28th October 2019

allaboutcircuits
13

What is TTL and CMOS voltage level?

However, real TTL gate circuits cannot output such perfect voltage levels, and are designed to accept “high” and “low” signals deviating substantially from these ideal values. “Acceptable” input signal voltages range from 0 volts to 0.8 volts for a “low” logic state, and 2 volts to 5 volts for a “high” logic state.

Can CMOS drive TTL?

In the LOW state, a TTL output can drive CMOS directly. However, the guaranteed TTL HIGH output level of 2.4 volts is not a valid input level for CMOS. If the TTL output drives only CMOS inputs, then essentially no current is drawn and the HIGH output may be 3.5 V or higher.
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