Casio J-20

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Calculators: Handheld: Casio J-20

		Size (approx):	100mm x 164mm x 30mm (max) (w,h,d) Weight g excluding batteries
		Power:	3V DC, 2 x AA size batteries. It accepts an adapter (3V DC 0.25W, type AD-2S) through socket on top side to the far left.
		Case:	Large slightly wedge shaped case made from two pieces of mixed matt and gloss black plastic. The raised and tilted display area has a neutral plastic filter, framed in black. On this is printed white text for the brand and model number with a border. The brushed aluminium keyboard surround is printed black and silver with reversed out text for the switch labels. The large keys are squashy with a return click and individual key escutcheons, typical of Casio.
		Display:	10 digit blue VFD with an eleventh digit for negative, error and memory indication. There are upper tick marks for the thousand separators
		Features:	Standard four functions with percentage, differential percentage, square root, change sign and four function memory. Switched decimal points and rounding mode.
		Age:	1985
		Manufacturer:	Casio Computer Co., Ltd., made in Japan, serial number (or part number?) 5P212A printed on the battery compartment cover.
		Comments:	More at home on the desk this big chunky calculator just fits in the hand. The logic is better than the other of this range (the Casio J-10) with no major bugs and only let down by input overflow and lack of constant on addition and subtraction.

Components:	1x cpu: Hitachi HD38401A 5J45 (date code Sep 1985), 36 pin DIL, 0.6" width black plastic 1 x 11 digit VFD: single tube flat face: FIP 11H5 No.4-1 8 2 x transistors 7 x diodes 8 x capacitors 4 x resistors 1 x transformer: TDK TC-81 59
Boards:	The main cpu board (G862-1 A36267-1) is held by a screw to the rear of the case and joined to the front-fixed keyboard assembly via a 24-way ribbon cable.
Construction:	To open the calculator remove the two screws from the rear. This does not really help as the case is held together by internal lugs. The easiest way I found was to lever out the rear at the side edge and pop the lug. After popping the side lugs, hinge from the bottom. Be very careful as damage can easily result. Eventually the front will lift off.

Logic comments:	(C) is used to cancel the last number entered and (AC) to clear the whole calculator.
	Input overflow is not suppressed, inputting an eleventh digit in an overflow that is not recoverable but (C) will allow you to input another number in a chain
	Negative numbers are represented by a “-“ in the far left (eleventh) digit thereby 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 and is recoverable using (C) which gives you a negative number
	There is a switch, which 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 like a mark-up key: (9)(X)(6)(∆%) will give “9.54” which is nine marked up by six percent, to discount 6% key in (9)(X)(-)(6)(∆%) to give “8.46”.

	The box measures 108mm by 180mm and is 40mm deep. It is therefore just big enough to accommodate the calculator in its case with manual and nothing much else. Of double tuck-end construction it is printed in blue and gold. There is a banner stating the number of digits – but having a model called J-20 with 10 digits is a little confusing		This image shows the calculator with the front removed, exposing the inside of the battery compartment and main cpu board. Quite a high component count for such a late date. Plenty of space inside for a nice big board and well separated components. The high number of link wires are also a surprise. You can just see the green ribbon cable at the bottom which connects to the keyboard assembly.