The Casio Mini CM-604 calculator is a handheld device that was manufactured by Casio Computer Company Ltd. in 1974. This horizontal format calculator features a two-piece case made of dark brown and beige smooth plastic, with an inset and slightly tilted neutral display filter framed by a white printed escutcheon. The calculator measures approximately 156mm x 80mm x 31mm (width, height, depth) and weighs 192g excluding batteries.
The CM-604 is powered by 6V DC, using 4 AA size batteries. It also has a socket on the top side to the far right, which accepts an adaptor (AD-4145, 0.4W, centre positive) for external power. The quoted battery life is eight hours for manganese dry batteries and 18 hours for alkaline batteries.
The calculator features a 6-digit blue VFD display, with no seventh digit for additional display. However, it has a shift-display key to help with this limitation. The display has an unusual way of representing zero, using only the lower half of the digit.
The CM-604 is a 4-function calculator with a display shift feature. It comes with a black and white manual measuring 148mm by 105mm, containing 12 printed pages with basic information and useful examples.
Inside the calculator, there are several key components, including a Hitachi HD32170AP 4F2 CPU (dated to the second week of June 1974), a 6-digit VFD display single glass tube, 5 transistors, 8 diodes, 15 capacitors, 2 SIL resistor arrays, 13 resistors, and a Fuji MCT0625A 4F10 transformer.
The keyboard board (ref 6P-E4A) sits loosely on the battery compartment and is fixed to the main CPU board by 9 stiff wires. The CPU board is crammed full of components, and due to the battery orientation, numerous cross wires are needed to wire them up in series.
Logic:
- The (C) button clears the last entry of a number, while the (AC) button clears the entire calculator.
- Overflow on number input is suppressed, and typing in a seven-digit number ignores the seventh digit.
- An overflow error is flagged with the answer displayed and viewable using the shift key (>), but it is not recoverable.
- Divide by zero results in “000000” being displayed, but using the shift key reveals a running clock. This error is also not recoverable.
- There is automatic constant on all functions.
- Negative numbers are shown by a “-” in the far left sixth digit until it gets too big, after which it is shown on the first digit of the shift display.
The Casio Mini CM-604 is a solidly built horizontal calculator from the early days of handheld calculators, featuring a quirky logic system and an annoying display (used to save money and power), but with the unique divide by zero bug that displays a running clock.
