Citizen 800D

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Calculators: Handheld: Citizen 800D

		Size (approx):	82mm x 136mm x 25mm (w,h,d) Weight 134g excluding batteries
		Power:	6.0V DC with 4 x AA size batteries. Accepts an adapter/charger (CTZ-800R 6V DC 0.35W, centre negative) through a socket on the top side to the far right. The on/off switch is on the top side, in the middle.
		Case:	Rounded two-piece smooth plastic case of light grey front and dark grey back. A flat, flush , warp-around green plastic display filter gives a good clean image. Just below this is a metallic sticker printed black with the brand and model number; which sits in its own recess. Inset keyboard surround is slightly rougher grey plastic. The subtle coloured keys are soft and squishy with a pronounced bounce-back click and work well enough.
		Display:	8 digit green VFD with a ninth digit for minus and error indicators
		Features:	4 function calculator with percentages, change sign, square root and pi
		Age:	1976
		Manufacturer:	Citizen Business Machines Inc., made in Japan. Serial number 141392
		Comments:	Well built calculator with a bright clear display that is very like some of the Decimo Vatman series et al. The logic is not bad and only really let down by the lack of overflow recovery, negative square roots and the archaic lack of input overflow suppression.

Components:	1 x cpu: NEC µPD940C R5X71M (Late 1975), 28 pin DIL 0.6" width black plastic 1 x 9 digit VFD single tube flat face 2 x transistors 5 x diodes 7 x capacitors 8 x resistors 1 x transformer: DCT-05B
Boards:	The keyboard assembly (800D 940C GK154-2 T-2, GICO Japan) (51.1.26 which is date code 26 January 1976) sits loosely on top of the main board and is joined by 12-way ribbon cable.
Construction:	Quite difficult as it is held together with poppers only. Try using lolly' sticks to lever apart the sides where the two cut-outs are located. Squeeze in the front section whilst levering the two sides apart. Eventually the front section will lift off.

Logic comments:	The (CE) key is used to clear the last entry of a number and (C) press clears the whole calculator.
	Input overflow is not suppressed, inputting a ninth digit causes an overflow which is recoverable using (CE)
	There is automatic constant on all four functions
	Negative numbers are shown by a "-" in the immediate left hand digit travelling into the far left (ninth) digit thereby allowing full eight digit negative numbers
	The change sign function can be used in mid number entry
	An overflow shows the result with a “C” (or “E” if negative) in the far left (ninth) digit and is not recoverable
	Divide by zero results in zero and an "E” in the far left (ninth) digit and is not recoverable.
	Negative square roots are allowed and result in a negative number
	No bugs found

This scan shows the front removed exposing the GICO keyboard assembly on top of the main board. In this case the cpu IC is under the square root key.

Very similar internal design to such calculators as the Decimo Vatman series.