Litton Monroe M1220

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Calculators: Desktop: Litton Monroe M1220

	Size (approx):	180mm x 210mm x 51mm(max) (w,h,d) Weight 720g excluding cable, quoted net weight is 900g.
	Power:	Mains (220V 50-60Hz, 5W) power only through a permanently fixed mains cable of considerable length.
	Display:	12 digit green VFD (Digitron) with an additional symbol cluster for overflow, negative and memory indication. Upper commas are used for thousand separators, lower commas not used.
	Features:	Standard four functions with percentages, square root, register exchange and four function memory. Switched constant and rounding functions. Switched fixed/floating decimal notation. Semi reverse polish notation (arithmetic).
	Age:	1980
	Manufacturer:	Monroe Systems for Business, a division of Litton Business Systems Inc., Litton Industries, Morris Plains, N.J., USA. Serial number 000812, made in Japan.
	Comments:	The logic is very good if a bit archaic with it semi-RPN mode. It does have an odd "retained negative zero" bug though. Nice bright display with large digits.

Case and Design:	Main body is made from two-piece matt finish light grey plastic with a third matt grey section for display surround. This surround contains a brown printed white label that sits in its own recess with the brand and model number. The heavily angled green plastic display filter is inset. The switch labels are embossed and painted brown.
Keys and Switches:	The nicely grouped keys are rather bland. Big clunky keys are long travel with a hollow sound and very easy to use. A useful (00) key for big number input is completed by easy to use switches for fixed/floating decimal points, rounding options and constant selection.
Construction:	Remove the two screws from the underside. There are then a series of lugs that need to be popped to separate the two halves. The ones on the lower edge are very difficult so be very careful not to cause damage. The front section will then come away, hinging at the bottom edge.

Boards:	The keyboard assembly (Tu Kasa EA229586) sits lose on plastic guide posts and has six diodes on it. A 16-way ribbon cable joins to the main (small) cpu board (211001241) which is fixed to the rear with one screw. Notable for what appears to be named QA stickers on both boards.
Components:	1 x cpu: NEC D1220C K0Y086, 42 pin DIL, 0.6" width black 1 x 12 digit plus symbol cluster VFD unit: NEC F1P13D10A No21 2 x transistors 11 x diodes (plus 6 on keyboard) 6 x capacitors 5 x resistors 1 x transformer: F41N 1 x mains transformer: 850801-4B 1 x fuse

Logic comments:	(CE) is used once to cancel the last number entered and (C) to clear the whole calculator.
	Operation is semi-Reverse Polish notation (arithmetic) input. To do 5-3 key in (5)(+=)(3)(-=)
	Input overflow is not suppressed, keying a thirteenth digit results in an overflow which is recoverable using (CE) or the right shift key (®)
	Constant function is switched in with the "K" switch and operates on multiple and divide only. The constant is the first entry on multiply and the second entry for division.
	Negative numbers are shown with a minus in the immediate left hand digit, travelling into the far right (thirteenth) symbol cluster thereby allowing full twelve digit negative numbers
	Divide by zero shows zero and "¬" in the far left symbol cluster and is recoverable using (CE)
	Overflow shows the result shifted and "¬" in the far left symbol cluster and is recoverable using (CE)
	Memory store is indicated by a "M" in the far left symbol cluster
	Use (RM) to recall the memory and (CM) to clear the memory
	A useful (00) key allows you to enter hundreds quickly
	Overflow in memory displays the shifted overflow but retains the original result and
	You can switch between round down, round up and round options by using the [¯ 5/4] switch
	You can switch between floating [F] or 4,2 and zero fixed decimal digit modes and [+] which is financial input: keying in (1)(2)(3) results in "1.23"
	(EX) is a register exchange key swapping the dividend and divisor registers
	Negative square roots are not allowed and show the result and "¬" in the far left symbol cluster and is recoverable using (CE)
	It suffers an odd form of negative zero bug: with [K] switched on key in (1)(-=)(/)(1)(0)(+=)(+=)(+=) etc to eventually give "0.". Now if you use square root you will get an error as it thinks you want the square root of "-0." even though it does not display this.

The image left shows the manual which measures 147mm by 210mm and is printed in black only with a half-tone cover. With 16 printed pages in total it has plenty of well spaced out instructions and examples.

Interestingly it refers to the decimal switch setting as "A" (and calls it Add Mode) rather than "+" as exhibited on the calculator and front cover picture. It also shows its American roots by stating "...punctuation is automatically set for dollars and cents.."

"Litton Monroe, Systems for Business, International Division, The America Road, Morris Plains, New Jersey, U.S.A. Printed in Japan, 2261-SF".