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4-Input RTL NOR Gate

### Introduction

In the previous experiment, we looked at the basic RTL inverter circuit. We found that it performed its job properly and well, and that the output voltage is not seriously degraded by a connection to a similar circuit, which then acts as a load on the first output.

Now it's time to use RTL technology to combine multiple logic signals into a single output signal. In this experiment, we will combine multiple RTL inverters into a single 4-input NOR gate. However, the method we will demonstrate here can be used for any number of inputs.

### Schematic Diagram

It is possible to create an RTL NOR gate by connecting multiple input resistors to a single transistor. However, because the base of the transistor does not operate at ground potential, there will be some interaction between input signals and a limit on the number of input signals that can be applied to one transistor. To avoid that problem, the circuit to the right is preferred. Here, each input signal is applied to its own transistor, just as in an inverter circuit. However, the transistor collectors are connected together with just a single collector load resistor. This way, the signals all combine only after inversion has occurred, so the input signals cannot interfere or interact with each other under any circumstances.

Any number of transistors can be used in this configuration to produce an RTL NOR gate of that many inputs.

### Parts List

To construct and test the 4-input RTL NOR gate circuit on your breadboard, you will need the RTL inverter circuit from the previous experiment plus the following additional parts:

• (3) 15K, ¼-watt resistors (brown-green-orange).
• (3) 2N4124 NPN transistors.
• (3) 0.3" black jumper wires.
• (1) 0.3" white jumper wire.

### Constructing the Circuit

You should still have your experimental RTL inverter circuit in place from the previous experiment. If you did not perform that experiment, you should do so now. Then, proceed with the assembly phase of this experiment.

### Circuit Assembly

#### Starting the Assembly

Before you begin installing new components, verify that your breadboard socket still has the RTL inverter circuit in place from the previous experiment, as shown to the right. If you did not perform that experiment previously, you should perform it now.

Click on the `Start' button below to begin assembling the circuit for this experiment.

#### Remove the White Jumper

Remove the white jumper currently connecting the inverter output to L0. Set this jumper aside for now; you will be installing it later in a new position.

Click on the image of the jumper you just removed to delete it from the assembly diagram and continue.

#### 0.3" Black Jumper

Prepare a 0.3" black jumper as shown 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.

#### 0.3" Black Jumper

Prepare a second 0.3" black jumper 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.

#### 0.3" Black Jumper

Prepare a third 0.3" black jumper 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.

#### 0.3" White Jumper

Prepare a 0.3" white jumper 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.

#### 15K ¼-Watt Resistor

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

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

#### 15K ¼-Watt Resistor

Locate another 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.

#### 15K ¼-Watt Resistor

Locate a third 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" as shown. 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.

#### 2N4124 NPN Transistor

Locate a second 2N4124 NPN transistor and form the leads to a separation of 0.1" as shown. 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.

#### 2N4124 NPN Transistor

Locate a third 2N4124 NPN transistor and form the leads to a separation of 0.1" as shown. 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.

#### 3" Orange Jumper

You should have a 3" orange jumper wire left over from a prior experiment. If not, cut a 3" length of orange hookup wire and remove ¼" of insulation from each end. Connect this jumper from logic switch S1 to the point indicated in the assembly figure to the right.

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

#### 6" Orange Jumper

You should have a 6" orange jumper wire left over from a prior experiment. If not, cut a 6" length of orange hookup wire and remove ¼" of insulation from each end. Connect this jumper from logic switch S2 to the point indicated in the assembly figure to the right.

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

#### 6" Orange Jumper

You should have a second 6" orange jumper wire left over from a prior experiment. If not, cut a 6" length of orange hookup wire and remove ¼" of insulation from each end. Connect this jumper from logic switch S3 to the point indicated in the assembly figure to the right.

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

#### 10" White Jumper

Locate the 10" white jumper wire you removed earlier. Connect one end to L0. Connect the free end to the point indicated in the assembly diagram.

#### 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 active four logic switches to provide a logic 0 to your experimental circuit. Then, turn on power and observe the state of L0. Record this state on the first line of the table to the right.

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.

When you have recorded the output state for all combinations of input states, turn off power and then look over your results. Is this the correct output pattern for a NOR gate?

Inputs Output
S3 S2 S1 S0 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

With all four logic inputs set to logic 0, all four transistors are turned off. The resulting output is therefore a logic 1. With all other input combinations, at least one input is a logic 1, and the corresponding transistor is therefore turned on. This pulls the output voltage down to a logic 0.

Having more transistors turned on makes no difference; the output cannot be pulled down any harder or any further. It remains at logic 0.

This gate is therefore verified to perform a logical NOR function upon its input signals.

When you have completed this experiment, leave your experimental circuit in place for the next experiment. You will be extending it without making any changes to the present circuit.

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