Discussion on TTL Logic signal voltage levels

Laser Driver engineer frequently encountered TTL design problems.As to laser light may occur cannot be completely closed.

TTL gates operate on a nominal power supply voltage of 5 volts, +/- 0.25 volts. Ideally, a TTL “high” signal would be 5.00 volts exactly, and a TTL “low” signal 0.00 volts exactly. 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. “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.

It is Hard to do in the actual design of perfect. Laser driver Board may 0.8V is considered to be high-Level, but some designs may 3.6V is still considered to be low-level. Resulting in some laser tube cannot be completely closed.

So different design engineers may be completely different.