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Calculators: Desktop: Busicom 121DN

(c) Emil Dudek 2011

Size (approx):

180mm x 210mm x 52(max)mm (w,h,d)
Weight 866g including cable and plug


Mains (220-240V 50/60Hz, 5.0W) power only through a fixed mains cable of considerable length.
Display: 12+symbol digit green VFD: the symbols (to the far left) are used for memory, error and negative indication. 
Features: Standard four functions with selectable floating/fixed decimal modes, switched rounding modes, switched constant, four function memory, register exchange, percentages, square root, hundreds entry and delete. Semi-RPN logic.
Age: est 1979-1981
Manufacturer: Busicom, made in Japan, serial number 7915112


A sturdy, medium-sized desktop from the later 70s which has a few extra useful functions.  The logic is quite robust with good recovery and no bugs.

Case and Design: Of three-piece construction made from matt light cream and dark brown plastic, which is quite solid.  The top of the case has a small cooling grid to keep the dust out.  A slightly inset and tilted green plastic display filter gives a bright and high contrast display.  This has a brown plastic escutcheon. The upper area has a black printed metallic sticker in its own recess with the model name and number.

Keys and Switches: The keys in orange, light grey and (very) dark grey keys are understated and simple. They are  long travel with a soft clunk on pressing and return. The function switches have raised lettering that are highlighted in black.
Construction: Remove the two screws from the rear.  Now things get difficult, so if in doubt, don't bother trying to open it.  There are two sturdy lugs on either side which are released by pushing in the rear (brown) section.  Once these have popped, there are two more on the lower edge which are very difficult to pop in the same way, some hinging of the case front is required.  The top section will eventually lift off. 
Boards: The whole calculator assembly sits in the rear section.  The sturdy plastic keyboard assembly (Inspector No. 2A. 147772 E T ukasa) sits loose on top of the main board and is held in place by plastic guides. It is joined to the main board by a 17-way ribbon cable.  This can be gently lifted away by sliding up and off.  The main board (211001241) is attached to the rear with plastic guides and holds plastic brackets for the display. A transformer has its own metal bracket mounting.  My particular example had the transformer plate lose as the two screws had come out of their weak mounting holes, this meant that the mounts were not earthed, so if your example rattles - check before plugging it in.
Components: 1 x cpu: NEC D1220C  K0X476, 42 pin DIL, 0.6" width black plastic
1 x 12 digit + symbol green VFD display, single tube flat glass face NEC FIP13D10
2 x transistors
12 x diodes (on main board)
6 x diodes (on the keyboard)
6 x capacitors
5 x resistors
1 x voltage converter F41N
1 x fuse
1 x mains transformer (850801-4B)

Logic comments: (C) is used to clear the calculator and (CE) as a Clear Entry function
Input overflow is not suppressed: keying a thirteenth digit results in an overflow that is recoverable using (->)
Negative numbers are shown with a minus in the immediate left digit, travelling in to the (far left) symbol cluster thereby allowing full twelve-digit negative numbers
There is an extra numerical key (00) for entering hundreds
There is switched (K) constant for divide and multiply only
Fixed/Floating decimal modes are selected by a multi-position switch for 0,2,4,F  or A.  The latter gives financial input that is displayed during number entry i.e. entering (2)(3)(4) is shown as "2.34"
There is a rounding switch for round down (v) and round (5/4) and up (^)
The logic is semi-RPN (Reverse Polish Notation) to do 2-3 key in (2)(+=)(3)(-=)
Negative square roots are not allowed and show the result with overflow which is recoverable using (CE)
The change sign key (+/-) can only be used at the end of number input
Divide by zero shows "0."  and "<-" in the far left symbol cluster and is recoverable using (CE)
Overflow shows the result and "<-" in the far left symbol cluster and is recoverable using (CE)
The (EX) keys allows to to swap primary and secondary registers
Memory storage is indicated by an "M" in the far left symbol cluster
There is switched memory auto-accumulation mode  "S" (sigma), which only accumulated results from a multiply or divide
Overflow in memory is flagged, is recoverable using (C) and maintains the original number
No bugs found