Casio 805-MR

Casio 805-MR Calculator

The Casio 805-MR is a desktop calculator from around 1976. It has a two-piece plastic case with a white top and black bottom. The brushed aluminum keyboard wraps around the bottom section. At 98mm x 160mm x 35mm and 222g without batteries, it is large enough to use on a desk but can still be held.

Power comes from 4 AA batteries or an optional AC adapter. The display is an 8-digit blue vacuum fluorescent tube, with a 9th digit that is not used. The large, squishy keys have a hollow sound when pressed.

Features are basic – just the four math functions plus square root, percentages, and a 3-register memory. An extra slide switch selects between floating point or fixed decimal with rounding.

Inside are simple 1970s era components – a NEC microprocessor, VFD tube display, transistors, diodes, capacitors and resistors. The keyboard connects to the main circuit board via a ribbon cable. Access just requires removing two screws.

Some notes on the logic:

  • The C button clears number entry, AC clears everything
  • 9 digit overflow is suppressed
  • Divide by zero shows E and locks up
  • Overflow shows E and locks up
  • Constant on multiply/divide but not add/subtract
  • Negative square roots allowed
  • 7 digit negative number limit
  • No memory store indication
  • Negative number indicated by minus sign
  • Has negative zero bug

Overall, the Casio 805-MR is a solid, well-built basic calculator typical of mid-1970s consumer electronics. The “cycling number” display provides some flair, though the logic has limitations like the 7-digit negative number handling. For basic office or school math it would have been a handy tool in its day.

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