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Thursday 17 July 2014

How 7401 Quad 2-Input NAND Gate Differ from 7400 Quad 2-Input NAND Gate?

by realfinetime  |  in TTL at  05:22

           We had already seen the circuits to control 7400 and 7401 using switches. If you are a starter in electronics, definitely, you will get confused about the difference between 7400 and 7401. It is very easy to create an application circuit using 7400. But using 7401, it is not that much easy. To handle 7401, you have to learn some more principles in electronics. I too confused about the same topic because, both are Quad 2-input NAND gate integrated circuits.

           7401 will not give the expected output, if the circuit is done as it is done for 7400. Some extra components had to be added in 7401 circuit, which is internally included in 7400 chip. That is the pull up resistor. In 7400, there is a built in pull up resistor. But in 7401 circuit, it should be included externally, because the outputs of 7401 are open collector as shown in the following diagram. That is, major difference between 7400 and 7401 is the pull up resistor.

Why pull up resistor?.

          A pull up resistor must be connected across the Vcc and output pin of logic gate. If pull up resistor is not connected, current will not flow through LED. Because current required to turn on LED is supplied from Vcc and the only path through which current can reach LED is through the pull up resistor. But in 7400, there is an internal connection inside IC through which current will reach the LED. So external pull up resistor is not required.
          Output pin of logic gate does not supply the voltage to turn on led. If both switches are ON, impedance of the gate will be very LOW. So current from the Vcc will flow through the pull up resistor to ground through GATE since current always choose low impedance path. This current will not flow through LED, because of the high impedance of  LED and current limiting resistor compared to GATE.

          Similarly, if any of the switch is OFF, impedance of the GATE will be very HIGH compared to the impedance of  LED and current limiting resistor and the current from Vcc will flow through the pull up resistor to LED. This will turn on the LED.

Advantages of Pull Up resistor.

          By using external pull up resistor, we can change the magnitude of load current. If you want to increase the load current, reduce the value of external pull up resistor and vice versa. In 7400, current will be fixed through a particular load at a particular voltage. So we will be forced to look for other methods, if this current is not sufficient to drive the load. But in 7401, simply adjust the value of resistance to get the required load current.

1 comment:

  1. The other advantage of open collectors is that the outputs of 2 such gates can be connected together so that they share a common load resistor.

    The outputs of the individual gates must both be high for the combined output to be high otherwise the output of the combination will be low. So the wiring together of the outputs performs an implied AND function.



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