The Soemtron ETR 220 Calculator

Soemtron ETR 220

Manufactured from 1966 to 1977 by V.E.B. Büromaschinenwerk Sömmerda, 155,142 Soemtron ETR 220's were built during the eleven year production run. With six functions, the processing logic of the Soemtron 220 calculator 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 Recall/Clear, Recall, Add and Subtract 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 15 digit display of Z750M "Nixie" tubes is multiplexed and uses a supply of around 180Vdc.

Soemtron ETR 220 core store

The Soemtron ETR 220 has a four plane (1248 BCD) "Ferritkernspeichern" or magnetic core memory 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 eleven year production run for the 220, 7.4Mb of magnetic core storage or nearly 60,000,000 miniature ferrite cores were hand threaded onto thousands of kilometers of copper wire.

The construction of very early model Soemtron 220's is reversed to those models we have in our collection. Confirmed by retired engineers from the Sömmerda factory in a recent visit to Germany, very early machines had a reversed board order, I.E. board 1 (memory) was in the middle of the machine close to the power supply, with board 12 (display driver) being on the outer left hand side. The power supply was also slightly different as the main series pass regulator transistor was internally mounted on a heatsink panel rather than on the base of the machine as in later models. It would appear that the reasoning for these changes stemmed mainly from the sensitivity of the Ferrite core memory to temperature and magnetic fluctuations caused by the power supply. Moving the regulator transistor to the base gave a simpler assembly and better operation as it moved a heat source to the bottom of the case away from the interior of the unit. See the 220 Gallery for photos of Philipp Maier's early reversed build Soemtron 220 S#27027 from around 1967-68.

Early model Soemtron ETR 220

Decimal point selection is by a 12 position thumbwheel switch to the left of the main numeric keypad which is indicated on the display by small incandescent lamps between each Nixie tube, with a range of decimal point selection signals being fed back to the logic circuits for use during calculations. This makes the Soemtron ETR 220 a "fixed point" machine rather than the usual "floating point" form found on modern calculators. Very early versions of the 220 did not have the thumbwheel switch but instead used a row of illuminated switches on the front panel just below the display window (picture right).

Circuit and logic diagrams of the Soemtron ETR 220, can be found on the Downloads page, or with each technical description below. Our original Soemtron 220, missing it's case and the vital power supply has a serial number of 105669, which we believe places it roughly two thirds through the production run of the Soemtron 22x series. This is reasonably backed up by the date codes found on the boards of between 17/73 to 22/73. We calculate serial number 105669 as being week 18 of 1973, presuming this date code is actually week and year. The second and complete, but damaged 220 is serial number 54555, so we think should have been made in about week 41 of 1969, we'll have a look and let you know !. The base panels of the Soemtron 220 and 222 all seem to be the same, so we think the serial numbers were assigned to the bases before the machines were actually assembled, this theory seems to fit with the serial numbers and date codes of the three machines we have, although admittedly it is a rather tenuous assumption.

The re-use of circuitry and clever logic design of the Soemtron 220 (carried over to the 222 and 224 models) produced a first generation electronic calculator that gave maximum functionality with the minimum component count. Microprocessors were not yet commercially available, more functions = more components = more cost. Design re-use was the norm, this might have given the end result some "quirkiness", Example - dual use of the Z counter for display and calculation give the Soemtron 220 and 222 calculators their distinctive "ripple" effect during larger calculations. Further gating or another counter would have meant a cleaner display but at the expense of more logic and components, increasing costs. A lesson probably lost in modern designs where the maxim of - throw more logic at the problem - is the norm, more is not always best.

In 1967 the Soemtron 220 was priced at £485.0.0d (about £6,600 at 2008 values !) and was described as - "the new SOEMTRON 220 . . . . in fact one of the least expensive machines on the market at this specification, delivery could be effected within 14 days".

May 2010 - In another trip recently made by Bernard to Sömmerda, a book entitled "BWS Sömmerda", written by Annegret Schüle (ISBN 3-9083931-1-9), was found. Pictures from the book show the 220 production line and test equipment, after a little research we have found the copyright owner and can now let you see the pictures, they are in the main Gallery page. We also picked up an ETR 220 for use as spare parts and a small supply of the rare Daro20 and Daro32 connectors.

Power supply - circuit
The power supply is a fairly conventional series regulated unit, with output supplies of -VE (-12V), BIAS (+12V), MEM (+13.5V) for the core memory circuits and drivers, and a fourth supply which provides approximately +180Vdc for the display nixies. This display potential is actually split into +/- 90V DC rails. Using a standard continental mains input connector for its age, the power supply has an internal input voltage selector for 110, 127, 220 and 240V AC. Our original Soemtron ETR 220 is missing it's power supply so initially we were unable to derive a circuit diagram, with the other three Soemtrons now in our possession, another 220, a 222 and a rare 224, all having their power supplies we now have working samples to design a replacement power supply from.

Keyboard - circuit
The ETR 220 keyboard consists of 32, V23 microswitches capped with Red keys for control and functions, and White keys for numeric entry (see photo above), arranged in 4 logical groups thus -

• Clear and decimal point keys - Clear  Lö , Clear entry  C  and decimal point  , 
• Numeric keys -  0-9 , arranged in the normal order as seen on modern computers and calculators.
• Arithmetic functions - Negate number  -# , Raise to power  Xn , Multiply  X , Divide  : , Add  + , Subtract  -  and Result  = 
• Register controls - for registers  I ,  II ,  III  - Recall with clear  ✶ , Recall  ∇ , Subtract  -  and Add  + 

All signals from the keyboard go through a 32 plug/socket arrangement (PL17/SK17 on the drawings), to the backplane PCB where they are routed to Boards 11, display Anode drivers and 12, display Cathode drivers, for encoding in diode matrices. The Numeric keys  0-9 , clear  Lö  and decimal point  ,  are routed to Board 12 to encode the key pressed into a 4bit BCD code  0-9  &  ,  with a separate signal for the Clear key  Lö . See the Board 12 technical description below for logic and circuit diagrams. Note that the diode encoding matrix does not include diodes for all of the keys pressed, presumably to save on components, and that the codes are encoded by some of the keys acting directly on the diode matrix outputs. All of the other remaining keys are routed to Board 11 for encoding into five control signal lines for the three memory registers and functions. See the Board 11 technical description below for logic and circuit diagrams. As per the numeric encoding scheme, some of the keys act directly on the encoding matrix outputs.

The PCB is marked 05-220-1097-3b.

Display - circuit
The display consists of 15, Z750M Nixie tubes, a power "ON" neon, "negative" result lamp and 10 fixed position decimal point lamps. Each Nixie tube displays the numbers 0-9 in 13mm (0.51") high digits using a supply of 180Vdc at approximately 2mA. Each digit is a stamped metal cathode of the required numeric shape and is commoned onto a multiplexed signal buss derived from Board 12, the display Cathode driver board. Each numerical digit has it's own individual inverter driver on board 12 with a slight difference for the "0" digit due to the Display decimal point logic on Board 4 used during a numerical entry.

Each Nixie tube anode is separately driven from Board 11, the display Anode driver board via fifteen individual drivers, whose control signals are again derived from a multiplexed data buss. The ten Decimal Point lamps are directly switched from the last wafer of the front panel decimal point selection switch via a short cable loom and interconnecting 13pin plug and socket (PL13/SK17 on the drawings). The other three wafers of the decimal point selection switch encode eight signal lines for the main calculator control logic.

Board 12 - Display Cathode Drivers, numeric key encoding - circuit - logic
14, PNP / NPN transistor pairs driving a common cathode multiplexed display buss from an internal data buss. Each pair consists of a PNP transistor inverting and conditioning the data buss signals, feeding an NPN transistor driving the commoned display cathodes, each with an output clamp diode. However the driver for the "1" cathode buss is quite different in that it does not have the inversion and conditioning PNP transistor stage, but drives the "1" cathode buss directly with a solitary NPN transistor. This difference is due to the decoding logic of the "0" and "1" digits and the ability to display both digits simultaneously as a "barred 0" to represent the current decimal point position during entry.

In this particular Soemtron ETR 220 the NPN transistors are type BS02 and the PNP transistors are type GS121. Board 12 also contains that part of the keyboard encoding circuitry concerned with the  0-9  numeric keys (BCD 0-9), the clear  Lö  (BCD 15), and decimal point  ,  keys (BCD 12). The numeric keys  0-9  and the decimal point  ,  are encoded into a 4bit BCD word with a separate signal line for the clear  Lö  key.

The PCB is date coded 22/73, marked 05-220-7012-3 0h on the component side and 3044 05-220-7012-3 0f on the solder side.

Board 11 - Display Anode Drivers, function key encoding - circuit - logic
15, NPN transistor pairs with addressing matrix and signal conditioning. Each Nixie tube Anode is controlled by a four input diode gate decoding eight BCD control signals (Z1, Z2, Z4, Z8 and their complements) from the Z counter on board 10, to successively enable each tube in turn for a set period, to coincide with data signals on the display data buss. The NPN transistors are type BS02. Board 11 contains the remaining keyboard encoding sections for the function and register key groups, see the board 11 logic and circuit diagrams for the encoding scheme.

Pressing any of the function keys generates a strobe signal ANYKEY, with any of the following signals :-

• Pressing any key of the three sets of register keys generates a two bit binary encoded register address SELREG1 and SELREG2.
• The four register functions  + - ✶ ∇  generate KY(ADD), KY(SUB) and ANYRGX (recall with clear), with all three signals absent being the register function  ∇  or RECALL.
• The remaining seven arithmetical functions  + - X : = -# Xn  are also encoded within the matrix on board 11 into KY(ADD), KY(SUB), KY(MUL), KY(DIV), KY(EQU), KY(NEG) and KY(RAISE) respectively.

The PCB is date code 20/73, and is marked 3043 05-220-7011-5 0g on the solder side.

Board 10 - 5 bit display "Z" counter - circuit - logic
Board 10 contains five cascaded flipflops connected as a five bit BCD counter. Each flipflop is constructed from pairs of cross coupled PNP transistors with associated control gating and signal buffering and pulse circuitry. Its function is to generate a four bit BCD count (0-15) for the nixie display and a five bit count (0-31) for calculations.

Complex input gating to the Z counter determines the initial state and operating conditions of the Z counter during calculations, but for display purposes, the counter purely cycles through a repetitive 1 - 2 - 4 - 8 sequence deriving the signals for the multiplexed display anode drivers on Board 11. This dual use of the Z counter for both display multiplexing and calculations give the Soemtron 220 and 222 calculators their distinctive "ripple" effect during larger calculations especially multiplication and division. Further gating or another counter would have meant a cleaner display but at the expense of more logic and components, increasing costs.

The individual flipflops on this board have test points labeled MPx, with MP being "MessPunt" in German. The flipflops are numbered with test points MP9, MP17, MP42, MP56, MP58, for flipflops are Z1, Z2, Z4, Z8 and Z16.

The PCB is date coded 21/73, and marked 3042 05-220-7010-7 f on the solder side.

Board 9 - F2,5-6 function latches, including register addressing - circuit - logic
Board 9 contains four flipflops, again constructed from pairs of cross coupled PNP transistors with associated control gating, signal buffering and pulse circuitry. On the logic diagram they are labeled F2, F4, F5 and F6. Part of board 9's function in this Soemtron ETR 220 has been ascertained as the two bit binary addressing for the three temporary memory registers (MI, MII and MIII) using flipflops F4 and F5, F5 having the test point label MP10 for bit two of the addressing scheme. Of the remaining two flipflops, F2 and F6, are unmarked on this board but their functions seem to be temporary flags for the Add, Subtract, Multiply, Divide, -# and Xn functions. This board also contains keyboard debounce and delay circuitry and master reset (Lö)circuitry for the flip-flops. There are a further three AND gates and a 21 input AND-NOR gate, currently all of undetermined function.

• F2 - Active during Add, Subtract and Divide.
• F4 - Temporary register address 1.
• F5 - Temporary register address 2.
• F6 - Active during Multiply, Divide, -# and Xn.

The PCB is marked 05-220-7009-2 0e on the component side, and 3041 05-220-7009-2 0h on the solder side.

Board 8 - F1, F3, VER and MZ function latches - circuit - logic
This board contains four flipflops constructed in similar manner to the other boards. Flipflop MZ is the Negative indicator flag driving the display negative lamp directly. The remaining three flipflops on this board are unnumbered and their functions are yet to be determined. There are a further two AND gates currently all of undetermined function.

The PCB is date coded 22/73, marked 05-220-7008-4 0f on the component side, and 3040 05-220-7008-4 0f on the solder side.

Board 7 - B1-4, VOR and R function latches - circuit - logic
Board 7 contains six flip-flops constructed from pairs of cross coupled PNP transistors with associated control gating, signal buffering and pulse circuitry. The flip-flops labeled B1-4, VOR and R on the logic diagram appear to be for the following functions - Add, Subtract, Multiply, Divide, Equality and raise to power functions and two as yet undetermined functions.

• B1 - Stores a pressed Add or Subtract key during entry.
• B2 - Stores the pressed Multiply key during entry.
• B3 - Stores an pressed Divide key during entry.
• B4 - Stores a pressed = or Xn key during entry.
• VOR - (function not determined).
• R - (function not determined).

There are a further nine gates of varying complexity ranging from single diodes to a four input AND-NOR gate, the functions of all these gates are yet to be determined.

The PCB is date coded 22/73, marked 05-220-7007-6 0g on the component side, and 3039 05-220-7007-6 0g on the solder side.

Board 6 - Control gating - circuit - logic
Board 6 contains seven major groupings of AND-NOR and gates whose functions have not yet been determined. In all cases between one and eight multiple input AND gates feed a corresponding OR gate followed by one or more inverter buffers deriving the OR and NOR functions, one gate has multiple buffered outputs.

The PCB is date coded 21/73, marked 05-220-7006-8 0g on the component side, and 3038 05-220-7006-8 0e on the solder side.

Board 5 - V, SCHRIEB, S1 function latches and 25Khz clock - circuit - logic
Board 5 would appear to contain three latches and various OR/NAND gates similar to Board 6, it also contains the 25KhZ system clock generator and master system clock flipflop. The function latches are labeled V, SCHRIEB and S1 on the logic diagram. The function of V and S1 are not determined yet but SCHRIEB is WRITE in English.

• V - (function not determined).
• S1 - (function not determined).
• SCHRIEB - Read / Write flipflop.

The PCB is date coder 21/73, marked 05-220-7005 1g on the component side, and 3037 05-220-7005-1h on the solder side.

Board 4 - Arithmetic unit E Register (input), Carry latch and digit drives 0/1 - circuit - logic
Board 4 contains five flip-flops constructed from pairs of cross coupled PNP transistors with associated control gating, signal buffering and pulse circuitry. Four of these flip-flops are the "E" register (Eingang or input) of the Arithmetic unit and are BCD encoded E1, E2, E4, E8, used during numeric entry and as part of the arithmetic unit in calculations. The fifth flipflop is the arithmetic unit's "Carry" function labeled on the logic diagrams as UBER for "Ubertrag" or carry.

• E1/E8 - Numeric code during entry, Arithmetic unit.
• UBER - Arithmetic unit carry.

Decoding logic for the special "barred 0" or "zero one" digit used to represent a decimal point in the display during a numerical entry is also on this board along with other display and control related logic.

The PCB is date coded 17/73, marked 05-220-7004-3 0h on the component side, and 05-220-7004-3 0i on the solder side.

Board 3 - Arithmetic unit A Register, digit drives 5-9 - circuit - logic
Board 3 contains four flipflops forming the "A" register (Ausgabe or output / result) of the Arithmetic unit and are BCD encoded A1, A2, A4, A8. The board also has five logic gates for decoding the digit signals for the 5, 6, 7, 8 or 9 digits in the display, and the decoding logic for the "C" buss, C1, C2, C4 and C8, part of the arithmetic logic unit.

The PCB is date coded 22/73, marked 05-220-7003-5 0e and on the component side, and 3035 05-220-7003-5e on the solder side.

Board 2 - Core memory drivers, Read, Write and Inhibit decoding - circuit - logic
This board contains twenty of the twenty two memory drivers need to operate the core memory of a Soemtron 220. Each driver consists of a decoding logic gate, pulse amplifier, isolation transformer and driver transistor which then activate a corresponding wire in the core matrix according to the required function. A 4uS monostable on the board generates timing signals T and /T which fed to all the driver input logic gates to ensure the correct pulse widths of the signals applied to the core matrix.

The PCB is date coded 22/73, marked 4638 05-224-7002-5c on the solder side. This pcb numbering therefore has this board being originally designed for an ETR 224 !.

Board 1 - Core store, read amplifiers and digit drives 2-4 - circuit - logic
Board 1 contains the display digit drivers when showing either a 3, 4 or 5 digit in any of the fifteen digit position of the display. Read amplifiers HV1, 2, 4 and 8 are also on this board, wired directly to the read wire connections in each plane, they convert the resultant emf seen on a read wire from a read operation. Two more core memory drivers are on this board, BL2 and BL4, which perform the "blocking" or inhibit functions for core planes 2 and 4, bits 2 and 4 of a BCD 1248 word.

The PCB is date coded MAY 73, marked ETR 220 on the component side, and 3033 05-220-7001-0 0e on the solder side. It appears that this board was used across the whole range of Soemtron 22x electronic calculators.

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