Why is Floating Input High in TTL Logic Circuits

In summary, in TTL based logic circuits, floating input is considered as HIGH because it connects the emitter of a transistor to 0V and turns it off, while grounding the input turns the transistor on. This was made possible by a resistor inside the chip that pulled the base of the transistor to Vcc. Unlike CMOS inputs, floating inputs in TTL circuits were not a problem due to the relatively low resistance. This information can be found in old datasheets for TTL gates.
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Why floating input is considered as HIGH in TTL based logic ckts ?
 
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Long long ago, a TTL input (for a simple example the 7400) was the emitter of an NPN transistor.

The base of said transistor was pulled up to Vcc (typically 5V in those days) by a resistor inside the chip.

Grounding the input connected the emitter to 0V and turned the transistor on.

Leaving the input floating or connecting it to 5V (usually via a resistor) turned the transistor off.

You can find the schematic of a TTL gate in old datasheets.

Since the input was relatively low resistance, there were none of the problems of CMOS inputs where allowing the input to float is a definite no no...
 
  • #3


Floating input in TTL logic circuits is considered as HIGH because of the way the transistors are designed in these circuits. In TTL logic circuits, a HIGH input is defined as a voltage level near the positive supply voltage, typically around 5 volts. When a floating input is connected to a TTL circuit, it is essentially disconnected from any external voltage source. In this state, the input is considered to be in a HIGH state due to the internal pull-up resistor present in TTL circuits. This pull-up resistor ensures that the input is pulled up to the positive supply voltage, thus resulting in a logic HIGH state.

Additionally, TTL circuits use bipolar transistors which require a certain minimum voltage (typically 0.7 volts) to turn ON. When the input is floating, the voltage at the base of the transistor is not high enough to turn it ON, resulting in a logic HIGH state.

Furthermore, floating inputs can be caused by a disconnected or open circuit, which can be interpreted as a logic HIGH state in TTL circuits. This can be useful in certain applications where a floating input can be used to trigger a specific response.

In summary, floating input in TTL logic circuits is considered as HIGH due to the internal pull-up resistor, the minimum voltage required to turn ON the transistors, and its potential for use in certain applications.
 

Related to Why is Floating Input High in TTL Logic Circuits

1. What does it mean for an input to be "floating" in TTL logic circuits?

When an input is floating, it means that it is not connected to a high or low voltage source. This can occur if there is no external connection to the input pin or if the input is not being driven by a voltage source.

2. Why is floating input high a concern in TTL logic circuits?

Floating input high can result in unpredictable behavior in TTL logic circuits. The input can pick up stray electrical signals, causing it to fluctuate between high and low states, which can lead to errors in the logic circuit's output.

3. How can floating input high be avoided in TTL logic circuits?

One way to avoid floating input high is to connect the input pin to a high voltage source, such as a pull-up resistor. This ensures that the input is always in a known state and prevents it from picking up stray signals.

4. What are the potential consequences of floating input high in TTL logic circuits?

Floating input high can result in incorrect logic states and unpredictable behavior in the circuit. This can lead to errors in the output and potentially damage the circuit components.

5. Can floating input high occur in other types of logic circuits?

Yes, floating input high can occur in other types of logic circuits, such as CMOS circuits. However, the specific consequences and methods for avoiding it may differ from TTL logic circuits.

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