Casio 121-MR

The Casio 121-MR is a large desktop calculator from 1975. It measures about 120mm x 170mm x 32mm (w,h,d) and weighs 294g without batteries. Power comes from 4 AA batteries or an optional AC adapter.

The case consists of different colored plastic sections – dark grey on the bottom, cream on top, and black across part of the top. The keyboard is surrounded by a brown painted metal plate that holds the Casio name and model number. The keys feel hollow and squashy but work well. This is a big calculator better suited for desktop use than portable use.

It has a 12 digit blue vacuum fluorescent display with no 13th digit, and a red LED to indicate negative numbers. Standard features include the four basic functions, percentages, square root, x/y exchange, sign change, and four function memory. There are switches for count/sum mode, rounding up/down, and setting a fixed decimal point.

Some logic notes:

  • Pressing C cancels the last number entered, while AC clears the whole calculator
  • Constant function works automatically on all four basic operations
  • Input overflow is suppressed – entering a 13th digit is ignored
  • The red LED shows negative numbers, allowing full 13 digit negatives
  • Divide by zero shows 0 and locks up the calculator
  • Overflow displays the result but locks up the calculator
  • There is no indication of memory use – you have to remember it’s there
  • Negative square roots work but lock up the calculator
  • Full precision square roots are calculated even with decimal point fixed at 0
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