typical voltage assignment

[Guest 4electros]

Voltage between 0.8V to 2V in 5 Volt CMOS and TTL Logic, this may cause error in a digital circuit. Today's digital circuits works at 1.8 volts, so this statement may not hold true for all the logic circuits

The above statements might be easy to understand , though i didn't stand well for it ..could anyone explain it in his special way and i will be thankful!

i've attached PNG file for it!

regards!

[audioguru]

0.8V or less is recognized as a logic low to a TTL input. Most TTL outputs produce a low voltage of 0.4V or less to provide 0.4V of immunity to noise.
2.0V or more is recognized as a logic high to a TTL input. Most TTL outputs produce a high voltage of 2.4V or more to provide 0.4V of immunity to noise.

1.5V or less is recognized as a logic low to a Cmos input. Most Cmos outputs produce a low voltage of 0V to provide 1.5V of immunity to noise.
3.5V or more is recognized as a logic high to a Cmos input. Most Cmos outputs produce a high voltage of 5.0V to provide 1.5V of immunity to noise.

The output voltage of TTL doesn't go high enough to drive a Cmos input high.
The output current of Cmos isn't enough to drive a TTL input low.