The Soemtron ETR 222 Calculator
Manufactured from 1970 to 1972 by V.E.B.  Büromaschinenwerk  Sömmerda, 3,203  Soemtron ETR 222's desktop calculators  were built during the two year production run. With six functions , the processing logic of the Soemtron ETR 222 calculator, derived from the ETR 220, is all transistorised, consisting mostly of pairs of PNP Germanium transistors coupled as Flip Flops with a few resistors and capacitors, and sets of diodes for gating functions. The keyboard is large and cumbersome, non multiplexed, utilising V23 microswitches with external supplementary springs for the keys themselves. The unit has three memories for intermediate calculations, each with their own set of Add, Subtract, Recall, and Recall/Clear keys, there is also a "C" key which clears the current entry and an "Lö" key which clears everything excepting the three operand stores. The multiplexed display of 15 digits is based on Z750M "Nixie" tubes and uses a supply of around 180V !.
Decimal point selection is by a 12 position thumb-wheel switch to the left of the main numeric keypad and is indicated on the display by small incandescent lamps between each Nixie tube, whilst a range of decimal point selection signals are fed back to the logic circuits for use during calculations. This makes the Soemtron ETR 222 a "fixed point" machine rather than the usual "floating point" form found on modern calculators. Memory on the Soemtron ETR 222 has a "Ferritkernspeichern " or magnetic core memory consisting of four planes (BCD) of 6 x 16 digit registers, a total of 384 bits or 48 bytes, showing the age of the machine from a time when magnetic core memory was THE "modern" technology before the advent of multi-megabyte semiconductor RAM. Over the two year production run for the 222, 144Kb of magnetic core storage or nearly 1,152,000 miniature ferrite cores were hand threaded onto probably hundreds of kilometers of copper wire.
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