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4-Input RTL OR Gate

### Introduction

Resistor-Transistor Logic (RTL) is a large step beyond Diode Logic (DL). Basically, RTL replaces the diode switch with a transistor switch. If a +5v signal (logic 1) is applied to the base of the transistor (through an appropriate resistor to limit base forward voltage and current), the transistor turns fully on and grounds the output signal. If the input is grounded (logic 0), the transistor is off and the output signal is allowed to rise to +5 volts. In this way, the transistor does invert the logic sense of the signal, but it also ensures that the output voltage will always be a valid logic level under all circumstances.

Because of this, RTL circuits can be cascaded indefinitely, where DL circuits cannot be cascaded reliably at all.

### Schematic Diagram

Since a transistor will inherently invert the logic sense of a signal, we can't use transistors to merely combine input signals in an OR function. What we can do, however, is to take the NOR gate we studied in the last experiment and re-invert the output, as shown to the right. This will give us a NOT-NOR function, which brings us back to an OR function.

The NOR output is still available as well, so this circuit provides complementary outputs if they're needed. Indeed, many standard RTL ICs provide complementary outputs for convenience and functionality.

### Parts List

The OR gate you'll be constructing for this experiment is mostly present on your breadboard socket already. You will simply be adding an RTL inverter — just like the one from your first RTL experiment — to the output circuit. You will leave the NOR output connected to L0, and connect the inverted output signal to L1. That way you'll be able to monitor both output signals at the same time.

To construct the additional inverter circuit for this experiment, you will need the following parts:

• (1) 1K, ¼-watt resistor (brown-black-red).
• (1) 15K, ¼-watt resistor (brown-green-orange).
• (1) 2N4124 NPN transistor.
• (1) 0.3" black jumper.
• (1) 0.6" white jumper.
• (1) 10" green jumper.

### Constructing the Circuit

You should still have your experimental RTL 4-input NOR gate in place from the previous experiment. If you did not perform that experiment, you should do so now. Leave all components and jumpers in place. Now, continue your assembly with the steps below.

### Circuit Assembly

#### Starting the Assembly

Before you begin installing new components, verify that your breadboard socket still has the complete 4-input RTL NOR gate in place from the previous experiment, as shown to the right. If not, go back and assemble that circuit now. If you did not perform the experiment previously, you should perform it now.

Click on the `Start' button below to begin installing the remaining components for this experiment.

#### 0.3" Black Jumper

Prepare a 0.3" black jumper as shown above and install it on your breadboard socket in the location indicated in the assembly diagram to the right.

Click on the image of the jumper you just installed to continue.

#### 0.4" White Jumper

Prepare a 0.4" (3/8" insulation) white jumper as shown above and install it on your breadboard socket in the location indicated in the assembly diagram to the right. Note that this jumper won't actually interfere with the transistor already installed, as the transistor body is necessarily spaced above the breadboard socket.

Click on the image of the jumper you just installed to continue.

#### 1K, ¼-Watt Resistor

Locate a 1K, ¼-watt resistor (brown-black-red) and form the leads to a spacing of 0.5". Install it on your breadboard socket in the location indicated in the assembly diagram to the right.

Click on the image of the resistor you just installed to continue.

#### 15K ¼-Watt Resistor

Locate a 15K, ¼-watt resistor (brown-green-orange) and form the leads to a spacing of 0.5". Install this resistor on your breadboard socket as shown to the right.

Click on the image of the resistor you just installed to continue.

#### 2N4124 NPN Transistor

Locate a 2N4124 NPN transistor and form the leads to a separation of 0.1". Install this transistor on your breadboard socket as shown to the right. Be sure to observe the orientation of the transistor as you install it.

Click on the image of the transistor you just installed to continue.

#### 10" Green Jumper

Cut a 10" length of green hookup wire and remove ¼" of insulation from each end. Insert one end of this jumper into the contact location indicated in the assembly diagram to the right. Connect the other end to the input resistor for LED indicator L1. Do not disturb the white jumper connected to L0 as you do this.

Click on the image of the jumper you just installed to continue.

#### Assembly Complete

This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, proceed with the experiment on the next part of this page.

### Performing the Experiment

Set all four logic switches to provide a logic 0 to your experimental circuit. Then, turn on power and observe the state of L1. Record this state on the first line of the table to the right. Also note the state of L0, and record this value to the right of your recorded value for L1.

Continue to set the four input switches for this circuit to all 16 possible combinations, and record your results for each combination on the appropriate line to the right. Be sure you test all possible combinations and record the output states of both LEDs. Then, turn off power before looking over your results.

When you have recorded both output states for all combinations of input states, look over your results. Does L0 still reflect the correct pattern for a NOR gate? Does L1 show the correct outputs for an OR gate?

Inputs Outputs
S3 S2 S1 S0 L1 L0
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1
1 1 0 0
1 1 0 1
1 1 1 0
1 1 1 1

### Discussion

You should have found that L0 produced exactly the same output pattern as it did in the previous experiment: logic 1 when all switches were set to logic 0, and logic 0 all the rest of the time. Thus, the addition of the inverter circuit did not affect the output state of the NOR gate.

L1 showed the opposite state from L0 at all times, indicating that this is indeed an inverter circuit, just like the inverter you built and tested in your first RTL experiment. Thus, L1 showed a logic 0 when all switches were at logic 0, and a logic 1 for all other switch combinations. This is correct OR gate behavior, showing that this circuit operates correctly for all possible input states.

This concludes our experiments with RTL gates. Make sure you have power turned off, and then remove all of the experimental components from your breadboard socket. Set these aside carefully; you'll be using them in future experiments.

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