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Calculators: Desktop: Decimo Vatman Desk Mate (aka 12DM)

(c) Emil Dudek 2011

(c) Emil Dudek 2011

Size (approx):

125mm x 200mm x 38mm (max) (w,h,d)
Weight 328g excluding batteries


6.0V DC using 4 x C size batteries.  It accepts an adaptor (undefined) through a socket on the left side in line with the display.
Case: Made from a two-piece smooth black and light grey plastic case this calculator is large and solid feeling.  The top area is raised in order to accommodate the batteries.  This part has thick printed text for the brand and model name.   A green display filter is tilted at about 45 degrees and give a clean bright image.  It has two silver painted borders and white printed reminder text.  The keyboard is surrounded by a brushed aluminium panel with black switch labels.  The concave keys are long travel, bouncy but do not work well on my example.

12 digit green VFD with a thirteenth digit for negative, memory and error indicators.  

Features: Standard four functions with percentages, change sign, register exchange, square root, reciprocal, squares, powers and four function memory.  Switch fixed decimal point, switch constant and semi-RPN logic.
Age: 1977
Manufacturer: Decimo, made in Japan, serial number 70300578


Chunky desktop calculator that was built to last.  Lots of functions and the logic is OK but let down by the lack of recovery and negative zero bug.  The case looks like it could have the top area punched our for an alternative display position.

Components: 1 x cpu: Mitsubishi M58623-23P 5910, 28 pin DIL, 0.6" width
1 x 13 digit green VFD; single tube flat fronted Futaba 13-ST-02A 7D (April 1977)
1 x transistor
3 x diodes
6 x capacitors
6 x resistors
3 x resistor arrays
1 x transformer P15
Boards: The main board (12DMa 13-ST-02a) sits loose inside guided by four plastic pillars that go through holes in the board.  It is connected to the keyboard assembly via a 18-way ribbon cable.  This unit is attached to the front by 4 plastic lugs and held together with 10 screws.
Construction: Remove the large screw from inside the battery compartment and the front will lift away easily.

Logic comments: (CE) cancels an incorrect numerical entry, whilst (C) clears the calculator
Input overflow is suppressed, typing in thirteen digits ignores the thirteenth
There is switched constant on multiply and divide only
Negative numbers are shown by a minus in the far left (thirteenth) digit thereby allowing full eight digit negative numbers.
Divide by zero results in zero and an "C" in the far left (thirteenth) digit and is not recoverable
Overflow shows zero with "C" (or "E" if negative) in the far left (thirteenth) digit and is not recoverable
Negative square roots are not allowed and show the result with an error flag which is not recoverable
Memory store is indicated by the decimal point of the far left (thirteenth) digit
There is a switch for floating, 0/2/4 fixed decimal points
Semi-Reverse Polish Notation takes a little while to get used to; to do 9-5 key in (9)(+=)(5)(-=) to give "4"
This calculator has the negative zero bug; try (1)(+)(2)(-) to give "-1" now (1)(+) gives "-0"