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According to this Datamath page (link valid 2024-01-13) the TI-74S is a special OEM version of the normal TI-74 BASICALC to allow for customization, hence the omission of all the keys’ secondary functions, allowing the use of special keyboard overlays. Without all the extra keyboard markings however, this Pocket Computer is difficult to use. I’m guessing (but I can’t find any confirmation on that) that it originally came with a handy keyboard overlay. The secondary functions of all keys can be seen on this Wikipedia page (link valid 2024-01-13) on the TI-74. They are accessible by using the "FN"-Key.
It looks quite similar to the TI-95 PROCALC but is an entirely different machine.
The TI-74S quite heavy, which isn’t necessarily a bad thing. It feels very solid and sturdy.
This calculator basically has two methods of operation. BASIC mode and CALC mode (selectable using the "MODE"-Key). Both modes can be used to enter calculations but using a different syntax. For instance, in CALC mode, the square root of a number can be obtained by simply pressing the "I" (which secondary function is the square root). The display conveniently shows the square root symbol next to the result to show your last operation. In BASIC mode it works a bit differently, to obtain a square root one has to type "SQR(number)", either by typing it out or using "FN", "I", number, followed by ")". The closing bracket is necessary, omitting it will cause an error.
The TI-74S uses an exponent that can range from -127 to 127. The biggest factorial that can be calculated therefore is 84! (3.314240135E+126). And quite quickly too!
When set to BASIC mode, be careful how you add lines. Spaces at the correct positions are required, to enter a line of BASIC type the line number followed by a space. To delete a specific line type "del" followed by the line number.
Commands can be entered in lowercase. They are automatically converted to uppercase. The keyboard has auto repeat.
It has 8 kB of onboard RAM.
This Pocket Computer can be extended by means of slide-in cartridges. There is a special socket at the top-right where these cartridges can be easily swapped. The one in my collection has an unlabeled 256 kB FLASH module.
There is even a special module to be able to use the Pascal programming language.
Extra information about this Pocket Computer can be found on this Thimet page (link valid 2024-01-13).