Why is Floating Input High in TTL Logic Circuits

[mkbh_10]

Why floating input is considered as HIGH in TTL based logic ckts ?

 

[zeitghost]

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...

 

[oswaler]

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.