Register to reply

TTL floating input ?

by mkbh_10
Tags: floating, input
Share this thread:
mkbh_10
#1
Mar4-08, 01:08 PM
P: 220
Why floating input is considered as HIGH in TTL based logic ckts ?
Phys.Org News Partner Engineering news on Phys.org
Student develops filter for clean water around the world
Developing the next evolution in underwater communication
Compact vibration harvester power supply with highest efficiency opens door to "fix-and-forget" sensor nodes
zeitghost
#2
Mar5-08, 02:35 AM
P: 157
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...


Register to reply

Related Discussions
Input Calculus & Beyond Homework 4
How to obtain 4 input or gate using 2 input gates? Introductory Physics Homework 3
, i need input General Discussion 4
Logic question, 3 input OR to 2 input OR Electrical Engineering 9
Looking for input on this General Discussion 4