National Semiconductor 600

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Calculators: Handheld: National Semiconductor 600

		Size (approx):	70mm x 130mm x 11-24mm (w,h,d) Weight 78g excluding batteries
		Power:	9V DC, 1 x PP3 size battery. It accepts an adapter (undefined but 2.5mm plug) through a socket on the top side on the far left.
		Case:	Odd looking two-piece tan coloured smooth plastic case. Whilst the overall shape is wedge-like, the keyboard is flat and just the sides rise up to the top battery holding area. The red display filter is angled steeply at about 45 degrees and gives a clear display. A brown printed metallic sticker sits in its own recess and holds the company name and switch labels. The keys are loud click short travel and work well. There is a reminder panel on the back which is just as well for those who do not understand RPN.
		Display:	6 digit red LED flat dot type with no seventh digit
		Features:	Standard four functions, reverse polish notation (hence the lack of an (=) key) with a fixed two point decimal switch
		Age:	c1972/3
		Manufacturer:	National Semiconductor Corporation, 2921 Copper Road, Santa Clara, CA 95051, USA. Serial No. 051747
		Comments:	Quite early pocket calculator which explains the RPN logic, six-digit display, no decimals and basic functions. Novel sunken keyboard is actually quite nice and the early LED display bright and clean with a better angle of view than the later bubble lens type.

Components:
Boards:
Construction:	I cannot find a way to open this one and don not want to risk damaging - can anyone help?

Logic comments:	(C) can be used to cancel the last numerical entry whilst a second press clears the calculator
	There is automatic constant on all four functions but this is natural for RPN
	Input overflow is not suppressed, inputting a seventh digit causes an overflow which is not recoverable
	Negative sign is shown in the immediate left hand digit but as there is no seventh digit you are limited to six digit negative numbers; the display shows "EE" and the remaining result and is not recoverable.
	Divide by zero (i.e. (9)(+)(0)(/) ) shows "EEEEEE" and is not recoverable
	Overflow is shown by a "E" and the remaining number and is not recoverable
	For those who do not know RPN; 3+4 is (3)(+)(4)(+), 3-4 is (3)(+)(4)(-) and 3x4 is (3)(+)(4)(X)
	It cannot perform real decimal point calculations, there being no (.) key to make any.
	There is a switch marked "Decimal" that changes from floating to fixed two point for financial calculations. It actual fact it just switches on the third digit's decimal point. On switching it displays ". 0"