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The Casio J-20 is a handheld calculator introduced in 1985 by Casio Computer Co., Ltd. This chunky desktop calculator features a large, slightly wedge-shaped case made from a combination of matte and glossy black plastic. The raised and tilted display area has a neutral plastic filter, framed in black, with the brand and model number printed in white text with a border.
The brushed aluminum keyboard surround is printed in black and silver with reversed-out text for the switch labels. The large, squashy keys have a return click and individual key escutcheons, typical of Casio calculators of that era.
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Measuring approximately 100mm x 164mm x 30mm (width, height, depth), the Casio J-20 is powered by two AA batteries or an optional 3V DC 0.25W adapter (type AD-2S) through a socket on the top left side. It features a 10-digit blue vacuum fluorescent display (VFD) with an eleventh digit for negative, error, and memory indication, as well as upper tick marks for thousand separators.
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The calculator offers standard four functions (addition, subtraction, multiplication, and division) with percentage, differential percentage, square root, change sign, and four-function memory. It also includes switched decimal points and rounding modes.
Under the hood, the Casio J-20 is equipped with a Hitachi HD38401A 5J45 CPU (dated September 1985), a single flat-faced 11-digit VFD tube (FIP 11H5 No.4-1 8), two transistors, seven diodes, eight capacitors, four resistors, and a TDK TC-81 59 transformer. The main CPU board (G862-1 A36267-1) is connected to the front-fixed keyboard assembly via a 24-way ribbon cable.
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Logic:
- (C) is used to cancel the last number entered, and (AC) to clear the whole calculator.
- Input overflow is not suppressed, but (C) allows you to input another number in a chain.
- Negative numbers are represented by a “-” in the far left (eleventh) digit, allowing full ten-digit negative numbers.
- There is automatic constant on multiply and divide only.
- Overflow shows the result and “C” (or “E” if negative) in the far left (eleventh) digit and is recoverable using (C).
- Divide by zero shows zero and “C” in the far left (eleventh) digit and is not recoverable.
- The change sign can be used in mid-number entry.
- Square roots of negative numbers are not allowed and result in the answer and “E” in the far left (eleventh) digit, recoverable using (C), which gives you a negative number.
- A switch changes the fixed decimal mode from 0, 1, 2, or 4 decimal places.
- Another switch selects between floating decimal [F], round down [CUT], and round up/down [5/4].
- Memory store is indicated by the decimal point of the far left (eleventh) digit.
- (MRC) will clear the memory with the second press.
- The delta percent key acts as a mark-up key.
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The Casio J-20 calculator, while lacking some advanced features, offers a solid set of basic functions and a sturdy, well-designed case, making it a reliable desktop companion for basic calculations.