Such a signal would be considered uncertain, and no logic gate manufacturer would guarantee how their gate circuit would interpret such a signal.Īs you can see, the tolerable ranges for output signal levels are narrower than for input signal levels, to ensure that any TTL gate outputting a digital signal into the input of another TTL gate will transmit voltages acceptable to the receiving gate. If a voltage signal ranging between 0.8 volts and 2 volts were to be sent into the input of a TTL gate, there would be no certain response from the gate.
“Acceptable” output signal voltages (voltage levels guaranteed by the gate manufacturer over a specified range of load conditions) range from 0 volts to 0.5 volts for a “low” logic state, and 2.7 volts to 5 volts for a “high” logic state: “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. 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. Ideally, a TTL “high” signal would be 5.00 volts exactly, and a TTL “low” signal 0.00 volts exactly.
TTL gates operate on a nominal power supply voltage of 5 volts, +/- 0.25 volts. However, in reality, logic signal voltage levels rarely attain these perfect limits due to stray voltage drops in the transistor circuitry, and so we must understand the signal level limitations of gate circuits as they try to interpret signal voltages lying somewhere between full supply voltage and zero. In a perfect world, all logic circuit signals would exist at these extreme voltage limits, and never deviate from them (i.e., less than full voltage for a “high,” or more than zero voltage for a “low”). Logic gate circuits are designed to input and output only two types of signals: “high” (1) and “low” (0), as represented by a variable voltage: full power supply voltage for a “high” state and zero voltage for a “low” state.
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