These are the special gates which give 3 kinds of outputs: low, high, high impedance (Z). Such gates have an extra input which is called enable input.
When enable input is high, switch is closed & tristate buffer act as a normal buffer and when enable input is low output is high impedance state (floating). Following is the truth table:
Dual Inline Packaging (DIP): All the digital logic gates are manufactured as integrated circuits means all the transistors and resistors are mounted on a single piece of semiconductor and then put in Dual Inline Packaging (DIP). There is an industry standard about naming the ICs. The general format for the TTL IC is as.
- First two letters are used to specify the manufacturers’ name e.g. ST is used for company named ST microelectronics.
- Then we have 74/54 for TTL ICs. 74 is used for normal use ICs while 54 ICs are military-grade ICs and have higher operating temperature and are robust in voltage levels.
- Next 2-letter tells us about the circuitry. E.g. LS is used for Low power Schottky i.e. schottky diodes and transistor are used in the circuitry to decrease the power supply.
- And next letter tell us about the function of the IC e.g. 08 is for AND gate
Hence we have the complete name as ST74LS08