ETR 220
The Soemtron ETR 220 Calculator
 Soemtron ETR 220 |
Manufactured from 1966 to 1977 by V.E.B. Büromaschinenwerk Sömmerda (1-2), over 155,000 Soemtron ETR 220's were built during the eleven year production run. Processing logic of the Soemtron 220 calculator is all transistorised, consisting 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 [1]. The 15 digit display of Z750M "Nixie" tubes is multiplexed and uses a supply of around 180V !.
 Soemtron ETR 220 core store |
Decimal point selection is by a 12 position thumbwheel 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 220 a "fixed point" machine rather than the usual "floating point" form found on modern calculators. The Soemtron ETR 220 has a "ferritkernspeichern" or magnetic core memory 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 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.
Circuit and logic diagrams of this Soemtron ETR 220, can be found on the "Downloads" page, or with each technical description below. The logic diagrams and descriptions are a work in progress and at the current rate of progress we expect that this process will probably not be completed until the middle of 2009 !. 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, 222 (and presumably 224's) 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 bit tenuous !.
[1] The function of the "C" and "Lö" keys is somewhat suspect at this time and will require further investigation in the future.
Bernard Green relates that "The Lösch button did on some machines clear the three memories as well as the working registers and that we did some mods to disable the Lö wire through those memory cores".
Thomas Richter notes ".... pressing Lö does not affect the content of the 3 memories"
Power supply - circuit
The power supply appears to be a standard series regulated unit, with we think, four supplies that for now we have called +VE (+12V we think), BIAS (possibly +15V), MEM (possibly +20V) for the core memory circuits and drivers, and a fourth supply which provides approximately +180V for the display nixies. We believe that this display potential may actually be split into +/- 90V DC rails. Using a standard German 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 we have been unable to derive a circuit diagram a yet, we will have to design a new on once we have finished the other circuits. The other two Soemtrons in our possession, another 220 and a 222, both have their power supplies so we now have a sample model to design the replacement power supply from.
| Lö | 7 | 8 | 9 | -# | Xn | I ✶ | II ✶ | III ✶ | |||
| C | 4 | 5 | 6 | x | : | ∇ | ∇ | ∇ | |||
| , | 1 | 2 | 3 | + | - | - | - | - | |||
| 0 | = | + | + | + | |||||||
Keyboard - circuit
32, V23 style microswitches 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.
- Arithmetical 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 Keyboard PCB is marked 05-220-1097-3b.
Display - circuit
The display consists of 15, Z750M Nixie tubes, a power "ON" neon, "-VE" 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 180V dc at approximately 2mA. Each digit is a stamped metal cathode of the required numeric shape, with all of the cathodes being multiplexed on to a common signal buss derived from Board 12, the display Cathode driver board. Each Nixie tube anode is separately driven from Board 11, the display Anode driver board. The 10 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 three remaining wafers of the decimal point selection switch encode eight signal lines for the main calculator control logic. See the Display circuit page (here) for a circuit diagram and logic table.
Board 12 - Display Cathode Drivers, numeric key encoding - circuit - logic
14, PNP / NPN transistor pairs driving a common cathode display buss from an internal multiplexed 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 signal conditioning PNP transistor stage, but drives the 1 cathode buss directly with a solitary NPN transistor.
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 numeric keys 0-9 , clear Lö , and decimal point , keys. 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 marked 05-220-7012-3 0h (22/73) on the component side and 05-220-7012-3 0h 3044 on the solder side and is date coded 22/73. See the Board 12 circuit page for a circuit diagram and logic table of the encoding.
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 (1, 2, 4, 8 and their complements) from the calculators internal logic, to successively enable each tube in turn for a set period, which coincides 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.
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 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 marked 05-220-7011-5 0g 3043 on the solder side and is date coded 20/73. See the Board 11 circuit page for a circuit diagram and logic table of the encoding.
Board 10 - 5 bit display "Z" counter - circuit
Board 10 contains five cascaded flipflops connected as a five bit (Johnson ?) 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. 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, whilst the flipflops are Z1, Z2, Z4, Z8 and Z16. The PCB is marked 05-220-7010-7f 3042 on the solder side and is date coded 21/73. See the Board 10 circuit page for a circuit diagram.
Board 9 - F2,5-6 function latches, including register addressing - circuit
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 its function in this Soemtron ETR 220 has been ascertained as the two bit 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. The remaining two flipflops, F2 and F6, are un-marked on this board and their functions are yet to be determined. This board also contains the keyboard debounce delay monostable and what looks to be some power on reset circuitry. There are a further two OR gates and a 17 input OR-NAND gate, currently all of undetermined function. The PCB is marked 05-220-7009-20e on the component side and is date coded 16/73, the solder side is marked - 05-220-7009-20h 3041. See the Board 9 circuit page for a circuit diagram.
Board 8 - F1/3, VER and MZ function latches - circuit
This board contains four flipflops constructed in similar manner to the other boards. One flipflop has been determined to be the Negative indicator flag driving the display negative lamp directly. The remaining three flipflops on this board are un-numbered and their functions are yet to be determined. There are a further two OR gates currently all of undetermined function. The PCB is marked 05-220-7008-20e on the component side and is date coded 16/73, the solder side is marked - 05-220-7008-20h 3041. See the Board 8 circuit page for a circuit diagram.
Board 7 - B1-4, VOR and R function latches - circuit
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, Multiply, Divide, Equality or raise to power functions and one as yet undetermined function. There are a further nine gates of varying complexity ranging from single diodes to a four input AND NAND gate, the functions of all these gates are yet to be determined. The PCB is marked 05-220-7007-6 0g on the component side and is date coded 22/73, the solder side is marked - 05-220-7007-6 0g. See the Board 7 circuit page for a circuit diagram.
Board 6 - Control gating - circuit
Board 6 contains seven major groupings of OR/AND and OR/AND/NAND gates whose functions have not yet been determined. In all cases between one and eight multiple input OR gates feed a corresponding AND gate followed by one or more inverter buffers deriving the AND and NAND functions, one gate has multiple buffered outputs. The PCB is marked 05-220-7006-8 0g on the component side and is date coded 22/73, the solder side is unmarked. See the Board 6 circuit page for a circuit diagram.
Board 5 - V, SCHRIEB, S1 function latches and 25Khz clock - circuit
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. See the Board 5 circuit page for a circuit diagram.
Board 4 - Arithmetic unit E Register (input), Carry latch and digit drives 0/1 - circuit
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. The fifth flipflop is a Carry function labeled on the logic diagrams as UBER for "Ubertrag" or carry. The board also contains two small gates and four gated drivers. The PCB is marked 05-220-7004-3 0h on the component side, and 05-220-7004-3 0f on the solder side. See the Board 4 circuit page for a circuit diagram.
Board 3 - Arithmetic unit A Register (output), digit drives 5-9 - circuit
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 gates for decoding the Digit drives for digits 5-9, and four gated drivers for signals C1, C2, C4 and C8. The PCB is marked 05-220-7003-? ?? on the component side, and 05-220-7003-? ?? on the solder side. See the Board 3 circuit page for a circuit diagram.
Board 2 - Core memory drivers, Read, Write and Inhibit decoding - circuit
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Board 1 - Core store, read amplifiers and digit drives 2-4 - circuit
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