How much voltage is defined as high level and how much voltage is defined as low level.

Relatively speaking, different systems have different reference levels at high and low levels.

This depends on what system it is and your design.

Conventional: the TTL levels of 5V and 3.3v are generally higher than 2V and lower than 0.8V.

The TTL level of 2.5v is approximately 1.7V, which is high, and below 0.7V, which is low.

The level of 5V COMS is high when it is above 3.5, and low when it is below1.5v.

Coms level of 3.3v is high when it is above 2V, and low when it is below 0.7V

The COMS level of 2.5V is approximately 1.7V, which is high, and below 0.7V, which is low.

The level of 0V ECL is roughly-1.24 or higher, and-1.36 or lower.

The level of 5ECL is generally higher than 3.78V and lower than 3.64.

The ECL level of 3.3v is generally high above 2.06V, and low below 1.94.

The gtl level of1.2v is generally high above 0.85V, and low below 0.75.

The level of 1.5V GTL is roughly 1.2V, which is high level, and below 0.8V, which is low level.

RS232 level:+15V means low level,-15V means high level.