# Logic low is not zero volts but another voltage?

1. ## Logic low is not zero volts but another voltage?

How do you know if the logic low state is another voltage other than zero volts?

What kind of circuits use the logic low state as another voltage other than zero volts?

Most Logic circuits logic low state is always zero volts right?

- - - Updated - - -

How would you know if the square waveforms low state was not at zero volts but at -5 volts? and the high logic state was at +5 volts?

•

2. ## Re: Logic low is not zero volts but another voltage?

There is no such thing as perfect zero in a practical
sense. Some logic gets mighty close at no load but
you'd still see microvolts or millivolts if you looked at
it close enough. And old TTL is nowhere close.

Your high and low states are specified as ranges,
and the input must accept wider than the output is
rated to produce.

3. ## Re: Logic low is not zero volts but another voltage?

How would you know if the square waveforms low state was not at zero volts but at -5 volts? and the high logic state was at +5 volts?

- - - Updated - - -

Troubleshooting examples:

how would a tech know if the

1.) low logic state was at -1 volts with a DC offset at +4VDC , logic HIGH state was at +9 volts?
2.) low logic state was at 2 volts with a DC offset at +6VDC, logic HIGH state was at +11 volts?
3.) Low logic state was at -5 volts , Logic High state was at +5 volts?
4.) Low logic state was at -3 volts, with a DC offset at +3 VDC, logic High state was at +6 volts?

How would a tech know this, since the logic LOW state is not zero volts

Where have you seen circuits or what kind of circuits have low logic states like this with DC offset or no DC offset

4. ## Re: Logic low is not zero volts but another voltage?

Each logic family specifies its input requirements to recognize a logic one or zero as VIL and VIH.
For example for TTL ViH =0.8v and VIH=2.0v. Any value less than .8v is considered a logic zero and Any value greater than 2.0v is considered a logic one. An input between .8v and 2.4v is invalid.
Similarly the output specs are VOL=0.4v and VOH=2.4v which means that a valid zero output is less than .4v and a valid one logic output is more than 2.4v.
Differences between this threshold define the noise immunity of the logic family.
This specs are different for each logic family. ECL for example has completely different levels.

So to know if a signal is a zero or one you must check the datasheets for the logic that you are using.

1 members found this post helpful.

•

5. ## Re: Logic low is not zero volts but another voltage?

Originally Posted by danny davis
Where have you seen circuits or what kind of circuits have low logic states like this with DC offset or no DC offset
Nowhere. Nobody designs circuits like that.

BTW, Did you used to post on this forum with the username walters? The style is very similar.

- - - Updated - - -

Originally Posted by albert22
Each logic family specifies its input requirements to recognize a logic one or zero as VIL and VIH.....

•

6. ## Re: Logic low is not zero volts but another voltage?

RS232 output has a square waveform of -3 volts for LOW STATE and + 3 volts for HIGH STATE

Have you seen any others like this, which has logic states like this?

7. ## Re: Logic low is not zero volts but another voltage?

Originally Posted by danny davis
RS232 output has a square waveform of -3 volts for LOW STATE and + 3 volts for HIGH STATE

Have you seen any others like this, which has logic states like this?
Not so.
RS232 spec states logic 0 = +5V to +25V, and logic 1 = -5V to -25V

8. ## Re: Logic low is not zero volts but another voltage?

RS232 spec states logic 0 = +5V to +25V, and logic 1 = -5V to -25V
The square waveform is alternating and bi-direction, has a positive cycle and negative cycle right?

Any other digital logic signals like this?

9. ## Re: Logic low is not zero volts but another voltage?

RS-232 is probably the last digital interface standard that
expected positive and negative 1/0. All the others that
come to mind are bilevel unipolar, aside from some kooky
stuff like transformer-coupled aircraft busses.

And you should distinguish between interface and logic;
I've never seen a direct RS-232-levels logic family, just
transmitters and receivers; logic is done elsewhere.

I'd bet that outfits like TI, National, OnSemi/FreeScale
have logic selection guides that list levels, supplies and
speeds etc.

•

10. ## Re: Logic low is not zero volts but another voltage?

Hi,
ECL logic operate with negative threshold. However implementation of ECL is not so comfortable. Therefore thought out replacement for ECL - so call PECL which operate with positive value. From my opinion ECL almost disappear.

11. ## Re: Logic low is not zero volts but another voltage?

All TTL (whether it is Shottkey or lower power) has an input threshold of 2 Vbe drops of 0.65*2 =1.3V.
This is also the voltage of a floating input which is considered a marginal logic 1 as there is no input current but susceptible to noise.

It is also the threshold of RS232 which uses bi-polar drive levels to improve noise immunity.

Low threshold CMOS uses the 30/70 rule for thresholds. Low <=30% and high >= 70% of Vcc

--[[ ]]--