The **Casio 103-MR** is a large desktop calculator from the mid-1970s. It measures approximately 4 x 6 x 1 inches and weighs 8 ounces without batteries. Power comes from 4 AA batteries or an optional 6V AC adapter.

The calculator has a two-tone plastic case, with a light beige base and black top. The front panel is brushed aluminum printed with branding and labels. It has a 10-digit blue vacuum fluorescent display, though only 9 digits are used. There is no overflow digit. The squishy plastic keys make a hollow sound when pressed.

The 103-MR performs the four basic math functions, square root, and has a 3-register memory. It can select between 6, 4, 3, 2, 1, or 0 decimal places. Additional switches allow rounding down, standard rounding, or floating decimal. Logic features include constant multiply/divide, input overflow suppression, and lack of divide by zero or overflow recovery. Negative numbers display with a minus sign rather than a leading hyphen. Negative square roots are allowed, incorrectly producing a negative result. Stored memory values are not indicated on the display.

Internally, the calculator uses a Hitachi HD3692 microprocessor and a NEC VFD tube. It has 2 transistors, 5 diodes, 23 capacitors, 7 resistors, and other components on two circuit boards. The keyboard connects through a 23-pin ribbon cable. There are 7 jumper wires on the keyboard matrix, implying issues with the original design.

## Logic

- (C) clears the last number entered and (AC) clears the whole calculator
- Input overflow is suppressed, inputting an eleventh digit is ignored
- There is selectable constant on multiply and divide only by double hitting the operator; i.e. (5)(X)(X)(=) gives “25” (=) gives “125” etc.
- Negative numbers are shown by a minus sign in the digit to the immediate left of a number but as there is no eleventh digit to travel into you are limited to nine digit negative numbers
- Divide by zero shows “E” in the first (far right) digit and is not recoverable
- Overflow shows “E” in the first (far right) digit and is not recoverable
- Memory store is not indicated – you have to remember it!
- Memory overflow will result in an error and zero the memory
- Negative square roots are allowed and result in a negative number
- There is a switch selecting 6,4,3,2,1,0 fixed decimal points
- Another switch selects between floating decimal point [F], fixed round down [CUT] and fixed round [5/4]

Overall, the 103-MR provides the expected basic functionality but suffers from limitations in error handling and advanced math. It relies on a proven but dated circuit architecture. As an early consumer calculator it is an interesting piece of vintage technology.