Types of digital signals and threshold voltage 

It is well known that there are only two values of digital signals ; Hi and Lo.

But do you know how much voltage they actually are?
So when we talk about a digital signal which uses 5V power source, the signal could be either “CMOS level” or “TTL level”.

They both use 5V power source, but they use different voltages to identify if the signal is Hi or Lo. *The voltage used to identify if it’s Hi or Lo is called “threshold”.
Thresholds value for each level are below:

The reason why the threshold values for input signals are different from the output signals is that some margin is needed in order to be able to identify the Hi and Lo properly, even if the voltage value possibly changes (goes lower) at the receiving end, being affected by the signal line impedance.

Ok, now let’s talk about what will happen if we try to send a signal with the combination of 2 levels;  “TTL level for output” and “CMOS level for input”.
The output side put 3.0V voltage on the signal line, meaning to send a “Hi” signal.

Although the input side is CMOS level. Which means it only identifies the incoming signal to be “Hi” only if it is over 3.5V.
In this case, there is a possibility that the signal will not be identified as “Hi” by the input side (CMOS level) , even though the output side (TTL level) tries to send out the “Hi” signal.

(The input side (CMOS level) identifies the signal to be “Lo” only when it is lower than 1.0V, so it also means that it could be identified to be either “Hi” or “Lo” anytime.
That would disrupt the normal communication.
So it is important to adjust the threshold voltage standards of the input and output to be the same in order to transmit a digital signal properly.

Lately, most of the circuits inside PC’s uses low voltages like 3.3V, but still, most of the communications with external devices requires 5V signals.  

So remember once again, it is important to check if the signal (voltage) standard for input and output are the same before you actually send a signal.