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Star Trek was still using tablets in the 90s.
And, we can see tablets were still being used in the Star Trek universe in the 1990s with Deep Space Nine.
And, 1968's 2001: A Space Odyssey also showed tablets off.
Star Trek isn't the only future with tablets. In Stanley Kubrick/s 1968 movie, 2001: A Space Odyssey, we also have tablets
The first real world tablets crude ancestors were around in 1961.
Of course, long before the iPad, real world tablets were already appearing, While you certainly couldn't call it portable, the basic idea of a tablet can be seen in 1961's RAND Tablet, aka Grafacon (for 'Graphic Converter').