Communication, computing, industrial controls, medical devices, spacecraft, consumer devices, whatever. TTL broke the link between one area of electronic circuit design and a traditional college degree in Electrical Engineering. Within a defined set of rules, TTL was a lot like LEGO blocks - you assembled a bunch of them to make something. With TTL, a huge group of people who understood logic but not electrons could design stuff that actually worked. That universal platform supported development of the microprocessor and all that followed.
TTL was the third logic family to be developed, and the first to achieve truly international standardization and acceptance. Some of the original TTL parts still are in production, but original TTL has mostly been replaced by later, more advanced versions (Shottkey, Advanced, Advanced Shottkey, Bus, etc.) and the many many CMOS families. Basically, almost anything that runs on +5 V will interface with EPROMs.
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