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Burroughs Model C-3350 Desktop Calculator
Updated 5/8/2004

This calculator is interesting in that it is a mix IC and transistor machine. The machine uses a sprinkling of SSI (Small Scale Integration) IC's made by Mitsubishi. These devices are early TTL (Transistor-Transistor Logic) IC's which make up some of the more complex circuit elements, such as flip flops. But, the vast majority of the logic of the machine is made up of a tremendous number of transistors and diode-resistor gates. Based on date codes on some of the devices in the machine, it appears to have been made in 1970, which is rather late for this type of calculator technology. The machine was originally designed in the 1967 timeframe, but due to decreases in component and manufacturing costs, the machine was able to maintain price competitiveness through the early 1970's, even though calculator technology was progressing at a breakneck pace. The implementation of the C-3350's logic places it in the technology timeline between all-transistor designs such as the Sharp Compet 20, and early calculators that used primarily integrated circuits, such as the Brother Calther 412 and Sharp Compet 16. Burroughs, in fact, did not design nor manufacture this calculator. Burroughs leveraged the calculator expertise of the Japanese electronics giant, Sharp Corporation.

The Facit 1125 and Addo-X Model 9958
Facit 1125 Image Courtesy Rainer Fredrich
Addo 9958 Image Courtesy Andreas Zinn

Through an agreement forged between Burroughs and Sharp (at the time, Hayakawa Electric) in early 1968, Sharp designed and manufactured the major components of some of its calculator designs to Burroughs, providing ready-to-sell machines customized for Burroughs. Along with Burroughs, Sharp also signed similar agreements with other companies. An example of this shows in a similar machine marketed under the Addo-X brand name, as the Model 9958. Addo-X was also a major OEM customer of Sharp, but in this case, Addo-X relied upon Sharp to provide a cabinet very similar to Sharp's cabinetry. Facit, a famous Swedish manufacturer of mechanical calculators, also OEM'd Sharp's machine and marketed it as the Facit 1125. It appears that these machines are a derivation of the design of Sharp's Compet 32 calculator, which utilizes the same technology, and has similar features, with only a small difference in the way that the two memory registers are accessed.

Burroughs C-3350 with Case and Keyboard Removed

The C-3350 is built upon three fairly large circuit boards which span the width of the case. A beefy stamped metal cage provides a backbone for the circuit boards, and it appears that the designers wanted the electronics assembly to be very stable and shock resistant, with many rubber blocks positioned strategically to serve as shock absorbers. The three circuit boards plug into a hand-wired backplane which provides interconnection and power supply distribution. The machine uses small 'pancake' ceramic-cased silicon transistors, identical to those in the Sharp Compet 20, along with other more conventionally-cased transistors with the same part numbers as used in the Compet 20. Three are other parallels between this Burroughs-badged machine and Sharp's early calculators, such as the [X] and [÷] keys which have indicators in them which light up to tell the operator when such a function is in progress.

Circuit Board Detail
Note mixture of Mitsubishi IC, ceramic "Pancake" transistors, and conventionally-packaged transistors

The C-3350 has a number of unusual features. The machine has two memory registers which are based on magnetic core memory (the memory registers retain their content when power is turned off). Each memory register can be added to or subtracted from independently, and there are two keys for recalling the content of a memory register. One simply recalls the register to the display (with the diamond-shaped nomenclature), while the other key recalls the register to the display, and clears the memory register. Two incandescent indicator lamps under yellow jewels (labeled I and II) on the keyboard panel show the status of the memory registers, lighting up when a memory register is non-zero. The astute observer will notice that the machine does not have [+] or [-] keys, only having a two [=] keys, one red and the other white. To be more intuitive, these keys should have been labeled [+=] for the white key, and [-=] for the red key, which would more accurately relate how they are used for addition or subtraction operations. When multiplying or dividing, the white [=] key is normally used to calculate the result, and the red [=] key can be used to negate the result. Another interesting quirk of this machine is that it has a square root function, though the only clue to this function existing is in the model number badge on the keyboard panel of the machine, which has a square root symbol on it. Entering a number, then pressing the [÷] key, followed by the white [=] key results in the square root of the number being calculated. Later Burroughs and Sharp machines actually put a small square root symbol next to the division sign on the [÷] key to show this functionality.

The machine is a fixed-decimal point machine, which isn't uncommon for machines of this vintage. What is uncommon, though, is that the machine has two different settings for fixed decimal point operation. The upper slide switch (labeled CDS) selects for 4 or 6 digits behind the decimal point for numbers on the display, and the lower slide switch (labeled MDS) selects 2, 4, 6, or 8 digits behind the decimal point for numbers stored in the memory registers. This is a slight departure from the single knob used on the Facit and Addo-X versions of this machine. The calculator has a constant function, activated by a push on/push off key labeled [K]. The constant function works only for multiply and divide operations. A key labeled [RC] provides an exchange operation, which swaps the display with the previously entered number, e.g., [1] [=] [3] [RC] would end up with '1' in the display and '3' in the hidden operator register. The [CD] key clears the display without affecting other registers, for use when entry errors are made. The [C] key clears everything except for the memory registers. A large red push on/push off button controls power to the machine. The keyboard works by using magnet-actuated reed switches, as was most-common for machines of this vintage.

Keyboard Detail of Burroughs C-3350

The machine uses a 16-digit Nixie tube display, with each tube containing the digits zero through nine and a decimal point. The tubes are held in place by a metal frame with a rubber-like material which provides shock isolation and alignment for the tubes. The sign of the result is indicated at the right end of the display, by an incandescent lamp which lights up a minus sign window when the number on the display is negative. The machine is vigilant about overflow and invalid operation conditions, with an incandescent lamp next to the memory status indicators, labeled with a red jeweled "EC", which lights up and locks the keyboard when such operations are performed. One quirk in divide operations is observed; Dividends which are more than 15 digits long end up resulting in quotients which are incorrect, with no error or other indication. This is not a fault with with the circuitry, but is related to the algorithm used to perform division. The most significant digit in the working register of the machine serves as a single-digit counter to tally subtractions during division operations. Because this one digit is reserved for this function, any digit in that position at the beginning of a division is lost. This method is quite commonly used in many calculator designs of the 1960's because it helps minimize component count by utilizing existing logic and register space rather that having dedicated circuitry to count soubtractions.

Detail of Nixie Display and Driver Circuits

Nixies in Operation

An intriguing feature of the machine is a cover plate on the bottom of the machine which, when removed, exposes a connector socket which is attached to the bottom of the keyboard assembly. This connector allows the calculator to be plugged into an external device which converts the machine into a programmable calculator. The external programmer can record and simulate keypresses on the keyboard, allowing sequences of keyboard operations to be recorded, then "played back" to automate complex or repetitive calculations. In the photo below, the programmer connector appears as a greenish connector mounted to the back of the keyboard assembly. Sharp offered an accessory called a "Memorizer" which connected to various Sharp calculators to provide this capability. It is not clear at this point if Burroughs offered a similar device to turn the C-3350 into a programmable machine. The other connector seen above the keyboard assembly is the connector that plugs onto an edge-card connector on one of the calculator boards, providing the connection between the keyboard and calculator electronics. If anyone knows more about any Burroughs-marketed programmer option/add-on to this machine, I'd be interested in hearing about it.

The "programmer" connector on the bottom of the keyboard assembly

Text and images Copyright ©1997-2015, Rick Bensene.