# Output of an TTL family gate for Vin=1.5V?

Recently I came to find out that TTL outputs are typically restricted to narrower limits of between 0.0 V and 0.4 V for a "low" and between 2.6 V and VCC for a "high".

But if input of an TTL gate is 1.5V then it will not fall in any region and produces an output which will be neither high/low. If due to noise any such condition occurs in design then what preventention can be done?

• Why is your input at 1.5V? Is that considered high for your system or low? Maybe this should be amplified to fit to a high or a low. – Dean Nov 5 '15 at 16:42

Generally we design to prevent that happening. e.g. by only connecting the input of one gate to the output of another, or by only using low impedance sources to our gates.

If we must allow less well defined voltages on logic inputs we can use gates with schmitt inputs (like the 74LS13), or take them to Analogue to Digital converters.

You've gotten confused here, a bit. This article explains things, but I'll also do it my way.

The two levels you've specified are output levels. A TTL gate which is not being abused will output either less than 0.4 volts or more than 2.6 volts. But this is only half the story.

TTL is only guaranteed to operate correctly for inputs of less than 0.8 volts or more than 2.0 volts. So normally a logic low will have a voltage of less than 0.4 volts while a 0.8 volt level would work, and the system has 0.4 volts (0.8 minus 0.4) of margin. Likewise, a high level will be 2.6 volts or more while 2.0 would work, providing 0.6 volts of margin. This allows for rejection of the inevitable noise pickup which occurs in any real system.

With this in mind, what happens with 1.5 volts in? As mkeith has answered, there is no way to tell. Although 2.0 is the nominal high threshold, 1.95 is almost certain to work, that is, produce a logic 1 in the receiving logic. So is 1.9, although with less certainty. The nominal midpoint is, of course, $$V_m = \frac{0.8 + 2.0}{2} = 1.4\text{ volts}$$ and the closer you get to this point the less certain the result.

I don't think your levels are correct. Those sound like the worst case output levels rather than the input thresholds. But the basic question is sound. What happens when the input is between VIN-lo (max) and VIN-hi(min)? The answer is simple. The output state is not guaranteed. It may be high or low or in between, and it may not be stable (it could oscillate). So, make sure you don't do that. If you have a specific case in mind, you should describe it in more detail so a practical solution can be suggested.