6 Casio Digital Diary
PDAs may seem outdated by our standards, but they actually replaced an even earlier subset of devices. Long before smartphones, people in the 1990s used a type of now-obsolete calculator-like computer called a digital diary or an electronic organizer.
With a small display and a keypad, they were most often used as an electronic diary but could also sport address books, to-do lists and calendars. They had no connectivity or messaging options, however. That fact alone eventually led to their demise when PDAs became popular in the 2000s.