Measuring voltage with reference to 1 and 0 in TTL logic gates standard

Thread Starter

evec

Joined Oct 21, 2018
3
Hi, I have quite simple problem, which stopped me for a while, because I read that logic gates in TTL standard have high, low and the forbidden between state and I saw the table, but I would like to measure it on my own. I'm using the multisim and I have troubles with building the circuit to measure when I have logic '1' on input and when on output (on output should be higher), so could someone show me how should it look like so I could build it and measure this values on my own?
 

Thread Starter

evec

Joined Oct 21, 2018
3


yea, but is that a chance to measure values like this on the image ?

because with circuit like that above I can only measure the moment when output switches between '0' and '1' and first: I've got nothing about Indeterminate Region, secound: on output I have 5V or 0V and on the picture there are shown compartments also on the output
 

danadak

Joined Mar 10, 2018
4,057
Depends on the completeness of the model. Basically this region
is caused by finite G thru the logic path and noise. Its easy enough
to add noise into input. Problem is finding TTL model that exhibits
"real world" gate behaviour.

Maybe some one else can help here with a model source. Or contact
vendor tech help to see if their models properly sim the signal path.
Problem you have is not too many folks, if any, still working/manufac-
turing TTL.

Regards, Dana.
 
Last edited:

MrChips

Joined Oct 2, 2009
30,720
Generally, you cannot measure this with a real TTL device with normal loads.
What the boxes represent are the acceptable range of input and output voltages for logic '0' and logic '1'.

You can force the output towards the upper range of logic '0' by having an unusually small pullup resistor on the output.
Similarly, you can force the output towards the lower range of logic '1' by having an unusually small pulldown resistor on the output.
Neither of these extreme cases are recommended in an actual circuit application.

This can also happen when you have an unusually large number of loads on the output of a gate. Again, for reliable circuit operation you want to avoid doing this.

Look up TTL fan-out.
 

ebp

Joined Feb 8, 2018
2,332
evec, before I try to explain much of this, are you familiar with the concept of "noise margin?"

You will not be able to measure anything that will directly show you the boundaries of the intermediate region for the input because it is really mostly a matter of a definition rather than actual circuit behavior. If you tested a large number of gates from different production batches, you would see some variation in the switching points as they appear in Dana's simulation. The simulation will always show exactly the same switching point for the same external conditions. It will change a little if you change the power supply voltage and if you change the temperature. The limits of the intermediate region will have been decided by engineers who know the details of the internal circuit of the gate and have analyzed what can be expected in terms of variation, but also from analysis of how the output behaves. It is a matter of the engineers saying "If you do this, the circuit will always work properly. If you don't follow the rules we set, it might work OK but it might not."
 
Last edited:

AnalogKid

Joined Aug 1, 2013
10,990
A properly working TTL gate cannon have an output voltage in the indeterminate state part of the right had chart in post #3. When the input is in the input chart indeterminate state voltage range, the output will be either a 1 or 0 state. Which state will vary from one part to another, which is why it is indeterminate and unreliable. In fact, the output might make a noise burst when the input is indeterminate. This is because a standard (non-hysteretic) TTL gate is actually a high gain linear amplifier with intentional clipping.

The same is true with CMOS gates. One part of a CMOS hex inverter can make cheap but noisy microphone preamp.

ak
 

ArakelTheDragon

Joined Nov 18, 2016
1,362
A proper osciloscope can show you the fronts. But I think that what you need to do is set an MCU pin as input, and just apply voltage with a voltage regulator or simple pot, to see how will the MCU react(of course this is not very accurate or usefull and the result depends on the MCU behaviour).
 
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