A portable Chalkboard also known as a Slate tablet. It has an x,y & z grid pattern that's ideal for applying the Cartesian_coordinate_system onto, along with the aid of a Slide_rule. Dave Chalk is one of several IT experts that's aware of the history & development of communication symbols & data image manipulation. A 1960s etch-sketch-pad was a partial improvement from a portable chalkboard. But of course the real jump would be into an electronic device.
The classic 1968 depiction of a future iPad-Tablet_PC. http://www.wired.com/epicenter/2010/03/the-key-to-apples-ipad-uh-oh-its-magic/ 
Some features of the Apple iPad.http://www.cnn.com/2010/TECH/01/27/apple.tablet/index.html?hpt=T1
http://ca.news.yahoo.com/s/capress/100127/technology/technology_us_tec_apple
http://www.engadget.com/2010/01/26/the-apple-tablet-a-complete-history-supposedly/
http://www.networkworld.com/slideshows/2010/012610-apple-innovations.html#slide1
http://www.youtube.com/watch?v=T6VS_oT62wc&feature=related
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http://news.bbc.co.uk/2/hi/technology/8483654.stm
The first movie depiction of a Tablet PC or an iPad like device can be seen in the 1936 film, Things_to_Come. The movie is based on the 1933 HGW book, The_Shape_of_Things_to_Come.
http://www.technovelgy.com/ct/AuthorSpecAlphaList.asp?BkNum=378
http://www.technovelgy.com/ct/Science_List_Detail.asp?BT=Display
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"In 1936, Zuse made a mechanical calculator called the Z1, the first binary computer."
http://inventors.about.com/library/weekly/aa050298.htm
http://inventors.about.com/library/blcoindex.htm
http://www.epemag.com/zuse/
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HGW: 1899 & 1910 The_Sleeper_Awakes
"DVD/VCR (Entertainment Player)
A device that accepts stored moving picture entertainment and makes it available for viewing."
http://www.technovelgy.com/ct/content.asp?Bnum=773
http://www.technovelgy.com/ct/AuthorSpecAlphaList.asp?BkNum=178
http://www.technovelgy.com/ct/AuthorTotalAlphaList.asp?AuNum=54
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"A similar principle is used in a technique known as makyoh topography, which is used to determine the flatness of manufactured materials such as semiconductor wafers."
Who really knows how far back some of these concepts go?
