The Soemtron ETR222 Calculator

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Soemtron ETR222, ©2007 Serge Devidts
Soemtron ETR222
©2007 Serge Devidts
Click image for a larger version

Manufactured from 1970 to 1972 by V.E.B.[1] Büromaschinenwerk[2] Sömmerda, 3,203[3] Soemtron ETR222's desktop calculators[4] were built during the two year production run from 1970 to 1972.  With seven functions[5], the processing logic of the Soemtron ETR222 calculator, derived from the ETR224, is all transistorised, consisting mostly of pairs of PNP Germanium transistors with a few resistors, capacitors and diodes coupled as Flip Flops (latches) with further sets of diodes for gating functions.

The keyboard is large and cumbersome, non–multiplexed, utilising large microswitches (thought to be V23's[6]) with external supplementary return 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 a "Lö" key which clears everything excepting the three operand stores.  The multiplexed display of 15 digits is based on Z570M "Nixie" tubes and uses a supply of around 180V.

One plane of a Soemtron ETR222 core store, ©2007 www.robotrontechnik.de, click image for a larger version
Soemtron ETR222 core store
©2007 www.robotrontechnik.de
Click image for a larger version
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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 most of the Nixie tubes giving the impression of a comma, whilst a range of decimal point selection signals are fed back to the logic circuits for use during calculations.   This makes the Soemtron ETR222 a "fixed point" machine rather than the usual "floating point" form found on modern calculators

Memory in the Soemtron ETR222 is a "Ferritkernspeichern"[7] 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 kilometres of copper wire.

To date we have found that very little information exists about the Soemtron ETR222.  Currently we think that possibly only five Soemtron 222's remain in existence -

1.Our machine currently undergoing restoration in the www.soemtron.org collection.
2.There was one at the Digital Computer Museum, a German on–line museum of digital equipment, a photo of their ETR222, however as of July 2020 these links seem to be not working.
3.One owned by Serge Devidts on his website, his ETR222.
4.An email from Steffen Kreuzberg in Germany has let us know of his non–operational ETR222.
5.Early 2020, another email from Manfred Böhm has let us know of his fully working ETR22 (S# 74775).  Manfred has forwarded a few photos and a video of the machine, all can be seen here.  We have since heard that this machine is for sale on Ebay.

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If you have any further historical information, circuits, drawings, photographs, data or manuals about the Soemtron ETR220, 221, 222 and 224 or their manufacture, test equipment (Prüfgeräte) or you just know more about the Soemtron companies in general then please let us know, contact us.


1 Wikipedia entry for V.E.B. = Volks Eigener Betrieb, or Peoples Owned Company V.E.B.[top]
2 Büromaschinenwerk. Office Machine Works.[top]
3 ETR222 production of 3,203 during 1970-1972 Page 389 "BWS Sömmerda", by Annegret Schüle ISBN 39-8039-311-9, ©1995 Desotron Erfurt.[top]
4 Desk top calculator
Calculator.
Tischrechner, rechenmaschine.[top]
5 Functions Return constant, exchange operands, Negate number, Add, Subtract, Multiply, Divide.[back] [top]
6 V23 microswitch Originally thought to be a V23 microswitch variant, the correct style is the ABB or Petercem, M & CM style switches (data sheets ABB, Petercem), but with right angle PCB pins.  Data for these particular right angle PCB mount switches however, does not seem to be available even though they are from the M/CM range.[back] [top]
7 Ferritkernspeichern Ferrite Core Memory.[back] [top]

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