chip from the top (note the notch in Figure 1.2). Below the notch is pin 1 to pin 7 and above the notch is pin 14 down to 8. Figure 1.4 shows the Pin-out numbers and schematics for all of the 7400-series chips that we have access to in our labs. For this lab, you should take a close look at 74LS04, 74LS08, and 74LS32.
CAUTION
You should always connect the power (Vcc; Pin 14 in 14-pin chips) and ground (Pin 7 in 14-pin chips) of chips to VCC and ground. Leaving the ground pin (Pin 7) unconnected is NOT the same as connecting it to ground, and your chip will not work correctly.
Lab Preparation
1.3.4 Bringing it all together
The components described above are used to implement the physical circuits that you will be designing below. In the first 15 minutes of the first lab session, the Teaching Assistants will give a short demo of how to use the protoboard and chips and wires and tools. Before you start working on the lab exercises below though, you should view the module on Quercus called “Lab Breadboard Demo”.
1.4 Lab Preparation
The pre-lab for Lab 1 consists of three parts. In Part 1, you design a 2-to-1 multiplexer. In Part 2, you design an unnamed logic function. In Part 3, you design a 2-to-1 multiplexer a little differently (peek at Section 1.5 for why). Make sure that, in Logisim Evolution, Enable Vcc and Gnd ports is set to Yes for each TTL chip in your schematic.
1.4.1 Part I
The multiplexer is a combinational circuit that chooses one of multiple inputs to be the output. The number of inputs to a multiplexer is typically a power of 2. Multiplexers are an extremely useful circuit with many applications and are used, for example, in the datapath of a CPU.
The Boolean function f = xs + ys is a 2-to-1 multiplexer. The s is called the control signal or “select”. As we can see, when select is 0, then the output is x. However, when s is a 1, then the output is y. Perform the following steps:
1. Write out the truth table for the design and include it in your report.
2. In Logisim Evolution, draw a schematic that implements the function, using only 74LS04 (NOT),
74LS08 (AND) and 74LS32 (OR) chips. Save your design with the name: lab1_part1.circ. Export the schematic as an image and include it in your report. Also remember to upload the schematic (i.e., the .circ file) as part of your required files.
3. Is there a cheaper implementation for your design, assuming you are still limited to using the same three chip types? Include your analysis in the report.
1.4.2 Part II

Consider the Boolean function: f = a + b + cb. Perform the following steps:
1. Write out the truth table for the design and include it in your report.
2. In Logisim Evolution, draw a schematic that implements the function, using only 74LS04 (NOT),
74LS08 (AND) and 74LS32 (OR) chips. Save your design with the name: lab1_part2.circ. Export the schematic as an image and include it in your report. Also remember to upload the schematic (i.e., the .circ file) as part of your required files.
3. Is there a cheaper implementation for your design, assuming you are still limited to using the same three chip types? Include your analysis in the report.
Lab Demonstration
1.4.3 Part III
Re-create the 2-to-1 mux from Section 1.4.1. Except, instead of using the TTL chips, use the NOT, AND, and OR gates directly (i.e., under Gates). Save your design with the name: lab1_part3.circ. Export the schematic as an image and include it in your report. Also remember to upload the schematic (i.e., the .circ file) as part of your required files.
1.5 Lab Demonstration
1.5.1 Part I
1.5.2 Part II
Reference figures
1.6 Reference figures

Figure 1.3: Digital board header pin assignment
Reference figures

Figure 1.4: Schematics of 7400-series TTL Chips