Soemtron ETR 222 Calculator
S#74005 - Technical Description

Soemtron ETR 222

This ETR 222, our third Soemtron calculator, is complete but with a damaged keyboard.  It appears to be one of only two Soemtron ETR 222's existing that we know of from the original 3,203 units that were built.  It has a serial number of 74005, which we believe places it roughly half way through the production run of the Soemtron 22x series, sometime in 1971 possibly about week eight or nine.  The base panels of the Soemtron 220 and 222's all seem to be the same, so we think the serial numbers were assigned to the bases before the machines were actually built, this theory seems to fit with the serial numbers and date codes of the machines we have, although admittedly this is a rather tenuous assumption !.

Power supply - circuit
The power supply is a fairly conventional series regulated unit, with outputs of -12V for the logic, +12V bias, and +13.5V for the core memory circuits and drivers.  A fourth supply provides approximately +180Vdc for the display nixies.  This display potential is actually split into +/- 90V DC rails.  Using the standard continental mains input connector for the time, the power supply has an internal input voltage selector for 110, 127, 220 and 240V AC.

Keyboard - with the advent of the Soemtron 222 the keyboard layout of the 220 was changed slightly for the new model, with the main differences being -

1. The sign or negate entry key (220 -#) is moved from the central functional key group to the space below the Clear key  C  previously occupied on the 220 by the decimal point key ( , ) - which then
2. moves to a new location under the numeric  1  key left of the now slightly smaller zero  0  key.  The current decimal point position (overlaid 0 and 1 digits to show as a "barred" 0), is now not visible in the display during numeric input.
3. The 220 "Raise to Power" function key ( Xn ) has been removed.  For powers on a 222 the base number X is first keyed in, then for the exponent n, the multiplication key is pressed n-1 times, then the equals key once.  This key position has become the "return constant"  ∇  key.
4. Lastly the key position on a 220 held by the "sign or negate entry key" (-#) becomes the "Exchange operands"  #  key which swaps the contents of both operand registers.

The ETR 222 keyboard consists of 34, V23 microswitches with Blue keys for control and functions, and White keys for numeric entry (see photo above), arranged in 4 logical groups broadly similar to the 220 thus -

• Clear and sign keys - Clear  Lö , Clear entry  C  and negate entry  - 
• Numeric keys -  0-9 , arranged in the normal order as seen on modern computers and calculators, and decimal point  , 
• Arithmetic functions - exchange operands  # , return constant  ∇ , Multiply  X , Divide  : , Add  + , Subtract  -  and Result  = 
• Register controls - for registers  I ,  II ,  III  - Recall with clear  ✶ , Recall  ∇ , Subtract  -  and Add  + 

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