Casio FX-20

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Calculators: Handheld: Casio FX-20


		Size (approx):	74.5mm x 132mm x 24.6mm (max) (w,h,d) Weight 124g excluding batteries.
		Power:	3.0V DC, 2 x AA size batteries, 0.25W. Also accepts an adapter (AD-2S) through a top side socket on the far left. Two alkaline batteries give about 20 hours continuous operation, manganese about 8 hours. The on/off switch is on the right hand side just above the display.
		Case:	Two-piece glossy black plastic case the front of which is dominated by a wrap around brushed aluminium panel. This panel is printed with black text for the brand and model number and scientific key labels. The top of the front has a fine ridged pattern. The neutral plastic display filter is slightly raised but flat and gives a nice bright image. They keys are typical Casio, squishy but positive and each one has its own plastic escutcheon.
		Display:	8 digit blue VFD with a ninth digit for negative indicator. Small zero notation (see below)
		Features:	Four standard functions, square root, change sign, powers, trigs, logs, sexagesimal conversion. Three function memory. 8 digit standard and 6+2 scientific display mode.
		Age:	1975/6
		Manufacturer:	Casio Computer Co. Ltd. Made in Japan. Serial number 9025426 on a sticker in the battery compartment and 2218622 on the inside of the door.
		Comments:	Fabulous quality calculator from a famous range. As they got it so right, this layout remained for years - well into LCD calculators. Sound enough logic - but annoying limited mantissa and error recovery. The original open top, black soft plastic case with Casio logo embossed on front and made in Japan on the back.

Components:	1 x cpu: Hitachi HD3678B 5K33 (October 1975), 28 pin DIL, 0.6" width black plastic 1 x 9 digit VFD display, round faced single tube; NEC LD8124 0 x transistors 0 x diodes 7 x capacitors 3 x resistors 3 x resistor arrays On the mini-board: 2 x transistors 5 x diodes 2 x capacitors 2 x resistors 1 x transformer
Boards:	The keyboard assembly (B8C-E4B) is fixed to the front by five screws and attached to the cpu board (B8C-1A) via 14 strong wires in two bunches. There is small extra aboard on the main one with the power regulator module.
Construction:	Remove the single screw for the battery compartment. Still quite difficult to open. Open the lug visible to the left of the compartment by gently levering out the rear plastic section. There are two lugs on each side that are very difficult to pop as the plastic sides are so stiff. Warning - very likely to damage it so be very careful.

Logic comments:	The clear key (C) is used to clear an input number, the all-clear (AC) to completely reset the calculator.
	Overflow on the input of a number is suppressed, typing in a ninth digit is ignored
	The constant function is invoked for multiplication and division by a double press of the operator.
	There is no indication of memory use - you have to remember it.
	The negative sign is to the immediate left of a number, it can push to the ninth digit to allow 8-digit negative numbers. However, in scientific mode if you have a six digit mantissa which you make negative, you lose the least significant digit as all nine digits are already being used.
	On overflow the display just shows "E" in the far right (first) digit and is not recoverable.
	On divide by zero the display just shows "E" in the far right (first) digit and is not recoverable.
	Negative square roots are not allowed and result in an error as above
	Oddly enough the scientific display mode is limited on input; if you input a greater than six digit number, you cannot enter an exponent.
	The change sign function can be used in mid number entry
	Scientific display mode shows six (or five if negative) digit mantissa and two digit (positive or negative) exponent between 10³⁹ and 10^-39.
	The (M+=) key is called the automatic accumulate to memory key; it obtains the answer in four functions and accumulates it into memory positively
	The Sexagesimal key will convert from DMS to decimal, but not the reverse.
	No other bugs found


This image shows the "small zero" notation that apparently made it easier to distinguish numbers if a few segments of the display were out. This format was common in earlier Casio and Sperry calculators, but rare past the mid-1970s.	With the rear of the case removed you can bend out the main cpu board for examination. Notice the cut-out hole for the glass display tube vacuum seal, which in this case is not needed as the pip lies above the board. You can also see the mini-board to the left with the high voltage power supply components on for driving the display. The keyboard assembly can just be seen at the bottom if the picture.	One colour black 20pp booklet in English only. 75mm high by 104mm wide. Quite detailed with a few good of examples of scientific calculations Printed in Japan