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Burroughs Model C-3350 Desktop Calculator
Updated 6/14/2022

This calculator is interesting in that it is an example of the time period when electronic calculator manufacturers were beginning to undertake the process of implementing the new technology of integrated circuits into the tried-and-true discrete transistor and diode technology of the early solid-state calculators. The result is a calculator with a small number of integrated circuits, used conservatively in the calculator's logic to reduce component count, and at the same time, minimizing the risk of the new technology of integrated circuits. This "in-between" period did not last very long, as it became abundantly clear in a very short time that integrated circuits not only decreased component count, reduced physical size, used less power, improved reliability, simplified logic design, and had matured to the point that using them was less expensive than implementing the equivalent functions with discrete components.

The Burroughs C-3350 uses a sprinkling of bipolar Small Scale Integration TTL (Transistor-Transistor Logic) integrated circuits made by Mitsubishi in Japan. Mitsubishi was one of the earliest bipolar integrated circuit manufacturers in Japan. Mitsubishi's devices were patterned after TTL (Transistor-Transistor Logic) integrated circuits that were first produced by Sylvania Electric Products Inc. in the US beginning in 1963. While Sylvania was the first to sell TTL ICs, it was Texas Instruments (TI) that popularized the IC technology, first with its 5400-Series military spec ICs starting in 1964, followed by the now-ubiquitous 7400-series devices that first came to market in 1966. The 7400-series of TTL devices became the dominant small- and medium-scale bipolar IC technology very quickly, becoming the de-facto standard, with equivalent parts made by Motorola, AMD, Fairchild, Signetics, and National Semiconductor in the US, along with numerous European semiconductor companies. There was even a line of 7400-series compatible integrated circuits that were manufactured in a number of countries of the former Soviet Union.

For a period of time in the late 1966 to late-1967, Japan's very powerful Ministry of International Trade and Industry (MITI) mandated that no electronics that utilized Japanese-made bipolar integrated circuit technology be exported to the US due to concerns over possible violations of Texas Instruments' and Fairchild Semiconductor's integrated circuit technology patents. As a result, Sony ended up halting the export of a portable radio using an analog integrated circuit they had developed. Hayakawa Electric (Sharp) had also halted shipping any of its Sharp Compet 31 (Model CS-31A) to the US due to its use of Mitsubishi ICs in the memory register circuitry of the calculator. The concern was that TI and/or Fairchild could potentially file a complaint against Hayakawa Electric and/or Mitsubishi Electric for violation of patents on IC technology.

The Compet 32 used the same Mitsubishi TTL ICs as the Compet 31, and because of this. Hayakawa Electric had to delay the export of its brand new Sharp Compet 32 electronic calculator, as well as calculators using the same basic design for OEM customers, including Burroughs, Facit, and Addo-X. The delays in shipping these Hayakawa Electric-manufactured calculators to the US caused Sharp some difficulties, as their production estimates had included sales of the calculators in the US, including those that had been manufactured for Burroughs. This resulted in a large number of Compet 32s and Burroughs C-3350s that were built for the American market sitting in warehouses in Japan awaiting the time when they could be shipped. This represented a significant amount of lost revenue which was eventually realized, but only after MITI and the US State Department had negotiated acceptable terms to allow the shipment of the products to commence as 1967 came to an end.

At the same time that the IC patent issues were being investigated, there were concerns by Burroughs over the use of Nixie tubes manufactured in Japan (primarily by NEC and Hitachi) being used in the calculators made in Japan, as Burroughs felt that they were owed royalties for each Japanese "copy" of Nixie tubes used in Japanese-made electronic calculators.

Burroughs had invented the Nixie tube in its research and development laboratory in 1953, and began volume production of Nixie tubes after it purchased a vacuum-tube manufacturer called Haydu Brothers in 1954, with the intent to leverage the vacuum-tube production facilities of Haydu Brothers to manufacture Nixie tubes for sale around the world. For a time, Nixie tubes were manufactured and marketed under the Haydu Brothers name until tooling was created to mark the parts as Burroughs products.

This situation concerning Burroughs' concern over Nixie tube copies made by Japanese manufacturers was rather odd since Burroughs would eventually sign with Hayakawa Electric Co., Ltd. (Sharp), a heavy user of Nixie tubes manufactured by NEC and Hitachi in Japan, to buy calculators made by Hayakawa Electric and sell them under the Burroughs brand, as with the Burroughs C-3350 exhibited here, which contains Nixie tubes made by Hitachi in Japan. These Nixie tubes were manufactured without any regard to Burroughs' patent rights as the inventor of the Nixie tube display technology.

In the end, the conflict regarding the Nixie tube was addressed by the agreement between Hayakawa Electric and Burroughs. The agreement forged between Burroughs and Hayakawa Electric in early 1968, provided for Hayakawa Electric to design and manufacture calculators for Burroughs, providing completely built and ready-to-sell calculators to Burroughs for them to market, sell, and support through Burroughs' business machine division all over the world. As part of the agreement, Hayakawa Electric would pay Burroughs a small royalty for each Japanese-made Nixie tube that was used in Hayakawa Electric-made electronic calculators. The agreement provided for exemption of the Nixie tube royalty on any calculators made for Burroughs that used Nixie tubes.

Not long after the agreement between Hayakawa Electric and Burroughs was forged, MITI and the US Federal Trade Commission were able to broker a settlement in the TI integrated circuit patent issue. At the very end of 1968, this settlement was agreed upon to provide a means by which Japanese integrated circuit manufacturers would pay a reasonable licensing fee to both Texas Instruments and Fairchild for to allow the Japanese manufacturers to continue to make integrated circuits, as well as for a small royalty to paid to both US firms for each applicable IC used in the manufacture of Japanese electronic devices.

Japanese semiconductor manufacturers NEC, Hitachi, and Toshiba, while dabbling in bipolar logic, opted to focus on Metal Oxide Semiconductor (MOS) integrated circuits, a technology that allows much easier fabrication of tiny transistors used in Integrated Circuits, allowing the transistors to be smaller and consume less power than equivalent transistors used in bipolar integrated circuits. It took these manufacturers some time, though, to get their MOS fabrication processes running smoothly, meaning that both Mitsubishi and Sony, who had decided to stick with bipolar integrated circuit technology and were not making investments in MOS IC development, were the primary manufacturers of small-scale bipolar integrated circuits in Japan. Sony only used the ICs they manufactured in their own products, and did not sell their ICs to others, leaving Mitsubishi, for a time, as the only IC manufacturer that had integrated circuits available for Japanese calculator manufacturers to buy. It was either use Mitsubishi-made ICs, or import American-made ICs (which was expensive), or continue to manufacture calculators using discrete component technology, which was costly and had limits in terms of just how much calculating power could be packaged into an ever-smaller calculator as demanded by the marketplace.

The simplified transistors in MOS ICs held the promise of being able to put many more transistors on a single chip, allowing fewer chips to combine to form a calculator. Initially, MOS chips contained about the same amount of logic as bipolar chips, but in a short time, MOS IC technology became the predominant technology for just about any kind of device that needed complex digital logic. Today's tremendously complex processor chips in desktop computers, laptops, tablets, and cellphones are made with derivatives of the MOS technology used in the early chips made by NEC, Hitachi, and Toshiba many years ago. While bipolar logic ICs are still made today, their use is relegated to special purpose equipment that has requirements that simply cannot be met with MOS ICs.

The ICs used in the C-3350 are all of the same type, part number M2340, with each 14-pin package containing four two-input NAND (inverted output AND) logic gates. The majority of these ICs are used to create simple flip flops, which are used to store state information for the control circuitry of the calculator. The ICs make up only a small portion of the logic of the calculator, with the vast majority of the logic made up of a lot of discrete transistors and diodes.

Based on date codes on some of the devices in the exhibited calculator, it appears to have been made in sometime in 1970, which is rather late for this type of calculator technology. The C-3350 was originally designed in the 1966 time-frame, 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 discrete 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 C-3350 with Cabinet Top and Keyboard Removed
Power Supply Subsystem Located At Front Edge of the Calculator

The Burroughs C-3350 is based on Hayakawa Electric's groundbreaking Sharp Compet 32(CS-32A) calculator which was introduced in August of 1967. The Compet 32 was a watershed machine for Hayakawa Electric, utilizing a completely new architecture compared to the company's earlier calculators. The Compet 32 utilized magnetic core memory for storage of the calculator's working registers, adopted a bit-serial processing architecture that dramatically reduced the number of components required for the calculating logic, leveraged the bit-serial architecture to implement a multiplexed display driving system that significantly simplified the display subsystem, also reducing component count, and used a small number of Matsushita-made small-scale bipolar integrated circuits to further decrease component count. The result was a significantly smaller and lighter calculator that had more features, was more reliable, and used less power than previous Hayakawa Electric-made calculators.

The C-3350's logic is implemented on three circuit boards that are horizontally oriented, with edge connector sockets toward the rear of the calculator that provide 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 shares most of the features with Sharp's Compet 32, but implements them in a slightly different way. The C-3350 has two memory registers that are based on magnetic core memory (the memory registers retain their content while the calculator is shut off, even if it is unplugged from the power line). Each memory register can be added to or subtracted from independently. 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 [*] keys recall the register to the display and clear the memory register. The content of the display can be added to a memory register using the memory [+] keys; or subtracted from the memory register with the [-] key 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, with the calculator having two [=] keys, one red and the other white. To be more intuitive, these keys perhaps should have been labeled [+=] for the white key, and [-=] for the red key, which would more accurately relate how they function for addition or subtraction operations. This is the nomenclature that was used on the Sharp Compet 32, but Burroughs opted to use the color of the key as the clue to the user that the red key was for subtracting, and the white key for adding.. When multiplying or dividing, the white [=] key is normally used to calculate the result, however, the red [=] key can be used to negate the result. Another interesting feature of this machine (which also is present in the Sharp Compet 32) is that it can calculate square roots. 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 square 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 better indicate this capability.

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 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. As with the Sharp Compet 32, the BC-3350 has a constant function, activated by a push on/push off key labeled [K]. The constant function operates 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 is the same keyboard technology 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 that 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 that lights up a negative sign 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 is labeled with a red jeweled "EC", that lights up and ignores the keyboard when such operations are performed. Pressing the [C] key will clear the error indication and restore keyboard input. One quirk in divide operations is observed; Dividends that are more than 15 digits result 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 subtractions.

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 some kind of external device that can serve as a proxy to "press" keys on the keyboard by electronically closing their switch connections. This could convert the BC-3350 into a programmable calculator. The external programmer could record and simulate key-presses 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" that connected to a similar connector on various models of 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. It is also not clear if a Sharp-branded Memorizer device would work if plugged into the connector on the Burroughs C-3350. 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 the Burroughs C-3350 calculator, I'd be interested in hearing about it. Click the Email button at the top of the exhibit to send the Old Calculator Museum an Email.

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

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