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SCM Marchant Cogito 240-SR

SCM Marchant Cogito 240-SR Electronic Desktop Calculator

Updated 7/31/2014


The Old Calculator Web Museum is very happy to add this amazing machine to its growing archive of early electronic calculator technology. This calculator has been one of the most challenging machines to be documented to date. The search for information on the history of this machine has been extremely circuitous, but the results of the research reveal a history to the Cogito 240 (SCM's initial entry into the electronic calculator market) that is truly fascinating. While the Cogito 240-SR (a slightly later introduction that added square root and grand total multiplication) is most certainly not the most capable, fast, elegant, or easy-to-use calculator of its time, the story behind its development and, most specifically, the man behind its design, is one of the most fascinating stories in the history of desktop electronic calculating machines.

Stanley P. Frankel

Stanley P. Frankel, 1959
Photo from IRE Transactions on Electronic Computers, Sept. 1959

The concepts behind the design of the Cogito 240 were developed by a brilliant man named Stanley P. Frankel. Frankel had numerous places in history along with his design of the Cogito 240, including a major role in the Manhattan Project (the secret government-operated project to develop the Atom Bomb); developing breakthrough methodologies for mathematical simulation of complex physical systems that helped create the design for the Hydrogen Bomb; contributing to writing the largest and most complex computer program run on ENIAC (the first all-electronic computer); and designing numerous early small computer systems, including a computer that could be considered one of the earliest personal computer systems (the Royal McBee/Librascope/General Precision LGP-30). The full story of Stanley Frankel is beyond the scope of documentation in this exhibit, but his part in the development of the Cogito 240 is critical, and thus some of his story is told here. For the most complete account on Stan Frankel's life, refer to Steve Leibson's extremely well-researched and written article, Stanley P. Frankel, Unrecognized Genius

To begin the story of the development of the SCM Cogito 240, let us first start with a little about the background of the company that ended up marketing the calculator.

In 1910, two brothers, Rodney and Alfred Marchant, began manufacturing mechanical calculators with the intent of selling them to a marketplace that was hungry for better ways of performing mathematical calculations. Incorporating in 1911, Marchant Calculating Machine Co. of Oakland, California, made a very successful business making and selling desktop mechanical and later, electro-mechanical calculators, that used innovative mechanical engineering and design to make the machines fast, reliable, and reasonably-priced. It is interesting to note that a luminary in the world of mechanical calculators, Carl Friden, worked as a principal designer for Marchant before leaving to start his own calculating machine company, Friden Calculating Machine Company.

Cogito 240-SR Brag Tag

Model Identification on Front Panel

In 1958, Smith Corona Co., the well-established typewriter manufacturer, acquired Marchant Calculating Machine Corp. Smith Corona's management felt that diversifying the company from just a typewriter manufacturer to other office machine product lines was a good growth strategy. Once the acquisition was completed, Smith Corona continued to market Marchant's famous rotary calculators and adding machines under the Marchant brand name, using the name "Smith-Corona Marchant". In 1962, Smith Corona made this name it's new corporate name, becoming Smith-Corona Marchant, or SCM for short. By this time, SCM was into numerous areas of business machines, not just calculating machines and typewriters. SCM kept the "Marchant" name on their calculating machines, leveraging the market recognition of the Marchant name.

After World War II ended, and the nuclear weapons projects at Los Alamos started to slow down, Stan Frankel did various things, including a stint at the University of Chicago's Institute for Nuclear Studies; forming a mathematics consulting company with his colleague from the Manhattan Project, Eldred Nelson; and in 1949, being appointed the head of a newly-formed Computation Laboratory at Cal Tech. During his time at Cal Tech, Frankel lost his security clearance due to questions (thought baseless) about some of his past acquaintances during the period of anti-communism spurred by Joseph McCarthy.

Though it was a blow to him to be blocked from the inner circle of nuclear physics, Frankel's interests had shifted somewhat away from nuclear physics, more toward electronic devices that could perform complex calculations. His work with early IBM Punched Card-based calculating equipment during the Manhattan Project, as well as performing complex programming on some of the earliest electronic computers had convinced Frankel that electronic computing was the only way man was going to be able to understand the myriad complexities of nature. Unfortunately, this distraction led Frankel a bit astray as far as his duties as the head of the Computation Lab were concerned. He became so consumed with the design of a personal computer that he never was able to gain tenure at Cal Tech, and left (with persuasion) in 1954. During his time at Cal Tech, he did consulting work on the side, and ended up developing the design for a small computer he called MINAC, that was eventually licensed from him to become the Royal McBee/Librascope/General Precision LGP-30, which became a successful vacuum-tube-based small computer used in educational, military, and business environments. He also developed (under contract) the design for another computer called CONAC that was developed for Consolidated Oil Company. He also contracted with General Electric doing design work for some of their early large-scale computers.

Sometime in the late 1950's, Frankel got interested in the motion of building an electronic version of the desktop electro-mechanical calculating machines that had been used by the "human computers" in the early days of the Los Alamos neutron diffusion calculations.

Enter a company called Electrosolids, Inc., in Sylmar, California. The company, founded in 1956, had gained a solid foothold in a niche of the fiercely competitive electronics marketplace of the time building high-efficiency solid-state power inverters. The demand for such devices was high in the aerospace industry, and Electrosolids was able to build a flourishing business designing, building, and marketing various types of power supplies. One of the founders of Electrosolids, Joseph Strick (7/2/1923-6/2/2010), had come to know Stan Frankel through Eckert-Mauchly Computer Company (EMCC), Strick's brother-in-law knew Frankel, who was consulting for EMCC at the time, and introduced the two. Strick kept in touch with Frankel, and in time the two become close friends. Frankel had often chatted with Strick about the merits of electronic computing, and mentioned his ideas about a desktop-sized electronic calculator. Frankel was quite passionate that such a machine could be built, and didn't take long to convince Strick of the market opportunity such a machine would create.

By 1958, Electrosolids was embarking on diversifying its business. The company had developed a some solid-state walkie-talkies, a talking doll, and some other electronic devices to help minimize the company's dependence on the power supply business. Strick believed that Frankel's calculator idea might be a great way to augment Electrosolids' market. Frankel had just finished the design of a computer for Packard Bell Electronics, that ended up being introduced in 1961 as the Packard Bell PB-250, an all solid-state mini-computer (though the term didn't yet exist) and was free to pursue a new project. It was decided that a subsidiary company within Electrosolids would be created, called Computron Corporation, with Stan Frankel as the President. Frankel was given budget to build a prototype of the calculator, and hire a technician to assist. Together, Frankel and his technician would build the prototype calculator. Strick, clearly a visionary, was convinced that a personal electronic calculator would have market potential far beyond the niche markets that Electrosolids had up to this point.

Frankel and his technician set up shop in a small area set aside for them in the Electrosolids plant. Immediately they set about building the prototype machine. The machine was a fully-transistorized design, using a magnetostrictive delay line for the working register storage. Numeric entry was through a 10-key keyboard arrangement, and the display was presented as digits on the face of an oscilloscope CRT. It took nearly a year for the prototype to be built and made fully operational. By late 1959, it had taken the form of number of circuit breadboards spread out over a large workbench. It surely wasn't a desktop machine at this point, but in theory, based on component counts, it could be built into a desktop-sized package. The plan was to build and market the calculator was the "CC 1200". The "CC" stood for "Computerized Calculator", and the 1200 represented the expected selling price of the machine, of $1200.

Electrosolids' Calculator Announcement

"Leak" article announcing Electrosolids' Calculator
Electronics Magazine, April 1961
Article courtesy Nigel Tout

At one point, Electrosolids realized that Integrated Circuit technology was going to become a force to deal with, allowing electronic calculators to be built less expensively. Discussions were carried out about reducing the price of the machine when it went to market to grab as much of the market possible before IC-based machines came around. Prices of around $500 or so were bandied about. Somehow this word leaked out to the press, which resulted in the short article shown above. Clearly, this revelation did not transpire until much later, but the article did grab the attention of a number of folks who were also in the market to produce an electronic calculator, and lit various fires around the industry.

Unfortunately, during the time that the prototype was being built, Electrosolids had embarked on an aggressive project to build a high-power inverter for a customer who had very tight technical requirements. A tremendous amount of money was invested in the development of this product. As it turned out, the final product did not meet the specifications of the customer, and it seemed no amount of rework would get the device to meet the specifications. The customer finally backed out of the deal, leaving Electrosolids in a real bind. The company needed some money. Coincidentally, the calculator prototype was pretty well perfected, and running reliably. Funds were such that there just wasn't a way to turn the prototype into a production reality, so it was decided to try to sell the calculator prototype and design to an established mechanical calculator company, in hopes that enough money could be raised to help save the company.

Sometime in mid-1961, a press release offering Electrosolids' prototype electronic calculator for sale was published in a number of technical publications, soliciting a buyer. With a working prototype to demonstrate, it should have been a pretty easy sell, as many mechanical calculator companies were just beginning to think about the possibilities of electronics eclipsing mechanical devices as far as calculating machines were concerned.

The response was quite immediate, with a number of the key mechanical calculator manufacturers responding that they wanted to see the Computron prototype. The first company to come look at the prototype was Friden Calculating Machine Co., a major player in the mechanical calculator market. Senior members of management and engineering for Friden came to see the prototype. The prototype worked flawlessly for about ten minutes, then suddenly quit working when a transistor failed. When this happened, the Friden crew was clearly amused. The Friden entourage left uninterested. Interestingly enough, at the time, Friden was in the process of coming up with their own ideas for an electronic calculator. While it will likely never be known for sure, it is interesting to contemplate the motivation behind Friden coming to see the Electrosolids/Computron prototype machine. Perhaps the visit by Friden wasn't so much out of interest in acquiring the design, but more one of seeing what potential competition Friden may have in the newborn electronic calculator marketplace. The breakdown of the prototype machine may have been viewed by Friden as an indication that the design was not yet ready for "prime time", leading them to believe that anyone who acquired the design had a lot of work to do to make it a production reality. One can wonder, though, if the folks from Friden may have got some ideas that were incorporated into thier eventual EC-130 calculator. The Computron prototype used a CRT display with a seven-segment digit representation. Recently information has surfaced that in late 1961, Friden submitted a request for proposal to Stanford Research Institute's Computer Lab, indicating a desire for SRI to develop a prototype CRT-based display system for use in an electronic calculator. Coincidence?

Marchant Calculating Machine Co. sent a number of representatives to view the prototype, and while they were quite impressed with the machine, they weren't convinced that enough attention inside Marchant could be generated for the company to make an offer. Later, the CEO of Monroe's parent company showed up, thinking that a move to electronics may make sense for Monroe, but he too went away without making an offer, likely because Monroe was in the early stages of planning an electronic calculator project that eventually resulted in the the Monroe EPIC-2000.

It was late in 1961, and the very beginnings of electronic calculating were starting to move ahead. The British firm Sumlock had collaborated with another British company, Bell Punch, to develop an all-electronic calculator, the Anita Mk8. While not a solid-state machine (it used gas-filled, cold-cathode Thyratron tubes), and quite simplistic in its capabilities, it clearly demonstrated the ability of electronics to dramatically improve the speed and accuracy over electromechanical calculators.

Hopes of a deal to sell the Computron calculator protytpe were starting to fall a bit, when a European connection came along. Ian Rose, the head of the calculator division of Diehl, manufacturer of brilliantly-designed complex electromechanical printing calculators (whom Stan Frankel would later end up designing the concepts behind Diehl's first electronic calculator, the Combitron), got in touch. Diehl had a remarketing agreement with Marchant, by which Marchant could market Diehl-made mechanical and electromechanical calculators under the Marchant brand name in North America. Mr. Rose managed to get the right people inside Marchant to listen to Computron's electronic calculator ideas, and this time, a deal was struck. Sometime in 1962, Computron Corp. was acquired by SCM, and Stan Frankel and his technician began work as SCM employees to make the calculator they had developed into a production reality.

In the meantime, Friden had been hard at work developing their own electronic calculator -- and in fall of 1963, they were showing prototypes of their machine, which became one of the earliest transistorized desktop electronic calculators. The Friden 130 took the world by storm, and catapulted Friden into a leadership role in the new world of desktop electronic calculating machines. It's possible, but not substantiated, that Friden's push to develop their own electronic calculator may have had its beginnings with the demonstration of the Computron calculator. By 1964, other manufacturers such as Sharp(Japan), Wang(USA), Mathatronics(USA), IME(Italy), and Olympia(West Germany) had also began marketing electronic calculators. The benefits of electronic calculators were very clear -- much faster answers and nearly silent operation. These benefits were valuable enough to many scientific and business users that they could justify the higher cost of the electronic machines. The shift away from mechanical calculating had begun in earnest by the mid-1960's, and the management of companies that sold mechanical calculators as their main source of revenue started getting very nervous.

Conceptual View from US Patent

Conceptual drawing of what became the Cogito 240-SR from US Patent 3518629

The work at SCM to get Frankel's Computron prototype design packaged into a desktop unit moved along at a rather slow pace. It wasn't until June of 1965 that the SCM Cogito 240 was instroduced, right at around the same time that Monroe introduced it's EPIC-2000 electronic calculator. It seems likely that the pre-sales announcement of the Cogito 240 occurred sometime in late '64 or early '65. The Cogito 240 did not have the square root or accumulative multiplication functions of the later exhibited here. The addition of the square root and accumulative multiplication functions were added into the design at SCM (either independently of Computron, or perhaps with input from Frankel, although Frankel is not listed in the patent for the square root version of the calculator), and introduced in the early part of 1966 as a follow-on machine called the Cogito 240-SR, which is exhibited here.

SCM Cogito 240

The SCM Cogito 240
Image Courtesy of the Smithsonian National Museum of American History, Kenneth E. Behring Center

There's an amusing twist to the story of the development of the Cogito 240. SCM was in need of electronics expertise to help build the machine that Frankel had designed. Ads were placed for a number of high-level electrical engineers, as well as a bunch of technicians and other support staff that would be needed to build the production version of the calculator. One of the engineers that responded was a very bright Electrical Engineering/Computer Science graduate from UC Berkeley, named Thomas Osborne. Osborne was hired on at SCM to help with the development of the Cogito calculator. After becoming familiar with the architecture of the Cogito, he became disenchanted with the design. He thought that the machine was too much like an electronic implementation of a mechanical calculator. Osborne's background in computer science told him that an architecture more like that of a small computer was a much more efficient way to build a personal calculating machine. Osborne tried to influence SCM management to scrap the architecture that Frankel had developed, but his arguments fell on deaf ears. After trying for a few months to influence change, Osborne went to his management saying that he wished to resign, but was willing to work on his own, unpaid, to develop his concepts for calculator design into a working prototype, which he would offer SCM the first opportunity to buy once it was completed. SCM wished Osborne luck, but made no committment to consider any developments that he made. With that, he left SCM and began work on his own to develop a calculator design that used computer science concepts as its basis.

Osborne went on to develop a wonderful little calculator prototype (called the "Green Machine", after the metallic- Green color that the cabinet of the machine was painted) that provided full floating point math to ten digits of precision, was very fast, and quite compact. To learn more about Tom Osborne and the development of the Green Machine and HP calculators, please read Steve Liebson's Reprints of some materials written by Thomas Osborne. In early 1964, Osborne started showing his machine to a number of potential buyers, (including IBM, Hewlett Packard, and Monroe to name a few), but got no real interest in anyone buying his design. This was in spite of the fact that the calculator was light-years ahead of anything else on the market at the time. Osborne didn't bother asking SCM if they might be interested, as it was clear to him that they were only interested in their own design. Over time, the word of Osborne's calculator eventually worked its way to the right ears at Hewlett Packard, to a group of folks who were tasked with developing an electronic calculator that would blow the doors off of anything else out there. Osborne was contacted by these folks, and he was asked to bring his prototype to Hewlett Packard to demonstrate to them. Once the right people at HP saw Osbrone's machine, they were hooked. HP offered to give Osborne a job on the spot, but Osborne was more interested in being a consultant. Osborne went on for a time as an independent contractor to HP, then later hired on as an employee, becoming a key contributor to HP's revolutionary calculator developments, including the groundbreaking HP 9100A, which incorporated many aspects of Osborne's little prototype calculator. SCM's loss was Hewlett Packard's gain. This single decision on the part of SCM's management likely contributes strongly to why HP is still very successful in the calculator market today, while SCM left the electornic calculator business in the 1970's without much success.

Architecturally, the Cogito 240 is a curious mix of mechanical calculator concepts, and digital electronic technology. The machine implements a three register architecture, similar to many mechanical calculators. One register (called K) is where keyboard entries are placed, another is an accumulator (P) that accumulates sums and products, and the third register is essentially a counter used for forming quotients and (later) square roots (Q). The innovation in the implementation of the Cogito 240 is that numeric representation and math processing means of the calculator is much more computer-like. Numbers are represented in Binary-Coded Decimal (BCD) form, and all arithmetic is done in BCD. Some early electronic calculators utilized electronic equivalents of the ten-step counters used in mechanical calculators. At the time the Cogito 240 was designed, the use of binary-based math in electronic calculating was a concept applied typically only to electronic computers. However, by the time that the Cogito 240 hit the market, binary-based (rather than decimal-based) architectures for calculators had pretty much taken over because of the increased efficiency and flexibility in the use of the binary number system.

In order to make the Cogito 240 more usable for complex math, a twist was added to the three-register architecture, in that each of the three main registers has what is called a surrogate register. The surrogate registers serve as scratch pad memory for each of the main registers. This feature allows more complex math to be performed, as the surrogate registers can contain intermediate results, mostly eliminating the need for manual recording and re-entry of intermediate answers. The surrogate registers are called K', P', and Q'. The surrogate registers are not displayed on the CRT, while the main registers are always displayed. While the surrogate registers were helpful in performing more complex math, their use was not inherently obvious. Keyboard functions are provided to store the content of a primary (visible on the display) register in its surrogate, to recall the surrogate to its primary register, and to exchange the content of the primary register and its surrogate. This method of providing intermediate result storage was rather complicated and non-intuitive, and while unique, could not compare to Friden's elegant Reverse Polish Notation (RPN) stack architecture. The use of the surrogate registers of the Cogito 240 will be explained in more detail in the operational section of the exhibit.

SCM Marchant Cogito 240-SR Internal View

SCM Marchant Cogito 240-SR With Clamshell Opened

From a technology perspective, the 240 is reasonably conventional for its time. The machine is implemented with all discrete transistorized circuitry -- there are no integrated circuits to be found in the machine. A total of six circuit boards, each measuring about 9" x 10", contain the main logic of the machine, with a few other boards performing the various analog functions (deflection amplifiers, scanning, and beam blanking) needed to drive the CRT display. Logic gating is done with tried-and-true diode-resistor gates (AND and OR), and transistors are used for level shifting (buffers and inverters) and various types of flip-flops. A magnetostrictive delay line serves as the working storage for the calculator. The delay line has a capacity of 480 bits which provides all of the capacity needed for the registers of the machine. All of the bits making up the six working registers circulate in the delay line continuously, and are gated to various sections of the logic as they are needed.

Delay Line

The Delay Line (in the bronze colored enclosure underneath the Read/Write Amplifier card with the silver cover)

SCM 240-SR Delay Line

The manufacturer's label on the bottom of the delay line (click image for larger view)

The delay line was made for SCM by a company called Digital Devices, Inc. of Syosset, New York. It appears that Digital Devices was acquired by Tyco Labratories sometime in the late 1960's to early 1970's. (If anyone out there knows anything about Digital Devices, or the Digital Devices Division of Tyco, I'd love to hear from you). The delay line, model 414-1002, has a nominal delay of 460 microseconds, and a maximum bit rate of 1.1 MHz. At the maxiumum bandwidth, the delay line could hold 506 bits of data. In practice, the delay line is run at a slightly lower speed to reliably allow it to store the 480 bits of data needed by the calculator.

Another early electronic calculator, the West German-made Diehl Combitron, used magnetostrictive delay lines made by this Digital Devices. The fact that Diehl also used delay lines made by Digital Devices in their Combitron electronic calculators was not a coincidence. Stanley Frankel also designed the Combitron for Diehl, and thus it made sense that Frankel would stick with delay line technology he knew worked well.

Delay Line Read/Write Amplifier

Delay Line Read/Write Amplifier Board

The calculator utilizes a bit-serial architecture, with all operations occurring on data one bit at a time. The data in the delay line is interleaved so as to present the bits of each register such that only a couple of flip flops are required as temporary single-bit storage registers to store the bits needed to perform a binary operation on two bits. The timing of the calculator is generated by a master clock generator that runs at approximately 1 MHz (1,000,000 cycles per second), which is divided down by a chain of flip-flops to generate the major and minor states of the control logic. The state control flip flops are connected together with complex chains of gating that guide the machine through the various steps of performing a function. The patent information on the machine indicates that the machine has a total of 64 different states that are generated by the state logic, with each state carrying out a particular portion of the operational steps involved with running the machine. Originally, the Cogito 240 (note the lack of SR) was designed as a four-function machine only (no square root function). After the Cogito 240 was on the market, design work was done that added some extra logic states (or used unused states of the original design), in order to provide the square root function while the original design did not have this ability. The calculator's logic is hard coded by the wiring of the various flip-flops and gates, as opposed to later calculators that used a generalized microcoded architecture.

SCM Cogito 240-SR Upper Cabinet

The CRT & CRT Drive Electronics in the Upper part of the cabinet clamshell

The 240-SR weighs 39 pounds, and chews up a sizable chunk of desktop space, with a footprint of 13-1/2" wide by 19" deep. The machine is not as large as the Friden 130 in terms of footprint, mainly because the Cogito 240-SR is more aggressive than the Friden in terms of packaging density. The cabinet is arranged in a clamshell, with the top part of the cabinet folding up, exposing the main logic, keyboard, and power supply in the lower half of the clamshell, and the CRT tube its associated drivers and high voltage power-supply circuity in the upper part. The cabinetry is all thick wall aluminum castings or heavy gauge stamped sheetmetal. The construction is of high quality, with many machined surfaces and high quality connecting hardware. The overall design of the machine shows a good deal of care in the mechanical design aspects. A large and heavy heatsink takes up the entirety of rear panel of the machine, dissipating heat from the power transistors and rectifiers that make up the power supply. A small synchronous motor drives a fan that forces air drawn through cooling vents in the lower part of the cabinet across the logic circuit boards to cool the electronics. The fan is quiet enough such that it is not obtrusive in an office environment.

SCM Cogito 240-SR Keyboard Mechanism

Cogito 240-SR Keyboard Mechanism

The keyboard of the 240-SR is a mechanical marvel, similar in design to the mechanically encoded keyboard of the Friden 130. The keyboard uses a complex arrangement of levers that activate individual micro-switches for each key to identify the key that is pressed. The levers also serve to provide lockout functionality to prevent multiple keys from being pressed at the same time. Unlike the Friden 130/132, the keyboard is encoded into a form the electronics can use by electronic means (diode arrays and gating). The Friden calculator encodes keypresses by entirely mechanical means. Like the Friden 130, the keyboard mechanism of the 240-SR is designed to lock the math function key down during the time that math operations are taking place. A small solenoid fires to release the key when the operation is complete. Given the length of time that the machine can take to perform some operations, this key locking prevents a fast operator from causing errors by getting ahead of the calculator. Keyboard operation is rather noisy, but overall, the machine is certainly much more quiet than the electromechanical machines it was designed to replace.

SCM Marchant Cogito 240-SR CRT Display

Cogito 240-SR CRT Display
Click Image to view a video of display in operation.

The CRT tube uses a medium-persistence phosphor, as the display is scanned quickly enough that a long-persistance phosphor isn't required in order to avoid flickering of the display. The tube uses a blue-green phosphor, with a blue filter positioned in front of the faceplate to provide a pleasing blueish color to the display. The CRT display is organized as four rows of numbers, with the top row being the K register, the next to the top row being the Q register, and the bottom two rows making up the P register. The capacity of the K and Q registers is 12 decimal digits. You may note on the display that 16 digits are actually displayed for these registers. The first four digits of the K and Q registers are always zero, and are used as place holders for decimal point positioning, as explained in the operator's manual. The P register consists of a total of 28 digits, 24 of which are useful, with the leading four digits again reserved for decimal point placement. The P register has double the capacity of the K and Q registers as it serves as the main accumulator for the machine, and must have enough digits to hold the product of two twelve-digit numbers. The digits are formed using the familiar 'pieces of eight' 7-segment rendition, with rather peculiar half-sized zeroes. The decimal point lights to the right of the digit position it is being displayed in. The sign of the number in the register is displayed as a '-' before the most significant digit of the register. Two other annotations show up on the display, in the form of two "streaks" which can appear at the left, right, or at both ends, just below the digits of a register display. These notations show up when the number contained in the register needs its decimal point repositioned manually to properly express the magnitude of the number being displayed. The rendition of digits on the display is controlled by a series of diode-resistor gates which essentially form a ROM that defines the sequences of strokes of the electron beam to form the display features. The display cycle continues during calculation, allowing the user to observe the steps of the calculation as it proceeds.

SCM Marchant Cogito 240-SR Power Supply

Cogito 240-SR Power Supply Detail

The power supply of the 240-SR is rather complex, though it is of conventional design. The main logic supplies are +20V and -20V DC, with various other voltages used in the analog portions of the circuitry that drive the CRT. The logic voltages are electronically regulated by use of zener diodes as references, and high current pass-transistors for regulation. A bank of sizable computer-grade electrolytic capacitors provides filtering to assure minimal AC ripple on the logic supplies. The high-voltage power supply for the CRT is an epoxy encapsulated module that provides the approximately 3000 Volt potential required by the CRT. The cooling for the dissipation devices in the power supply comes via convection from the large heatsink at the back of the machine.

SCM Marchant Cogito 240-SR Interconnect Detail

Circuit Board Mounting and Interconnect Detail

The electronics of the machine were clearly not designed for ease of service. There is no backplane to speak of. All of the circuit boards are individually wired together with plug-in harmonica-style connectors, and a maze of wire between the connectors. The circuit boards have arrays of connector pins on one end onto which the harmonica connectors plug in to provide a connection. The connection pins are tin plated, as are the sockets in the harmonica connectors. The circuit boards are made of fiberglass, with traces on both sides of the boards, and plated-through feedthroughs connecting traces on each side of the board together. Components are mounted only on one side of each circuit board.

SCM Marchant Cogito 240-SR Circuit Board

Cogito 240-SR Circuit Board Detail

The boards are connected together in a stack, with insulating sheets of a stiff fiberboard-type material between the boards to keep components on the top of one board from shorting on the traces of the board above it. The boards are secured in the stack by a series of six plastic spacers that hold the boards seperated from each other. This means that in order to replace a given circuit board, all of the boards above it in the stack must be removed to get to the board that is to be replaced. Keeping track of the maze of harmonica connectors that had to be unplugged when replacing a board had to be one of a service technician's most difficult things to have to do. The other difficulty with this design is that it makes it very difficult to troubleshoot in the field, as gaining access to circuit board components and traces while the machine is running is next to impossible. The upside to this design approach is that it is quite efficient in terms of space, allowing the machine to be somewhat smaller than it would have been had a more serviceable design been used.

SCM Marchant Cogito 240-SR Keyboard Detail

Cogito 240-SR Keyboard Layout

Due to the rather unusual architecture of the machine, operating the 240-SR is different than just about any other calculator ever made. The architecture of the machine is not quite intuitive insofar as how to determine the input for a sequence of calculations in order to arrive at the right answer. While the Friden 130 required folks to learn the Reverse Polish stack method that it uses, once the basics were understood, it became quite easy to perform complex chains of operations with little trouble. The arithmetic methods of the 240-SR aren't quite as easily understood.

Numeric entry is pretty straightforward, using the numeric keys and the [DECIMAL] key to enter a decimal point. Negative numbers are entered by pressing the [NEG] key prior to entering the number. An unusual inclusion is the [POS] key, which forces the number to be positive, useful to correct the inadvertent entry of a negative number. The decimal point may be entered anywhere in the number, and the machine automatically adjusts the position of the decimal point on the display. By default, numeric entries are entered into the K register, beginning at the left-most (not including the place holder first four zeroes) digit, and advancing digit at a time to the right as digits are entered. It is possible to directly enter numbers into the P and Q registers by pressing the [Q] or [P] key (located in the left bank of keyboard keys) prior to making digit entry. The [CLEAR K] key clears the K register, and is used mainly to correct erroneous entries.

Moving register content between the registers is performed by a group of four keys; [Q->K], [Q->P], [P->K], and [P->Q]. The transfer from one register to another results in the source register being cleared, and the destination register receiving the former content of the source register.

The [EXCHANGE], [ENTER], and [RECALL] keys control the transfer of data between the main set of registers, and the surrogate registers. By default, the [EXCHANGE] key exchanges the content of the K register with the K' register. The [ENTER] key takes the content of the K register, and stores in into the K' register, leaving the K register clear. Lastly, the [RECALL] key moves the content of the K' register into the K register, clearing the K' register. The [Q] and [P] keys can be pressed before an [EXCHANGE], [ENTER], or [RECALL] key to perform the function on the designated register. For example, to exchange the Q and Q' registers, the [Q] key would be pressed, followed by the [EXCHANGE] key.

Cover of Cogito 240-SR User's Manual

Cover of Cogito 240-SR User's Guide

The arithmetic function keys, grouped at the right end of the keyboard, are a little unusual in that the multiplication, division, and square root functions each have their own individual "=" key that causes the result to be calculated. Addition and subtraction operate arithmetically, with the [+] key causing the content of the K register to be added to the P register, and the [-] key subtracting the content of the K register from the P register. After addition or subtraction, the K register is left undistrubed, allowing for simple constant addition or subtraction.

Multiplication and division operate by entering the first number then pressing the [X] or [÷] key. In the case of multiplication, the content of the K register is copied to the Q register, and the K register is cleared for entry of the multiplicand. In the case of division, the K register is copied into the P register, and the K register cleared. After this, the second number is then entered, and then the [X=] or [÷=] key is pressed to calculate the result. In the case of multiplication, the product is left in the P register, and the multiplicand is left in the K register, and the multiplier is left in the Q register. In the case of division, the quotient is stored in the Q register, the remainer is left in the P register, and the divisor is left in the K register.

The square root function of the 240-SR is most interesting -- it is a two step operation. Most calculators that perform square root operate simply by typing in the number to calculate the square root of, then pressing the square root function key. The answer is calculated and presented to the user. In the case of the 240-SR, the user must supply an estimated value as a second operand to help the machine in the calculation. To calculate a square root, the number to have it's root extracted is entered, then the key with the square root symbol is pressed, which moves the number to the P register, clearing the K register. Next, the guess value is entered into the K register, and the [square root =] key is pressed to begin the calculation. The Q register and P' registers are cleared, and the extraction of the square root begins, using the guess value as a starting point. The square root is presented in the Q register, and the radicand remains available in the P register (as well as the P' register). The square root calculation is extremely interesting to watch, with numbers zipping all about the display (see the VIDEO for an example). The method used for calculating square roots is based on the Newton-Raphson method of successive approximation. The root extraction calculation progresses slowly enough to observe the trial roots converging on each other, until they are close enough together that the calculation stops. Sadly, this method for calculating square roots, on a calculator that is already rather slow, makes for long calculating times, especially if the estimate provided is inaccurate. Some square root operations can take nearly a half-minute to complete.

At the right end of the keyboard are three switches labeled "÷= ACCUM", "X ACCUM", and "X= ACCUM". These switches are used to accumulate sums of quotients, multipliers, and products. These functions are useful for performing operations such as averages, sums of products, and other types of statistical operations. When the "÷= ACCUM" switch is on, the calculator accumulates results of division operations in the Q register. The "X ACCUM" switch enables the accumulation of multiplicands in the P' memory register. The "X= ACCUM" switch enables the accumulation of products in the P register. In addition to these accumulation modes, the [GRAND TOTAL =] key provides an additional means for accumulating sums of products. This key functions similarly to the [X=] key, except that an ongoing sum of products is accumulated in the P' register. The [GRAND TOTAL] key is used to recall the accumulated sum of products to the P register, by transferring the P' register to the P register, clearing the P' register.

Lastly, a unique feature, an addition to the original design of the machine as designed by Frankel, is designed to help with performing statistical correllation functions. This feature was added by SCM engineer Jorge Hernandez, and patented seperately under US Patent #3553445. The switch, located to the lower right of the numeric keypad, selects what is called "Dual Multiplication Control". When this switch is on, the function of the [DECIMAL] key is changed, such that five zeroes are indexed when it is pressed. These five zeroes serve as a seperator to allow the K register to contain two numbers, seperated by five zeroes. For example, to simultaneously calculate the following summations, x2, 2xy, y2, x, and y, the "X ACCUM" and "X= ACCUM" switches would be turned on, along with the "DUAL MULTIPLICATION CONTROL". For the two groups of numbers X and Y, let us use (40, 98, 81, and 40) for X, and (21, 57, 45, 26) for Y. To begin, 40 is entered, followed by the [DECIMAL] key (to seperate X from Y), then 21 is entered (with the K register display showing "oooo40.ooooo21ooo"), the [X] key is pressed, follwed by the [X=] key (which calculates the square of X and Y and presents the result of 1600 (for x2) and 441 (for y2), and 168 for 2xy as "oooo16oo.ooo168ooooo441oooooo" (using the 'o' to represent the half-sized zeroes rendered on the display) in the P register. Continuing by entering the remaining numbers the X and Y lists using the same method results in a final answer in the P register of "oooo19365.oo22222ooo6391ooooo", which indicates the sum of x2 of 19365, the sum of 2xy as 22222, and the sum of y2 of 6391. Pressing the [GRAND TOTAL] key provides the summation of the X and Y lists of numbers, displaying "oooo259.oooo149oooooooooooooo" in the P register, indicating the sum of the X list is 259, and the sum of the Y list is 149. The results take some intepretation, but via this functionality, a lot of simultaneous calculations can be done in a comparatively simple manner.

Overflow is handled rather uniquely on the 240-SR. Unlike other early calculators that simply roll over (e.g., on a 10-digit machine, 999999999 ÷ 1 = 0000000000), or stop the calculation, the 240-SR always tries to perform the calculation as best as it can. For results that overflow the capacity of the register that holds the answer (the Q or P registers), a system utilizing 'streaks' displayed below and to the right or left end of the register is used. In the case of large numbers (> 1011), a streak at the right end of a register indicates that the decimal point should be positioned 16 places to the right of the displayed decimal point position to provide an approximation of the result. If the streak is at the left end of the display, the decimal point would be positioned 48 places to the right of its displayed position. If both left and right streaks are displayed, the decimal point in the approximate answer is located 32 places to the right of the displayed decimal point location. In the case of small numbers (less than .000001), a left streak indicates that the number to be read should be preceded with four zeros plus one zero for each place to the right of the decimal point. A right streak indicates that the number to be read should be preceeded by 36 zeroes and one zero for each place to the right of the decimal. If both left and right streaks are visible, the number should be read with 20 zeroes and one zero for each place to the right of the decimal. The system can be a bit tedious, but it does allow the machine to provide approximations of very large or very small numbers.

Profile View

Profile View of the Cogito 240-SR

The 240-SR is most definitly not a speed demon. A general weakness of the design is the main reason why the machine isn't particularly fast. When decimal point positioning must occur in the P register, the only way that the shifting of the number a single place to the left or right can occur is to circulate the content of the register completely through the register, which takes a significant amount of time. An example of this can be seen in the video. A typical addition that does not require decimal repositioning occurs quite quickly. However, that changes quickly when a decimal repositioning has to occur, with the shifting process taking an operation that is almost instantaneous to one that can take almost a full second to complete. The same issue plagues multiplication and division. Simple multiplications take around 3/4-second, and more complex multiplications can take up to two seconds. Division operations complete in a somewhat slower times, with the most complex division of 999999999999 ÷ 1 taking just over four seconds to complete. The square root operation is by far the slowest, with some calculations requiring nearly 25 seconds to complete. The reality of the final product is that, in some cases, the electromechanical calculators that preceded the 240-SR may actually be faster!

While the Cogito 240-SR was not particularly stunning in terms of its performance, its legacy as an example of the brilliance of Stanley Frankel still lives on to this day.


This exhibit is dedicated to the memory of Stanley Frankel, 1919-1981
Profound thanks to Mr. Joseph Strick, founder of Electrosolids, Inc., for providing a great deal of information about Stan Frankel, who became a close personal friend of Mr. Strick.

Sincere thanks to Victor Yuhasz for the opportunity to acquire this wonderful artifact.


Thanks to Allan Frankel, son of Stanley Frankel, for providing a great wealth of information about his father.

Text and images Copyright `©1997-2014, Rick Bensene.