At which voltage do TTL IC's switch logic?

A dedicated voltage comparator chip, such as the LM339, is designed to interface directly to digital logic (for example TTL or CMOS). The output is a binary state, and it is often used to interface real world signals to digital circuitry (see analog to digital converter). The LM339 accomplishes this with an open collector output. When the inverting input is higher, the output of the comparator is connected to the negative power supply. When the noninverting input is higher, the output is floating (has a very high impedance to ground). With a pull-up resistor and a 0 to +5V power supply, the output takes on the voltages 0 or +5 and can be interfaced to TTL logic: