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Two, Two-Input DL OR-AND Gate

Introduction

With this experiment we will complete our study of Diode Logic (DL) gates. We have already seen that single AND and OR gates work acceptably although with some signal degradation. We have also seen that the AND-OR concatenation of gates works after a fashion, but with some serious problems concerning signal degradation.

Now we will explore the operation of an OR-AND gating structure, and compare its operation with the other gates. Especially, we will compare the OR-AND gate operation with the AND-OR gate we just examined.



Diode Logic OR-AND Gate

Schematic Diagram

In the previous experiment we built and tested a Diode Logic circuit to combine the outputs of two AND gates with an OR gate. Now we will reverse this construction, and use an AND gate to combine the outputs of two OR gates.

The schematic diagram to the right shows the required circuit for this experiment. It is very similar to the AND-OR structure of the previous experiment, except that all diodes are reversed and resistor connections are swapped between +5v and ground.

Once we have constructed this circuit, we will find out how well it works as compared to the AND-OR circuit.



Parts List

To construct and test the 2, 2-input DL OR-AND gate on your breadboard socket, you will need the following components, of which all but a single 10K resistor should be left over and correctly formed for this experiment:



Constructing the Circuit

You'll be using all but one of the parts left over from the previous experiment to construct this circuit. That one part is a 10K resistor, because you'll need two of these at 0.3" spacing, and you currently only have one formed to fit. The remaining components are already formed correctly, and will be installed in the same locations as before. However, they will all be connected differently for this experiment.



Circuit Assembly

Start assembly procedure









Starting the Assembly

Your OR-AND gate circuit will fit in exactly the same place as the AND-OR gate from your previous experiment. You will use mostly the same components to assemble it, but you will install them differently. Therefore, be very careful to observe the orientation and placement of all components. They will seem familiar, but will involve small but important changes.

Click on the `Start' button below to begin assembling your experimental circuit.

10K, ¼-Watt Resistor

You should already have a 10K, ¼-watt resistor (brown-black-orange) formed to a lead spacing of 0.3", left over from your previous experiment. If not, form a 10K, ¼-watt resistor to a spacing of 0.3". Install this resistor in the location indicated in the assembly diagram.

Click on the resistor image to continue with your assembly.

10K, ¼-Watt Resistor

You should also have a 10K, ¼-watt resistor (brown-black-orange) formed to a lead spacing of 0.5". Again, prepare one if necessary. Install this resistor as shown to the right.

Again, click on the resistor image to continue with your assembly.

10K, ¼-Watt Resistor

Locate a new 10K, ¼-watt resistor (brown-black-orange) in your stock of components and form the leads to a spacing of 0.3". Install this resistor in the location indicated in the assembly diagram.

Once more, click on the resistor image to continue to the next step.

1N914 Silicon Diode

Locate one of the four 1N914 diodes that you have already formed to a lead spacing of 0.3". Or, if necessary, prepare a diode for this lead spacing. Install this diode as shown in the assembly diagram. Be careful to observe the orientation of the diode.

When you have installed the diode correctly, click on its image to continue.

1N914 Silicon Diode

Locate one of the two 1N914 diodes that you have already formed to a lead spacing of 0.4". If you don't have one, prepare a diode for this lead spacing. Install this diode as shown in the assembly diagram. As before, be careful to observe the orientation of the diode.

As before, click on the image of the diode you just installed to continue.

1N914 Silicon Diode

Locate another 1N914 diodes that you have already formed to a lead spacing of 0.3", or prepare a diode for this lead spacing. Install this diode as shown in the assembly diagram. Once again, observe the orientation of the diode.

When you have installed the diode correctly, click on its image to continue.

1N914 Silicon Diode

Locate a third 1N914 diode that you have already formed to a lead spacing of 0.3", or prepare a diode for this lead spacing. Install this diode as shown in the assembly diagram. As before, observe the orientation of the diode.

As before, click on the image of the diode you just installed to continue.

1N914 Silicon Diode

Locate a fourth 1N914 diode that you have already formed to a lead spacing of 0.3", or prepare a diode for this lead spacing. Install this diode as shown in the assembly diagram. Once more, observe the orientation of the diode.

Following the normal procedure, click on the image of the component you just installed to continue.

1N914 Silicon Diode

Locate the second 1N914 diode that you have already formed to a lead spacing of 0.4", or prepare a diode for this lead spacing. Install this diode as shown in the assembly diagram. One more time, observe the proper orientation of the diode.

As usual, click on the image of the diode you just installed to continue.

3" Orange Jumper

Locate one of the 3" orange jumpers that you have used in past experiments. Install the jumper as indicated to the right, to connect one of the input diodes to S0.

As usual, click on the image of the jumper to move on to the next step.

3" Orange Jumper

Use a second 3" orange jumper to connect an input diode to S1, as indicated to the right.

As usual, click on the image of the two jumpers to move on to the next step.

10" White Jumper

Locate the 10" white jumper you have used in prior experiments. Connect one end to the L0 input, and the other end to the point indicated on the assembly diagram.

Click on the image of this jumper to the right to move on to the next step.

6" Orange Jumper

Connect one of your 6" jumpers to S6, and the other end to an input diode as indicated to the right.

As before, click on the image of the jumper to move on to the next step.

6" Orange Jumper

Connect a second 6" orange jumper to S7, and then connect the other end to the last input diode, as indicated to the right.

As usual, click on the image of this jumper to continue.

Assembly Complete

This completes the assembly portion of this experiment. Take the time now to check your work carefully against the assembly diagram to the right. Especially make sure that all diodes are installed with the correct orientation, since they do not all point the same way on the breadboard socket.

When you are ready, scroll down to the next part of the page and begin the actual experiment.

Restart assembly procedure


Performing the Experiment

Before applying power to your experimental circuit, determine the output voltage you would expect this circuit to produce if all input diodes are open-circuited (no input signal connected to any input diode). Assume a diode voltage drop of 0.65v and that resistance and voltage values in the circuit are precisely accurate.

When you have calculated this value, enter it in the top row of the table to the right.

Next, calculate the output voltage you would expect from this circuit if all four input diodes were connected to ground (logic 0). Enter this value in the second row of the table to the right.

Then, calculate the output voltage you would expect to see if any one diode input is connected to +5v (logic 1 input). Record this voltage in the third row of the table to the right.

Now turn on power to your experimental circuit. If you have a voltmeter, measure the output voltage of the circuit under the three sets of circumstances given above. Compare your measured results with your calculated predictions. How would you account for any differences?

Set S0, S1, S6, and S7 to each of the 16 possible combinations of logic signals from four inputs. Note the resulting state of LED indicator L0, and record that state on the appropriate row of the table. Continue until you have tested your circuit for all input combinations and recorded your observed results in each case.

When you have completed your determinations, turn off the power to your experimental circuit.

Expected
Open-Circuit
Output
Voltage
 v
Expected
Output
Voltage
for All 0 Inputs
 v
Expected
Output
Voltage
for One 1 Input
 v

Inputs Output
S7 S6 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

This experiment produced results that were probably highly unexpected: L0 remained on at all times, suggesting that the output of this gating structure is always a logic 1, regardless of the input states. This result is clearly wrong. But what happened? Let's find out.

In the first part of this experiment, you determined the output voltage of the OR-AND gating structure under various conditions. With an assumed diode voltage drop of 0.65v, we see 5 - 0.65 = 4.35v dropped across a combination of three 10K resistors. The two resistors connected to ground are effectively in parallel, so their combined resistance is 5K. This gives us a calculated output voltage of 4.35 × (5K/15K) + 0.65 = 2.1v.

If all inputs are grounded (logic 0), the input diodes will all be reverse biased, and the output voltage will remain at 2.1v. However, if any one input is pulled up to +5v (logic 1), it will pull the output of its OR gate up as well. This will cause the corresponding AND gate input diode to be reverse biased, effectively disconnecting that 10K resistor from the output circuit. Now we have only two 10K resistors dropping 4.35 volts, and the output voltage will be just half of the total voltage drop, or 4.35 × (10K/20K) + 0.65 = 2.825v.

As before, the connection of this circuit to L0 causes the L0 input resistor to be an extra load on this circuit, reducing the output voltage a bit more.

In any case, here we see the real problem of cascading Diode Logic gates. An output voltage of 2.1v is below 2.5v, and is therefore technically a logic 0. However, we learned in the previous experiment that the gate in the LED indicator circuit (L0) will be turned on by this voltage. Thus, L0 remained on for all input combinations; it never turned off.

We could re-design the L0 indicator circuit to set a specific threshold between logic 1 and logic 0, but the only threshold that makes any real sense is 2.5 volts. In that case, both the AND-OR and OR-AND gates must be considered unusable, since their output voltages can't help but stray into illegal ranges.

The end result is that Diode Logic can only be used reliably for single AND and OR gates, and must be immediately followed by an active stage of some kind. Compound DL gates are not reliable, and should not be used.

When you have completed this experiment, remove all experimental components from the right hand side of your breadboard socket. Leave the circuits on the left (power supply, LED indicators, and logic switches) in place, for use in future experiments.


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