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Burroughs Model C-3350 Desktop Calculator
Updated 12/25/2020

This calculator is interesting in that it is a mix small-scale integrated circuit and transistor logic. The machine uses a sprinkling of SSI (Small Scale Integration) IC's made by Mitsubishi in Japan. Mitsubishi was one of the earlier integrated circuit manufacturers in Japan, with their first mass-market commercial IC's being TTL (Transistor-Transistor Logic) bipolar devices, with NEC, Toshiba, and Hitachi all focused on Metal Oxide Semiconductor (MOS) integrated circuits. The ICs are all of the same type, containing four two-input NAND (inverted output AND) gates. The majority of these ICs are used to create flip flops, used to store state information. The ICs make up only small portions of the logic, though, with the vast majority of the logic of the machine made up of a large number of transistors and diode circuitry. 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 1966 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. ]urroughs leveraged the calculator expertise of the Japanese electronics giant, Hayakawa Electric Co., Ltd. (later known as 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 Hayakawa Electric in early 1968, Sharp designed and manufactured calcultors for Burroughs, providing ready-to-sell calculators. Along with Burroughs, Sharp also signed similar agreements with other companies. An example of this is another version of the Sharp Compet 32 calculator marketed by the Swedish company Addo-X, as the Model 9958. Addo-X was also an OEM customer of Sharp, and in the 9958, it appears that Sharp provided the entire machine, including the cabinet, with minor changes between the 9958 cabinet and the Compet 32 cabinet, along with color scheme changes.

Along with Addo-X, Facit, a Swedish manufacturer of wonderful mechanical calculators. also OEM'd a version of the Compet 32, marketing it as the Facit 1125. These calculators, like the Burroughs C3350, are based on electronics manufactured by Sharp, and sold under the various brand names as their own unique product.

The Sharp Compet 32
Image Courtesy Takaharu Yoshida

Burroughs C-3350 with Cabinet Top and Keyboard Removed
Power Supply Subsystem Located At Front Edge of the Calculator

The C-3350's logic is implemented across three circuit boards that are horizontally oriented, with edge connector sockets toward the rear of the machine that make up the backplane connections between the boards. A sturdy stamped metal cage provides a backbone for the circuit boards. The electronics assembly is 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 that provides interconnection and power supply distribution. The machine uses many small 'pancake'-format ceramic encased Silicon transistors made by Nippon Electric Co., Ltd. (NEC), identical to those in many of Sharp's second-generation electronic calculators. Along with the pancake transistors, there are also a great many conventionally-packaged Silicon transistors. Three are other parallels between this Burroughs-badged machine and Sharp's equivalent, such as the [X] and [÷] keys that have indicators in them that 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 that 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 by pressing either of the [⋄] keys, while the [*] keyssss recall the register to the display and clears the memory register. The memory registers can also be added to with the memory [+] keys, or subtracted from the memory register with the [-] keys for each memory register. In these cases, the content of the display is added to or subtracted from the memory register without affecting the number in the display. 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 reader may note that in the math function section of the keyboard there are no [+] or [-] keys, only having a two [=] keys, one red and the other white. To be more intuitive, these keys perhaps should back 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. This is the nomenclature that was used on the Sharp Compet 32. 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 the √ symbol on it. Entering a number, then pressing the [÷] key immediately followed by the white [=] key results in the qquare root of the number in the display being calculated. Later Burroughs and Sharp machines actually put a small √ symbol next to the division sign on the [÷] key to show this functionality.

The C-3350 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 departure from the single selector used on the Compet 32. 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. The [RC] key provides an exchange operation, which swaps the display with the previously entered number, e.g., [1] [÷] [3] [RC] [=] would end up performing 3÷1, with 3 as the result. 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 uses magnet-actuated reed switches, as was most-common for machines of this vintage, and was the same keyboard techology used by Sharp on the Compet 32.

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 right-hand 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 ignores the keyboard when such operations are performed. One quirk in divide operations is observed; Dividends that are more than 15 digits 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 calculator 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 records 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 ©1997-2020, Rick Bensene.