What is Diode Logic ? = A diode is a two-terminal electrical device that allows current to flow in one direction but not the other. It is like a pipe with an internal valve that allows water to flow freely in one direction but shuts down if the water tries to flow backward.
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The diode's two terminals are called the anode and cathode. In the diode symbol, the arrow points from the anode (flat part of triangle) toward the cathode (point of the triangle). The schematic diagram for a diode is shown in Figure B.5.
The device operates by allowing current to flow from anode to cathode, basically in the direction of the triangle. Recall that current is defined to flow from the more positive voltage toward the more negative voltage (electrons flow in the opposite direction). If the diode's anode is at a higher voltage than the cathode, the diode is said to be forward biased, its resistance is very low, and current flows. The diode is not a perfect conductor, so there is a small voltage drop, approximately 0.7 V, across it. If the anode is at a lower voltage than the cathode, the diode is reverse biased, its resistance is very high, and no current flows.
We can construct simple gates with nothing more than two or more diodes and a resistor. See Figure B.6.
At the left of the figure is a diode AND gate, and at the right a diode OR gate. Let's examine the AND gate first. If one of the inputs A or B is grounded, current flows through the diode and the output node X is at a low voltage. The only way to get a high output is by having both inputs high. This is clearly a logical AND function.
Now we turn to the OR gate. Whenever one or the other of the inputs A and B are high, current flows through the associated diode. This brings the output node Y to a high voltage. This circuit clearly implements a logical OR.
Unfortunately, it is difficult to cascade circuits of this kind into multiple levels of logic gates. The voltage drops across the diodes add up as they are cascaded in series, leading to significantly degraded voltage levels.
For example, suppose we wire up five diode-resistor AND gates in series. If a string of inputs are logic 0 and the series diodes are conducting, then the output from the final stage should be recognized as a logic 0 as well. But because each diode adds a 0.7-V drop, the measured output would actually be at 3.5 V. This is pretty far from any voltage that would be recognized as logic 0.
One solution is to increase the power supply voltage, redefining the range that is recognized as a logic 0 and logic 1. Of course, the higher the voltage, the higher the power consumed and the more heat the circuit generates. And no matter what you set the power supply to, there is still a limit to the number of logic levels that can be cascaded. This is hardly an adequate solution.
Also note that it is not possible to construct an inverter with only diodes and resistors. AND and OR functions by themselves are not a complete logic without NOT. Thus, there are some logic functions that cannot be implemented in diode-resistor logic. Fortunately, transistors solve all of these problems.
Now we turn to the OR gate. Whenever one or the other of the inputs A and B are high, current flows through the associated diode. This brings the output node Y to a high voltage. This circuit clearly implements a logical OR.
Unfortunately, it is difficult to cascade circuits of this kind into multiple levels of logic gates. The voltage drops across the diodes add up as they are cascaded in series, leading to significantly degraded voltage levels.
For example, suppose we wire up five diode-resistor AND gates in series. If a string of inputs are logic 0 and the series diodes are conducting, then the output from the final stage should be recognized as a logic 0 as well. But because each diode adds a 0.7-V drop, the measured output would actually be at 3.5 V. This is pretty far from any voltage that would be recognized as logic 0.
One solution is to increase the power supply voltage, redefining the range that is recognized as a logic 0 and logic 1. Of course, the higher the voltage, the higher the power consumed and the more heat the circuit generates. And no matter what you set the power supply to, there is still a limit to the number of logic levels that can be cascaded. This is hardly an adequate solution.
Also note that it is not possible to construct an inverter with only diodes and resistors. AND and OR functions by themselves are not a complete logic without NOT. Thus, there are some logic functions that cannot be implemented in diode-resistor logic. Fortunately, transistors solve all of these problems.
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