Wired logic connection

Wired logic connection

A wired logic connection is a logic gate that implements boolean algebra (logic) using only diodes and resistors. It differs from a digital logic
integrated circuit which utilizes transistor-transistor logic (TTL). A wired logic connection can create an AND or an OR gate. A major disadvantage of wired logic connection is its inability to create a NOT gate.

The Wired AND Connection

The wired AND connection electrically performs the Boolean logic operation of an AND gate.

Connection Requirements:

* A positive voltage source
* Pull up resistor
* One diode per input

AND Connection Process

The positive voltage from the source is directed away from the output "C" and towards "A" and "B" via the diodes connected directed towards the inputs.
When positive logic that is equal to or greater than that of the source is applied to all inputs the source voltage is directed to the output. The AND
gate is capable of an infinite number of inputs but only one output.

Open Collector

The wired AND function can be achieved by simply tying gate outputs together to a common-collector pull-up
resistor. The wired AND function below is achieved by using open-collector TTL gates.

Output

The output "C" is determined by the inputs at "A" and "B" according to the truth table (right) and square wave (below).

Application

Examples of AND logic being used are everywhere, especially in the area of safety.
* A microwave will not turn on until the start button is pressed AND the door is closed.
* A vending machine won't dispense a candy bar until the proper amount of money is
inserted AND the buttons are pressed.

The Wired OR Connection

The wired OR connection electrically performs the Boolean logic operation of an OR gate.

Connection Requirements:

* Pull down resistor
* One diode per input
* Connection to ground

OR Connection Process

Voltage from any input is directed through its corresponding diode directly to the output "C". If no voltage/logic exists on any input there will be no output. The positive voltage source that is present in the AND connection is replaced by a ground in the OR connection. Also, notice the positioning of the diodes as compared to the AND gate. Similarly, the OR connection is capable of an infinite number of inputs and only one output.

Output

The output "C" is determined by the inputs at "A" and "B" according to the truth table (right) and square wave (below).

Application

Just like its AND counterpart the OR logic is also all around us.
* Your computer will print by mouse clicking the print icon OR by dropping the file menu
and selecting the print option OR pressing the print key on the keyboard.
* A cell phone works on battery power OR when it's plugged in.

The Wired NOT Connection

This connection is not possible.

Implementation

Applying the logic inputs from the truth table to the circuit, the output represents NOR ((A+B)’) using positive logic, which is AND (A’B’) using negative logic. Replacing the NAND gates with inverters and placing an AND gate at the junction has an identical truth table as the circuit with the NAND. The two open-collector NAND gates are connected as inverters. Inverters can not perform ANDing functions. It is evident that the ANDing is being done at the common connection point.

Practicality

The inability to perform the NOT operation reserves the wired logic connection as mostly an educational tool. By observing the operation
of these two simple electrical circuits a student can learn the basics of diodes and the voltage divider. Wired logic reduces power consumption and cost. A diode is a much simpler component than an integrated circuit requiring no input voltage to work in the circuit.

AND gate using DRL

Construction

Two p-n diodes are used as input and connected as shown in the figure and a resistor is connected to the positive terminal of the 5v battery permanently. The output voltage Y at C is with respect to earth.

Working

1.When both A and B are connected to earth no diodes D1 and D2 are forward biased and hence conduct. There occurs no voltage drop across both diodes as they are considered to be ideal. Therefore, entire 5V drop occurs across the resistor and Voltage at C is still zero with respect to earth and hence the output is 0.2.When A is earthed and B is connected to positive terminal of battery 5V the Diode D1 conducts but D2 doesn’t. Again considering Diode D1 as ideal 5 volt drop occurs across D1 and also across resistor. Voltage drop at C with respect to earth is again zero. Thus the output is 0. 3.When A is connected to the positive terminal of the battery and the B is earth, the Diode D2 conducts but Diode D1 doesn’t.. Since diodes are ideal no voltage drop takes place across D2 and entire voltage drop of 5V occurs across resistor. Voltage drop at C is zero with respect to ground and again the output is 0.4. When both A and B are connected to the positive terminals of the battery, none of the diodes conducts and hence no current flows through. Consequently, no current flows through resistor R and the voltage drops at C which is 5V with respect to earth. Now the output is 1.

See also

*diode
*Boolean logic
*AND
*OR
*NOT
*electrical circuit
*resistor
*truth table
*square wave
*voltage source
*ground

References

1.Digital Techniques, Heathkit Educational Systems,19902.Fundamental Physics, K.L Gomber and K.L Gogia,Pradeep Publications, 2005


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