In this laboratory exercise, students will test several logic chips to verify their functions. Students will also test an “unknown” chip to determine the Boolean function it realizes. Background material for this and the following experiments, as well as chips connections layout, can be found in chapter 6 of this manual.
Background material for this and the following experiments, as well as chips connections layout, can be found in Chapter 6 of this manual
To perform this experiment, students will need to become familiar with the logic probe they will use.
This is described in this section.
We will use a logic probe to test the functions of our chips. First we must connect the logic probe’s Vcc and Gnd clips so the probe has power. We can clip them directly to the terminals of the protoboard. Then we touch the tip of the logic probe to the pin we wish to test. The logic probe has two LEDs that will be used in this experiment. When the probe detects a 0, its “Low” LED lights up; the “High” LED lights when a logic 1 is detected. We can use the logic probe to check the input and output values of a chip. Figure 4.1 shows a typical logic probe.
To test our chips, we will need to input values to the gates. To input a 1, we will connect a wire between the chip’s input and Vcc. We can input a 0 by connecting the wire from the chip’s input to Ground.
An important point students sometimes forget: Each chip has to be supplied with power, which means that designated pins (not only inputs and outputs) must be connected to Vcc and Ground. For example in chips such as 7408 pin 7 is ground and pin 14 is Vcc. If these connections are not made the chip will not work
In the first part of this exercise, students will test several logic chips. Each group of students has a protoboard and three known chips: a 7408 (AND gates), 7432 (OR gates), and 7486 (XOR gates). Place the chips into the protoboard, ensuring that each chip straddles one of the channels. This ensures that there is no direct connection between the leads on the two sides of the chips. Connect Vcc and Gnd for each chip.
Starting with the AND gate, connect both inputs to Gnd, producing a logic 0 for each input. Use the logic probe to verify these input values, and then check the output values for the gate. Record your output values on the worksheet for this exercise.
Next, change one of the inputs to Vcc, changing it to logic 1. Again verify your input values and determine your output value using the logic probe, and record your output value on the worksheet. Repeat this for all possible combinations of the input values, and then verify that your values match those in the truth table for the AND function.
Repeat this procedure for the OR and XOR gates.
The protoboard for this exercise also contains a chip which has tape covering its surface. Here you will determine its function without removing the tape.
To do this, follow the same procedure you used in the first part of this exercise. Vary the values of the inputs to the chip and record its outputs on the worksheet. Then compare the truth table for your unknown chip to those given earlier in this chapter and find the table that matches the unknown chip’s table. What is the function (type) of your unknown chip?
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GATE TYPE