MITS Model 7440 Desktop Scientific Calculator

Updated 4/1/2022

In the world of electronic calculators, the company named MITS may not ring bells for many people. However, those who were involved in the infancy of personal computing, or those who have an affection for the early days of personal computing know the name well. MITS, (pronounced letter at a time) is an acronym for Micro Instrumentation and Telemetry Systems, effectively created the "personal computer" market with the introduction of the "Altair 8800" mini-computer (as they called it) in early 1975. The Altair computer was heralded as a breakthrough in the world of computers, with a kit selling for a mere $397 in 1974. There were other "hobbyist" computers available at the time, but they were not as powerful (relying on earlier microprocessor chips), nor as well publicized as the Altair. The Altair computer was featured on the front cover of the January, 1975 edition of Popular Electronics magazine, with a series of articles on the design and construction of the machine. Orders for the Altair poured in at a tremendous rate, propelling MITS and the Altair into the spotlight of personal computer history.

MITS started up as a small four-man company in late 1969. The principals were Ed Roberts, Forrest Mims III, Stan Cagle, and Bob Zahler. In January of 1970, the company was formally incorporated. Initial discussions came to the conclusion that there was a market in the model rocketry field for various electronic devices to go up with the rocket, at first, strobe lights to aid in recovery (for night launches), and later, to provide simple radio telemetry. These devices were designed such that hobbyists could build them from a kit, or buy pre-assembled. Later, MITS developed a novel wireless "walkie talkie" that used a beam of infrared light to send voice between two transceivers, up to 1000 feet apart.

Cover of November, 1971 Popular Electronics Featuring MITS 816 Prototype

While these products generated a modest revenue, by the early part of 1971, MITS was looking for a way to break into new markets. At the same time the fledgeling Large-Scale Integration (LSI) MOS Integrated Circuit manufacturer, Electronic Arrays, had recently introduced a six-device chip set that provided a great majority of the logic for a complete four-function electronic calculator. In mid-1971, the MITS team realized that there was tremendous market potential for low-priced electronic calculators. MITS contacted Electronic Arrays to arrange for some production samples. Soon, a prototype machine was built. MITS arranged volume pricing of the chip set, further reducing the cost. Since MITS had experienced some earlier success with Ed Roberts and Forrest Mims writing articles for various hobbyist publications, especially Popular Electronics, it was hoped that the calculator could make its debut as a featured construction article in that publication. Popular Electronics was contacted, and shown the prototype of the calculator in the fall of 1971.

Production MITS 816 Calculator - Note differences between production and prototype on magazine cover above.
Photo Courtesy of Steve Shepard

The calculator was a big hit with the magazine's publishers(Ziff-Davis), and made the cover of the November, 1971 issue of the magazine, with the lead story proclaiming "Electronic Desk Calculator You Can Build". At the time, the machine was called the "Popular Electronics Calculator", but internally to MITS, this machine was designated as the model 816. The 816 could be ordered as a complete, assembled calculator for $275, or as a ready-to-build kit for $179. The model number 816 was based on the fact that the calculator had an eight-digit vacuum-fluorescent display, but could perform calculations to sixteen digits. At the time, the major players in the calculator market were charging between $400 and $600 for an electronic calculator with similar functionality.

The Electronic Arrays chip set consisted of six LSI integrated circuits containing over 8,000 transistors. The six devices had specific functions; input(keyboard processing), output(display generation), arithmetic(math processing), register storage, control ROM(microcode), and control logic. While it isn't entirely clear at this time, the chip set used in the MITS 816 appears to be the same chip set used in the ICM-816 calculator, which was introduced by a subsidiary of Electronic Arrays called International Calculating Machines, in early 1971. The ICM 816 initially sold for $495.

Inside view of MITS 7440

MITS proceeded to develop a number of other desktop (908DM, 1200-Series and the 1440), and later, handheld calculators, all of which were sold as kits or fully-assembled. While the calculator line was initially successful, intense competition in the electronic calculator marketplace, along with the introduction of single-chip calculator IC's by various large chip makers such as Texas Instruments, Rockwell and General Instrument, not to mention stiff competition from Japanese electronic calculator manufacturers Sharp, Casio, and Canon, it became ever more difficult for MITS to remain price-competitive in the market. There was also tough competition in the electronics kit market from Heathkit, which offered a number of high-quality build-it-yourself calculator products. By the late part of 1973, it was possible to buy a mass-market calculator fully assembled for less than it cost to buy one of MITS 816, 1440, or 908DM desktop calculator kits.

Cover of the Famous January, 1975 Popular Electronics, with Altair 8800

As a result of these market pressures, MITS management realized that they were going to have to come up with a new product to recapture the revenue that the sagging sales of their calculators was causing. The result was the Altair 8800 computer, a "complete" (albeit, quite limited without additional expansion) Intel 8080 microprocessor-based computer kit that sold for $379, or in fully-assembled for $479. Another front-cover feature in Popular Electronics magazine, and MITS was on its way to an astounding (yet daunting to the small company) success. MITS' place in history was cemented as the maker of the first "minicomputer" replacement, or "personal computer" at a price that made it affordable to a mass market of computer hobbyists.

The last desktop electronic calculator offered by MITS was the Model 7440, exhibited here, introduced in early 1974. The 7440 was a scientific calculator, providing many of the features of earlier expensive desktop scientific calculators such as those made by Wang, Hewlett Packard, Compucorp, and others. However, the 7440 was a bit dated and out-classed as soon as it was introduced, as in mid-1972, Hewlett Packard had introduced the revolutionary shirt-pocket, scientific, rechargeable battery-powered HP-35 calculator, which provided essentially the same functionality as the 7440 in a very high-quality, portable package, for $395. While the 7440 was priced at $199.95 for the kit, and $299.95 for the fully-assembled unit, the allure of a "take anywhere" calculator like the HP-35, with HP's incredible reputation for quality and innovation, was worth the extra $120 to many professionals who were interested in a handheld turn-key solution from a high-quality company, rather than buying a pre-built desktop machine from a small time player like MITS. While those on a budget likely found the $199.95 price for the 7440 calculator in a kit form attractive, the market was changing so fast that the allure of the 7440 proved to be short-lived.

The MITS 7440 Programmer (click on Image for View of 7440 Programmer Ad)

At the same time the 7440 calculator was announced, MITS also announced the "7440 Programmer", a device about the same size as the 7440 calculator that added programmability to the 7440 calculator. The Programmer was based on small and medium-scale TTL integrated circuits, along with new at the time Intel 1101 RAM chips for storing the program steps. The 7440 Programmer was offered as a complete assembled unit for $299.95, and as a kit for $199.95. The 7440 Programmer offered 256 steps of program memory. The instruction set, which consisted of seven bit hexadecimal operation codes, provided function codes for each of the keys on the 7440 calculator, limited test and branch capabilities, and the ability to light or extinguish six individual LEDs on the front panel of the Programmer to serve as prompting for the user of a program to enter data points, as well as halting the system to allow user input to the calculator, and also to mark the end of a program. For an additional $129.95 (in assembled form, $79.95 for the kit), the capacity of the Programmer could be expanded to 512 keystrokes through the addition of seven 110A RAM chips that would plug into un-populated sockets on the Programmer's memory circuit board. The combination of an assembled 7440 and the base 7440 Programmer would cost just under $600.

In comparison, shortly before the introduction of the 7440 calculator and its companion Programmer, Hewlett Packard had introduced their HP-65, a more capable version of the HP-35, adding, among other things, 100-step learn-mode advanced programming capability along with the ability to load and store programs on small but durable magnetic strips through use of a built-in miniature magnetic strip reader/writer. The HP-65 had an introduction price of $795, a mere $195 more than the combination of the MITS 7440 and base 7440 Programmer. The 7440 Programmer had no way to load or save programs except from the its hexadecimal keyboard. The Programmer has a place inside the cabinet for a standard 9 Volt battery to be connected that would provide power to retain the content of the program memory when the Programmer was not powered from wall-socket power. A fresh battery would keep the memory content safe for approximately one hour, with its primary purpose being to keep the memory alive when moving the calculator/programmer combination to a new location. The magnetic strip reader/writer in the HP-65 calculator made the pocket-able machine far more useful than the MITS programmable calculator system. Not only could a program be stored on a magnetic strip for later re-entry into the calculator in a matter of a few seconds, the magnetic strip reader made it possible to have a library of programs stored on individual magnetic strips, with any of the programs able to be loaded into the calculator and ready to use in a just a few moments.

The combination of the 7440 scientific calculator and 7440 Programmer took up a significant amount of space on the desktop, had somewhat limited programming capabilities compared to the HP-65, and very importantly, was not able to be carried around in a shirt pocket. The 7440 calculator's transcendental functions were not particularly accurate, an important consideration in complex engineering calculations. The 7440 Programmer was truly more of a curiosity for someone who had purchased or built a 7440 calculator and was interested in being able to write programs for it, either as an educational pursuit to learn programming concepts, or for specific types of calculations that the owner had in mind. The Programmer really wasn't a product that could come close to the capabilities of already existing programmable calculators on the market, even at its relatively low price. The 7440 calculator itself was a useful calculating machine at a reasonable price, but its limited accuracy in its transcendental functions was a pretty serious handicap for use of the calculator in calculations where accuracy was a serious consideration.

It isn't clear how many 7440's were sold, but the number had to be somewhere in the hundreds of machines at the most. The Programmer device probably saw far fewer sales, due to its limited audience. While MITS had a lot of success initially with their calculator products, the realities of the marketplace made it impractical for MITS to continue to be competitive by the end of 1973.

The MITS 7440 exhibited here was originally purchased in kit form by Mr. Bruce Franklin (May 17, 1945 - June 3, 1991). Mr. Franklin was a journeyman electrician in Baltimore, Maryland. He was the type of person that always loved to learn new things, and signed up for a correspondence course in which the MITS 7440 kit was a part of the curriculum. Mr. Franklin was attracted by both the challenge of building a scientific calculator from a kit, and learning about the wonders of cutting-edge electronic technology. It is clear that Mr. Franklin was well-versed in the construction of electronic devices, as the quality of his assembly is extremely high. His wife, Barbara, said that he cherished the machine, which is very obvious, as the calculator is in wonderful condition both physically and operationally after all of these years. While it is unknown exactly when Mr. Franklin took the correspondence course, or whom the course was offered by (MITS did not provide such learning material), it is clear from the serial number of the calculator, D10063, that it had to have been sold very early after the introduction of the machine. The first advertisements for the 7440 appeared in Popular Electronics magazine in March of 1974. The date codes on the MOS Technology chip set in the machine are from the 12th and 13th weeks of 1974, which were in mid-March. It's my guess that this machine was assembled from the kit sometime in the late spring to early summer of 1974.

MITS 7440 Keyboard Layout

The 7440 provides a nice complement of scientific functions, including Trigonometric (Sin, Cos, Tan and inverse functions), Logarithmic (Natural and Base 10 Logarithm, and e^x), as well as raising an arbitrary number to an arbitrary power, square root, reciprocal, and the usual basic four math functions. The machine has ten significant digits of accuracy, with the ability to automatically switch to scientific notation (with an exponent ranging from -99 to +99) when the number to be displayed exceeded 10 significant digits. The display is made from fourteen early LED (Light-Emitting Diode), seven-segment display modules manufactured by LED pioneer Litronix. Litronix was a spring-1970 spin-off from Monsanto, where the seven-segment LED display first began mass-production.

Close-Up of Display (note discrete LED at left indicating calculator is in Radians mode)

The left-most digit of the display is reserved for the sign of the number, as well as error indication. The next ten digits were used for the main numeric display, be it a floating point number, or the mantissa in the case of scientific notation. Three more seven segment displays were used to display the exponent when the calculator was in scientific notation (one digit for sign of the exponent, and two digits for exponent itself).

The MOS Technology MCS2525 & MCS2526 LSI devices that provide the calculating power of the MITS 7440

The 7440 is based on a two-chip LSI chip set made by MOS Technology (not to be confused with Mostek, which was a different company), with each ceramic-packaged chip having 28 pins. As noted earlier, the chips are date- coded the 12th and 13th weeks of 1974. The part numbers of the chips are MCS2525-001 and MCS2526-001. It is possible, though unverified, that MOS Technology was the first IC manufacturer to cram the logic of a fully-featured scientific calculator onto only two chips. This chip set went through a number of revisions over its lifetime with the MCS2525 going to version 004, and the MCS2526 going to version 005.

The chips as mounted on the circuit board

This chip set was used by a variety of calculator manufacturers including Commodore, Kings Point, Netronics, Summit and Qualitron. Earlier scientific calculators, including the famous handheld machines from HP, utilized three or more Large Scale Integration devices. MOS Technology's primary customer for its calculator chip sets was Commodore, which eventually acquired the company (renaming the company to Commodore Semiconductor Technology) to make chips for Commodore's calculators. Before being acquired by Commodore, MOS Technology had become quite famous for the development of the 6502 microprocessor, an simple yet elegant 8-bit single-chip CPU that ended up being the processor of choice for Steve Wozniak and Steve Jobs' prototype computer which became the Apple I microcomputer, the genesis of Apple Computer. The 6502 carried on to Apple's enormously successful Apple II line of personal microcomputers. The 6502 was also used in Atari's fabulously successful game consoles, and later, Atari's 400 and 800 microcomputer systems.

Internal Circuit Board Layout

The MITS 7440's design uses three circuit boards. The main board, occupying most of the base of the calculator, contains the power supply circuitry (except the small transformer, which is mounted directly to the sheet metal baseplate of the machine), the two calculator IC's (in high-quality sockets), clock generation and transistor-based digit drive circuitry.

Back view of Internals. Note hand-wired connections between Main and Display Boards

The second board, mounted to the main board at an angle, hosts the LED display modules, each with seven segments and a left(not used) and right-hand decimal point. The digits have a height height of 1/2 inch, making the display quite readable, even from a distance. A single discrete LED is also included to indicate the calculator is performing trig calculations in radians (when lit) or degrees. Along with the LED devices are transistorized segment driver circuits. The display is multiplexed at a high speed, making it appear that the display is continuous, although in reality, the digits are presented to the LED displays a digit at a time. The display provides leading and trailing zero suppression in the mantissa portion of the display, however, exponents when the calculator switches to scientific notation do not have a leading zero suppressed. Results are right-justified on the display.

Lastly, a third circuit board makes up the keyboard assembly. Two identical "blocks" of key-switch assemblies are mounted to this circuit board. Each key-switch assembly contains 18 key-switches, which utilize gold-plated wiper-type switch contacts for long life, and minimal actuation bounce. While magnetically-actuated micro-switches were the most reliable mechanism for keyboard switching, the switch modules used by MITS were of high quality, and still work very nicely to this day, with no key bounce (which results in multiple entries for a single key press) observed. The key-switches are wired in an X-Y array, which is scanned rapidly by the chip set to determine which key is depressed at any given time. The chip set contains logic to ignore depression of more than one key at the same time, to prevent input errors. The key-caps are made of high-quality plastic with embedded nomenclature to prevent the key labels from wearing off with use. Interconnections between the circuit boards are made with many individual wires, requiring a bit of patience for those who opted to build the machine from a kit.

At the right side of the keyboard circuit card are empty connections for a cable assembly that would provide the connectivity for the 7440 Programmer. The Programmer included a cable assembly which would have to be soldered in place, and routed through an extra hole in the bottom of the calculator case (which is covered by an adhesive "patch" in machines without the programmer facility).

Etched Identification on Main Circuit Board

The circuit boards are made of fiberglass, with tin plated copper traces on both sides of the board. Plated-through feed-throughs provide connectivity between each side of a circuit board. Yellow silk-screened component identification and other informative information exists on all of the boards, making the job much easier for kit builders. The quality of the boards is not bad, but not great. Care had to be taken when soldering in components so as not to overheat the circuit board, which could lead to de-lamination of the pads and traces from the circuit board. Component layout and density is conservative, likely to allow for easier construction by those who opted for the kit version of the machine, as well as easier manufacturing of the machines that MITS offered in assembled form.

Bottom View of Cabinet (to show original color)

The calculator's cabinet is made from a decent quality molded plastic (likely ABS) in a light gray/beige color. The surface is a wrinkle texture that is molded into the cabinet, except the area around the keyboard. A red filter surrounded by a black bezel is glued into place, positioned in the cabinet in front of the LED displays to make the circuit board and components less visible to the user, while allowing the LEDs to shine brightly through.

Power Switch with MITS Nomenclature

On the top surface of the cabinet, a high-quality rocker switch provides the means for turning the machine on and off. It seems that this cabinet was a general-purpose cabinet, as it appears to be identical to that used on the earlier MITS 1440 calculator, as well as being the same cabinet (with different cutouts for the keyboard) used on the 7440 Programmer. The baseplate of the calculator is a black-painted stamped piece of sheet metal, with stamped holes for ventilation, and drilled holes for securing fasteners. Four rubber feet provide stable, non-skid footing for the calculator. The serial number tag is affixed on the bottom side of the baseplate. The cabinet attaches to the baseplate with a set of six machine screws.

Serial Number Tag [Note "Old-Style" Original MITS Logo, Pre-Altair]

From a user perspective, the 7440 is quite simple to use. It utilizes algebraic logic, with two-levels of parenthesis nesting to ease more complex calculations. For example, performing 14 X ((6 - 5) / 2), one would simply enter the problem exactly as shown, pressing the "=" key at the end of the problem to display the result of "7". The calculator does not process problems by the mathematical rules of precedence, rather it simply executes the operations left to right as entered, with the parenthesis over-riding this rule. The 7440 offers a single memory register. The content of the display can be stored into the memory register by pressing the [=] key followed by the [M] key. Recalling the memory register to the display is done by simply pressing the [M] key alone. There is no indication provided to show when the memory contains a non-zero content. The only way to clear the memory register is to either power-cycle the calculator (the memory register is automatically set to zero on power-up), or to explicitly store zero into the register using the key sequence [0] [=] [M]. The trigonometric functions can be carried out in either degrees or radians. A [DEG/RAD] key on the keyboard toggles between these two modes, with a single LED lighting at the left end of the display panel to indicate that the calculator is operating in radians mode. Inverse trigonometric functions are calculated by first pressing a key labeled [ARC], followed by the particular trig. function desired. Typical of most calculators of the period, the [X↓Y] key swaps the two operating registers of the machine. The 7440 has no means for performing constant calculations, which is somewhat surprising. The calculator operates in full automatic floating decimal mode, always positioning the decimal point to maximize the accuracy of the displayed result, with leading zeroes suppressed. If the number to be displayed is too large to fit within ten digits, the display automatically switches to scientific mode, and will also switch back to standard floating point mode if the calculation results return to a number within the ten significant digit capability of the machine.

Display Indicating Error Condition

The MOS Technology chip set catches all error conditions, including division by zero, extraction of the square root of a negative number, raising a number to a negative power, nesting of parenthesis more than two levels deep, and calculation over/under-flow. The machine does not provide input overflow detection, simply ignoring any digit entries in excess of its ten significant digit capacity. Error or overflow conditions are indicated by the left-most digit in the display showing an unusual combination of segments. When such a condition exists, the machine ignores any further keyboard presses until the [C] (Clear All) key is pressed to reset the error condition. The calculator performs an automatic clear operation at power-up. Occasionally, though, the power-up initialization fails, and gibberish appears on the displays, requiring a press of the [C] key to force the machine into a normal operating state. This could be a result of component aging, or perhaps a minor design flaw either in the MOS integrated circuits or the power supply.

Cover of MITS 7440 Operation Manual (Click Image to View Manual, 36 Pages, 28MB)

The 7440 (at least at the early production level of this machine) came with a 36-page Operation Manual [WARNING-28 Megabyte PDF Download], spiral bound, with a heavier grade textured paper front and back cover. It appears that the document was printed, rather than just photo-copied from the typewritten master document. While the document is primitive from a production standpoint, it is well-written, and provides a good summary of the operation of functions of the machine, a section dealing with accuracy and error detection, and lastly, a section with a broad selection of example problems and how they can be solved with the machine.

The 7440 blanks the display while calculations are in progress. While not nearly as fast as the 1968-vintage HP 9100 desktop calculator, or the early '70's Wang 700-series machines, answers are typically almost instantaneous, with a few exceptions (logarithmic and trig. functions), some of which can take up to 3/4 second to perform. The 7440 provides a result of 9.08210803 to Mike Sebastian's Calculator Forensics calculation, indicating that the accuracy of the trigonometric functions leaves a bit to be desired.