Decimo 9100 Realtone

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Calculators: Desktop: Decimo 9100 (aka Realtone 9100)


		Size (approx):	114mm x 193mm x 45mm (max) (w,h,d) Weight 514g excluding batteries but including strap
		Power:	6.0V DC, 4 x C size batteries. Also accepts an adaptor (Type 2Q11B, 6V DC, 200mA) through a top side socket on the right. Remove adaptor to operate with batteries. There is an auto power off feature to save batteries. Innovative two key on/off system.
		Case:	Large heavy battery-desktop machine that was built to last. The case is made from two-piece mottled black plastic top and bottom and has thick metal panels on the sides, top and keyboard surround. The latter wraps around the bottom and has some black printed text for the switch labels. The top front has a large plastic sticker with the Decimo branding. The area below has a recessed metallic sticker, printed in black and blue, with the Realtone brand and "auto shut off" text. The keys are large, long travel and hollow sounding but work well enough. The deep blue display filter is slightly inset and heavily tilted with white text for the numerals and symbol display. The rear has a large sticker with loads of calculation info. There is a metal lug on the left hand side for the black plastic carry strap.
		Display:	8 digit blue VFD display – with a ninth used for negative, overflow and battery low indicators
		Features:	Standard four function with switched constant, switched fixed decimal, semi-RPN logic.
		Age:	1973
		Manufacturer:	Rear labels states Realtone only, made in Hong Kong. Serial No. 5143160 from a sticker inside the battery compartment.
		Comments:	Solid and heavy calculator with a lovely deep blue display (which has the ninth digit separated by a conspicuous gap) and unusual two-key on/off system. Obviously a re-branded Realtone calculator as they did not even bother removing the branding. The archaic RPN logic appears sound enough but there is no recovery and a strange input overflow state that nearly works. This machine was branded by a number of companies such as Soundesign (8300/Mach II), Morse Electrophonic (Cal-801), Mayfair (2W01B), Kings Point and probably others. I wonder who the real manufacturer was and whether they specialised in supplying office equipment companies. Still, not a bad job for such an early date - lovely!

Components:	1 x cpu; Texas TMS0105NC B7307 (week 07 of 1973) 28 pin DIL, 0.6" width in its own socket 9 x 1 single digit VFD tubes 19 x transistors 22 x diodes 9 x capacitors 2 x variable capacitors 21 x resistors 3 x resistor arrays 1 x transformer
Boards:	The main cpu board (PC-P521) is fixed by five screws over the keyboard assembly and joined by a 20-way edge connector. The keyboard assembly is held to the front with seven large screws with an additional nine smaller screws holding the keys in place. A sturdy metal bracket hols the display tubes in place.
Construction:	Remove the two long screws from the back and the two short ones inside the battery compartment. The rear section then lifts off easily.

Logic comments:	The (CE) key is used to cancel an entry whilst the (C) key clears the whole calculator
	Input overflow is not suppressed as inputting an ninth digit results in "C" (or "E" if negative) being displayed in the far left (ninth) digit which is not recoverable. However, you can still perform a calculation on this number (with the "C" or "E" still showing) but only once; subsequent results appear to be nonsense.
	Negative numbers are shown with a "-” in the far left (ninth) digit, thereby allowing full eight digit negative numbers.
	There is an switched constant on multiply and divide only by using the (K) switch.
	Overflow shows the result and “u” (or "o" if negative) in the far left (ninth) digit and is not recoverable
	Input overflow followed by result overflow will show "G" (or "6" if negative) in the far left (ninth) digit and is not recoverable
	Divide by zero shows zero and “u” in the far left (ninth) digit and is not recoverable
	You can select floating decimal point mode or fixed two and four decimal points by using the 4.2.F. switch
	My example switched off after 1 minute and 5 seconds when you lose the information being displayed. Recovery is only by pressing the on key.
	It operates semi-Reverse Polish Notation logic: to calculate 5-3 key in (5)(+=)(3)(-=) to give "-2"
	It suffers the divide to negative zero bug, key in (with constant switch on): (1)(-=) to give "-1" then (/)(1)(0)(=)(=)(=) etc to eventually give "-0" one more equals goes to "0", showing the extra internal digit of precision.
	When the battery power is low the right hand upper segment of the far left (ninth) digit lights, thereby draining even more battery power

	This scan of the main cpu board removed from the calculator is fascinating. The early date of 1972/1973 is given away easily by the high component count (especially the transistors). Metal mounting brackets, sturdy keyboard assembly and main IC socket shows that no expense was spared. Notice the individual VFD display units with the far left (ninth) digit separated by some distance - a practice that was no copied much. At the bottom left you can see the PCB tracks that connect to the keyboard by the (removed) edge connector.