# Logic clarification

Discussion in 'Homework Help' started by adriano08, Jun 7, 2010.

Apr 25, 2010
33
1
Hi.. i know it would sounds silly but i guess i need someone to clarify this with me again.

Does power supply source, 5v, 9v, 12v or whatever voltages directly connected to the input of a logic gate always mean a logic of 1?

What if i tied my Vcc source to a switch, then connected to a logic gate input like 7404 inverter. Will the output always be a logic 0?

Vcc ----o___o---- i/p ==> o/p of '0'?
Vcc ----o o---- i/p ==> o/p of '1'?

On side note, i mean does Gnd always implies a logic of 0 while Vcc a logic of 1?

2. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
Usually. It depends on the $V_O_H$ (Voltage output high) and $V_O_L$ (Voltage output low) of

your logic gate. For example, if I set $V_O_L = 1V to 2V$ and $V_O_H = 4V to 5V$, then the only point at which I get a logic "1" is when my voltage input is greater than 4V and less than 5V.

Again, the post above still applies here. If your Vcc was at 4.5V, then yes, it would invert appropriately.

You can think of Vcc as simply the voltage required to power your logic gate. Ground is always 0V, and may not be necessarily a "0" to a logic gate.

I would suggest that you take a look at some of the logic gate datasheets to get an idea of how this works.