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Bohn Omnitrex 8 Electronic Calculator

Updated 9/4/2012

On the surface, this calculator didn't seem to be anything too exciting as an addition to the museum, but after receiving it and looking it over, it turned out to be quite a surprise. When I first saw the calculator available, I thought it was a fairly generic vacuum-fluorescent display calculator from the early 1970's. However, there was an intuitive feeling about the machine that triggered an interest, so I decided to acquire it. Once the machine arrived, I pretty much figured I had got what I expected: A neat little machine, with an unusual flip-up display panel, interesting styling, but alas, nothing terribly stunning. Then, when I opened the machine up, low and behold, I found out that it wasn't quite what I had expected. The machine turned out to be a late 1960's design made Japanese business machine company, utilizing early small-scale DTL integrated circuits and a small core memory array for storage of working registers. Needless to say, I was quite happy that I had gone ahead and acquired the machine for the museum.

Profile view of Bohn Omnitrex 8

The Omnitrex 8 was marketed by Bohn Duplicator Company, of New York City, New York. The history of Bohn Duplicator Co. is somewhat of an unknown to this author. The company is known for having sold various types of office equipment including spirit duplicators, copying machines, and mechanical calculators (an example being the Bohn Contex). Like many manufacturers of mechnical calculating machines, it appears that Bohn was caught off-guard by the explosion of electronic calculating machines in the late 1960's, and had to find someone with electronics expertise to design and manufacture electronic calculators for them.

The ID Tag on the Bohn Omnitrex 8

Enter Eiko Business Machine Co., Ltd. in Japan. As with Bohn, the history of Eiko is unclear, but it is known that the company started in the business machine arena with electro-mechanical adding machines and cash registers, and later shifted over to electronic technology for their calculating devices. Most of Eiko's late 1960's electronic calculator designs were sold through OEM (Original Equipment Manufacturer) or distributorship agreements with other companies, such as Bohn and SCM, as well as through Eiko's own brand name, Unitrex.

The Unitrex IC 8
Image Courtesy Serge Devidts

Eiko's version of this machine was marketed directly in Japan and other countries under the Unitrex brand as the Unitrex IC 8. At the time this exhibit was prepared, it is not clear if the Unitrex branded calculator was sold in the North American market.

A variation in color theme of the Omnitrex-8

Along with the Unitrex brand, the same machine was marketed by a company called Deltek, as the Deltek IC-8. It appears that a number of variations were made on the theme, depending on the date of manufacture and the brand name the machine was sold under. The variations were in the color themes of the machine, with differing keyboard bezel and keycap colors. In fact, the Omnitrex-8 has been observed with differing color schemes itself as shown in the photos.

The Deltek IC-8
Image Courtesy Eric Holma

In the early 1970's, Eiko made a name for it's Unitrex brand in the US calculator market, when they flooded the market with low-cost desktop, and later, handheld calculators that set new low price-points for basic office and home electronic calculators. The Bohn Omnitrex 8 was a result of an OEM agreement between Bohn and Eiko, where Eiko produced the machines and OEM customers such as Bohn simply put their own brand name and serial/model number on the machine and sold it through their own distribution channels. It's very clear that this was the case with the Bohn Omnitrex 8, as the circuit boards inside the calculator have "UNITREX" clearly labeled on each board.

Closer View of Keyboard of the Bohn Omnitrex 8

The Omnitrex 8 also turned out to be a bit of a surprise in terms of how it operates. The calculator is quite unique from a number of standpoints. First, the machine has no decimal point key on the keyboard, nor does it have decimal points in the large vacuum-fluroescent display tubes. A number of other calculators in the museum forsake fractional number input or output for the sake of saving costs, including machines like the Denon DEC-311, and the Casio 121-A. The Omnitrex 8 can provide fractional results, but only for viewing. There is no way to carry any fractional result forward in further calculations. Fractional results, along with results of calculations that exceed the eight-digit capacity of the machine can be viewed by successive depression of the [=] key.

23 Result Display

For example, performing 2 ÷ 3, then pressing the [=] key results in "0" being displayed. Pressing the [=] key again results in "66666666". The machine can display up to sixteen significant digits by two depressions of the [=] key. This functionality can also display results of calculations that are greater than eight significant digits. For example, performing 999999 X 999999 will display "9999" on the display when the [=] key is pressed. Pressing the [=] key again will display "98000001", providing the remaining digits of the product, for a result of 999998000001. The extended precision results are only available for viewing, the extra digits are discarded when performing any successive math operations.

The Unusual Multiplication/Division "Half-Size Zero" Indication

Along with the unusual extended capacity of the Omnitrex 8, there are numerous other interesting operational quirks of the machine. Pending multiplication and division operations are indicated by lighting a "half-size" zero in the left-most digit of the display. This indiciation consumes a digit position, meaning that the multiplier or dividend can not have more than seven significant digits. Any digits entered in excess of seven digits causes the most significant digit to be discarded, shifting the new digit in at the least-significant digit position. The calculator provides no lock-out or indication for input overflow, meaning that it is possible for data entry errors to go un-noticed, resulting in answers that can be incorrect. For example, performing 1 X 12345678 would result in a display of "o2345678" (with "o" representing the half-sized zero "operation pending" symbol at the left end of the display) before the [=] is pressed. A careful user may note the seven digit limitation, but someone in a hurry or otherwise not paying close attention to the display could miss the fact that the most-significant digit ("1") of the multiplier was lost. Pressing the [=] key at this point would provide the result of " 2345678", which, based on the original problem, is quite incorrect.

The Omnitrex 8 doesn't detect or indicate overflow in any way, making it crucial that the user keep tabs on the results the machine gives. Operations that overflow the machine generate results that have any overflows simply discarded. Division by zero does not cause the machine to give any indication, nor does the machine get confused like other machines of similar vintage do. Division by zero simply results in zero.

Unusual Negative Number Display

The Omnitrex 8 indicates negative numbers in an unusual fashion, lighting "-" signs in all digit positions prior to the result. For example, performing 2 - 3 (which would be entered as 2 [+] 3 [-]) results in "-------1" being displayed. The largest correctly indicated negative number the machine can represent is "-9999999", as the "-" indication consumes a digit on the display. If one is subtracted from this number, the result of "10000000" is displayed, but there is no room for the "-" indication. Adding one to this result shows that the sign is properly maintained internally, even though it can't be displayed, as the result shows as "-9999999". The machine does not properly carry a negative number through multiplication or division operations. An example would be performing 1 - 7, then dividing the result (-6) by 2. The correct answer would be -3, but the calculator incorrectly displays 3 as the result. The negative indication of "-------6" (the result of performing 1 + 7 -) is immediately cleared to " 6" when the [÷] key is pressed.

Addition and subtraction on the machine operate adding machine-style, with the operation being entered after the number. For example, to subtract 10 from 20, one would enter 20, press the [+] key, then enter 10, and press the [-] key, with the display immediately indicating the answer of " 10". Leading zeroes are suppressed at all times, making for easier reading of results. Multiplication and division operate algebraicly, with the [=] key finishing the calculation and displaying the result. The machine has an unusual constant capability that retains the number entered before a function is carried out and will re-apply it if no intervening number is entered. This constant function operates on all four math functions. For example, pressing "1" [+], followed by successive presses of the [+] key will increment the dislpay by one each time the [=] key is pressed. Similarly, the multiplier in a multiplication problem is retained and applied to differing multiplicands. For example, entering "2 [X] 6", and pressing the [=] key will result in 12 as expected. Then, pressing "7", and pressing the [=] key will result in 42...retaining the "6" from the last calculation as the constant, and multiplying it by 7.

Bohn Omnitrex 8 Circuit Boards. Note Core Memory Array on right Circuit Board

The Omnitrex 8 uses small-scale DTL (Diode-Transistor Logic) integrated circuits for the logic, and a 6x4x4 (96 bit) ferrite core memory array for storage of working registers. The IC technology used in the machine simply was not of a high enough level of integration to provide for the data storage needs of the machine, and still meet the size and cost requirements for the market the machine was designed to sell into.

Closer View of Core Memory Array

As a result, the IC's make up the calculating and control logic, and the core memory stores the working registers of the machine. This combination of technologies was used in a number of other calculators of the time, ranging from the Busicom-designed NCR 18-2 office calculator, the simple programmable Casio AL-2000, and the highly-advanced Wang 720C. Integrated circuit technology advanced very quickly in the late 1960's and early 1970's, such that the working registers of calculating machines no longer required specialized storage such as core memory or acoustic delay lines, with all of the working registers of a calculator implemented in integrated circuit shift registers or randon-access memory.

The Mitsubishi Small-Scale IC's in the Omnitrex 8

The 88 IC's in the Omnitrex 8 are all made by Mitsubishi, members of their M59xx-series, second generation small-scale DTL IC family. The Omnitrex 8 provides an interesting contrast to another calculator in the museum that shares similar Mitsubishi small-scale integrated circuits, the Brother Calther 412. The Calther 412 uses the first-generation Mitsubishi devices, the M53xx-series. The levels of integration between the M53xx and the M59xx series IC's do not seem to be all that different, all devices contain a few logic gates, or a couple of flip-flops. It seems that the M59xx series may have simply used an improved manufacturing process to reduce cost and/or increase yields. From a logic perspective, the M53xx and M59xx all use DTL/TTL compatible logic levels. Interestingly enough, the Calther 412 does not use core memory for register storage -- it uses registers implemented in the IC's (flip flops connected as shift registers) for all register storage, which partially accounts for the price difference between the Calther 412 ($795), and the Omnitrex 8 ($389). The Calther 412 is a somewhat earlier design then the Omnitrex 8, which also accounts for the price difference, as price competition in the marketplace continually drove prices down over time.

The Omnitrex 8 Less Cabinet

The keyboard of the Omnitrex 8 is a common magnetically-activated reed switch design. Something about the design makes it feel somewhat mechanical. The keys have a fairly short throw as compared to other calculators with similarly-designed keyboards, and the keys do not have as smooth an action as others, but the design seems robust and is very reliable. The keyboard works as well as it did the day the machine is new, with no sign of any wear. The keycaps are made of a hard plastic material, with molded-in key identifiers to prevent wear. The keycap material is very durable, and shows virtually no sign of wear from use during the calculator's life. The keyboard connects into the rest of the machine via a wiring harness that provides the connections between the power supply, keyboard, and the two logic boards.

Detailed View of 10-Segment Display

The display on the Omnitrex 8 is composed of eight individual vacuum-fluorescent display tubes mounted in a pivoting display panel.. A greenish-yellow colored, apparently polarized plastic screen covers the tubes. The display tubes are somewhat unusual, utilizing a 10-segment rendition, allowing the display of ones that are centered within the digit, and the four with a 'tail'. The tubes do have decimal points to the right of each digit, which are not used in this application.

The "Backplane" Wiring Harness

The power supply of the Omnitrex 8 is very conventional, with a small transformer feeding rectifying diodes, smoothing capacitors, then into transistorized voltage regulation circuitry. The power supply takes up the rear section of the machine. A metal plate provides a heatsink base for mounting the transistors used for voltage regulation.

Bohn Omnitrex 12
Image Courtesy Jared Tichy

Eiko offered a very similar machine that provided twelve digits of capacity, added full automatic decimal support, along with a [K] key for entering a constant versus the automatic constant of the eight digit machine. This machine was marketed by Eiko as the Unitrex IC-12, and by Bohn as the Bohn Omnitrex 12. It isn't known if Deltek offered the twelve digit machine. If you have any information about this machine, the museum would love to hear from you. Click HERE to contact us.

The Omnitrex 8 is a rather fast calculator, delivering virtually instantaneous results to all calculations. The most complex multiplication, 99999999 X 9999999 gives a result in barely The displays are left active during the calculation, but the machine calculates quickly enough that about the only noticeable indication that the machine is working is a slight flicker of the display.

Text and images Copyright ©1997-2023, Rick Bensene.

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