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Calculators: Desktop: Decimo Vatman 120D

(c) Emil Dudek 2011

Size (approx):

175mm x 210mm x 20-30mm(keyboard), 50mm(max) (w,h,d)
Weight 675g excluding cable


Mains (240V 50Hz, 5W) power only through a permanently fixed mains cable of considerable length.
Display: 12 digit green VFD with additional symbol cluster on the left for memory (M), error (<) and negative (-). The display has upper commas for thousand notation (and unused lower ones).
Features: Standard four functions with percentages, square root, four function memory, register exchange, change sign and double zero entry.  Switches constant, rounding and fixed/floating decimal.
Age: Estimate 1978
Manufacturer: Decimo Limited, made in Japan, serial No. 84108766.  An additional rear sticker has the number 8410.


This late mains-only VFD desktop is smaller than most and dispenses with the more obscure accounting functions.  Still retains the annoying (+=) type keys, however, which take some getting used to and the chunky keys of years before.  Otherwise the logic is sound with good recovery ability.  Probably one of the last desktop VFD mains models.

Case and Design: The base is made from smooth black plastic whilst the top section is a one-piece orange (may originally have been cream) plastic.  The s-shaped green display filter is angled steeply to give a clear bright image. Above the display is a black printed metallic label that sits in its own recess.  

Keys and Switches: They keys are well laid out, large hollow sounding click with long travel.  A row of switches above them switch for decimal point selection (A024F), rounding (down, 5/4, up) and constant (K). The raised labels for these switches are printed black. The on/off switch is on the right hand side, nearly at the top.
Construction: Remove the two screws from the bottom  and the whole front section will come off after popping the two side lugs and hinging off the two bottom lugs - not easy.  Be careful not to lose the switch dust covers as they just sit loosely on the switches and are the only thing that stops them moving off their contacts.  As always with this type of unsealed keyboard, my example had accumulated loads of dust.  The whole assembly is bolted to the rear section with sturdy screws and metal brackets.  

Boards: The main keyboard assembly sits loosely on top of four plastic supports and is joined to the main cpu board via a 17-way ribbon cable.  There are a surprising amount of diodes (six) on the key matrix area.  The small main cpu board (ref 211000821) is attached to the rear with a couple of screws and houses the bracket for the display.  Most of the components are for power supply and display driving.
Components: 1 x cpu: NEC D1220C 8406KY, 42 pin DIL, 0.6" width
1 x 12 digit green VFD display: flat tube with left symbol clusters: Itron FG139N7 TDA Japan
2 x transistors
18 x diodes (6 on the keyboard assembly)
6 x capacitors
5 x resistors
1 x mains transformer: 850801-4
1 x board transformer: F41N

Logic comments: (CE) is used to cancel the last number entered and (C) to clear the whole calculator.
(00) allows more rapid number entry , especially in financial decimal mode (A on the points switch)
Input overflow is not suppressed, inputting a thirteenth digit results in an overflow that is recoverable using (CE)
Constant function is switched in with the "K" switch and operates on multiple and divide only
The rounding switch can select round down (v), round up/down (5/4) and round up (^)
The decimal point switch can be selected between floating "F", fixed "0,2,4" and  financial mode "A" for $x.xx entry
Negative numbers are shown with a minus in the far left symbol cluster allowing full twelve digit negative numbers
Negative square roots are not allowed and display the result and error which is recoverable using (CE)
Divide by zero shows a "<" in the far left symbol cluster and is not recoverable
Overflow shows the result and "<" in the far left symbol cluster and is recoverable using (CE)
Memory storage is indicated by "M" in the far left symbol cluster
The register exchange key (EX) exchanges the answer for the first number entered
The add keys are rather different.  To do 3-4 you input (3)(+=)(4)(-=) or (3)(+=)(4)(+/-)(+=)
There is only one minor bug that I can find, the hidden negative zero bug.  Key in (1)(+=)(2)(-=)(1)(+=) to give "0." as one would expect, however the square root of this zero results in an error.  If you use (+/-) before the root then the correct zero result will show on square rooting this zero.