Page "Transistor–transistor logic" Paragraph 21

Standard TTL circuits operate with a 5-volt power supply.

A TTL input signal is defined as " low " when between 0 V and 0. 8 V with respect to the ground terminal, and " high " when between 2. 2 V and 5 V ( precise logic levels vary slightly between sub-types and by temperature ).

TTL outputs are typically restricted to narrower limits of between 0 V and 0. 4 V for a " low " and between 2. 6 V and 5 V for a " high ", providing 0. 4V of noise immunity.

Standardization of the TTL levels was so ubiquitous that complex circuit boards often contained TTL chips made by many different manufacturers selected for availability and cost, compatibility being assured ; two circuit board units off the same assembly line on different successive days or weeks might have a different mix of brands of chips in the same positions on the board ; repair was possible with chips manufactured years ( sometimes over a decade ) later than original components.

Within usefully broad limits, logic gates could be treated as ideal Boolean devices without concern for electrical limitations.