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MITS Model 7440 Desktop Scientific Calculator

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 telemetery. 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 chipset 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 chipset, 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 chipset 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 chipset used in the MITS 816 appears to be the same chipset 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 and 1440), and later, handheld calculators which were sold as kits or fully-assembled. While the calculator line was initially successful, the intense competition in the marketplace, along with the introduction of single-chip calculator IC's by various large chip makers such as Texas Instruments, Rockwell and General Instruments, made it difficult for MITS to remain price-competitive in the market. There was also tough competition in the electronics kit market from Heathkit, who 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 came to the realization 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 for $379, or fully-assembled for $479. Another front-cover feature in Popular Electronics magazine, and MITS was on the way to an astounding (yet daunting to the small company) success, and MITS' place in history as the maker of the first "minicomputer" replacement 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, and very 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 HP-35 calculator, which provided essentially the same functionality, 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" machine 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 in a kit attractive, the market was changing so fast that the allure of the 7440 was short-lived.

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

At the same time the 7440 was announced, MITS also announced the "7440 Programmer", a device about the same size as the 7440 calculator that added programmability to the 7440. The 7440 Programmer 0as offered as a complete assembled unit for $299.95, and as a kit for $199.95. The 7440 Programmer offered 256 keystrokes of program memory, and a limited group of test and branch capabilities in its base form. For an additional $129.95 (in assembled form, $79.95 for the kit), the capacity of the Programmer could be expanded to 512 keystrokes. The combination of an assembled 7440 and the base 7440 Programmer would cost just under $600. Just prior to the introduction of the 7440, in January of 1974, Hewlett Packard introduced its HP-65, essentially a more capable version of the HP-35, adding, among other things, 100-step advanced programming ability and the capability to load and store programs on magnetic strips. The HP-65 had an introduction price of $795, a mere $195 more than the combination of the MITS 7440 and 7440 Programmer. The 7440 Programmer had no way to load or save programs except from the keyboard, and when the power was turned off, the program was lost. The combination of the 7440 and 7440 Programmer took up a significant amount of space on the desktop, had fairly limited programming capabiltites as compared to the HP-65, and its transcendental functions were not as accurate as the HP65, an important aspect in complex engineering calculations.

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 even fewer sales, due to its more technical audience. While MITS had a lot of success initially with their calculator kits, the realities of the marketplace simply 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 bleeding-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 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 chipset 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 compliment of scientific functions, including Trigonometric (Sin, Cos, Tan and inverse functions), Logarithmic (Natural and Base 10 Logarithm, and ex), 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 "Brains" of the 7440

The 7440 is based on a two-chip LSI chipset 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 to cram the logic of a fully-featured scientific calculator onto only two chips. This chipset 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 chipset 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 chipsets was Commodore, who eventually acquired the company (renaming the company to Commodore Semiconductor Technology) to make chips for Commodore's line of calculators, and later early their personal computers (the PET, Commodore 64, and later, the famous Amiga). Before being acquired by Commodore, MOS Technology had become famous for the development of the 6502 microprocessor, an elegant single-chip CPU that ended up being the processor of choice for Steve Wozniak's and Steve Jobs' prototype computer that became the genesis of Apple Computer.

Internal Circuit Board Layout

The MITS 7440's design utilizes 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), master clock generation, and transistor-based digit driver 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, provides the LED displays (each a module, with seven segments and a left(not used) and right-hand decimal point, with a digit height of 1/2 inch), a single LED to indicate whether the calculator is operating in degrees or radians (radians when lit), and the 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 digit-at-a-time. The display provides leading and trailing zero suppression in the mantiassa portion of the display, however, exponents when the calculator switches to scientific notation do not have a leading zero suppressed. Results are right-justfied on the display.

Lastly, a third circuit board makes up the keyboard assembly. Two identical "blocks" of keyswitch assemblies are mounted to this circuit board. Each keyswitch assembly contains 18 keyswitches, which utilize gold-plated wiper-type switch contacts for long life, and minimal actuation bounce. While magnetically-actuated microswitches were the most relaible 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 keypress) observed. The keyswitches are wired in an X-Y array, which is scanned rapidly by the chipset to determine which key is depressed at any given time. The chipset contains logic to ignore depression of more than one key at the same time, to prevent input errors. The keycaps 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 feedthroughs provide connectivity between each side of a circuit board. Yellow silkscreened 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 good. The 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 for the pre-built machines that MITS offered.

Bottom View of Cabinet (to show original color)

The calculator's cabinet is made from a quality moulded plastic (likely ABS) in a light gray/beige color. The surface is a wrinkle texture, moulded into the cabinet, except the area around the kayboard. 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. The baseplate of the calculator is a black-painted stamped piece of sheet metal, with cutouts for ventilation, and drilled holes for securing fasteners. Four rubber feet provide a 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 machine 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 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 properly 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 chipset 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 locks out any further operations until [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

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 as a result of Mike Sebastian's Calculator Forensics calculation, indicating that the accuracy of the trigonometric functions leaves a bit to be desired.


This exhibit is dedicated to the memory of Mr. Bruce Franklin (1945-1991), the original owner and builder of this calculator.

Sincere thanks to Mr. Russell K. Hobbie for making it possible for the Old Calculator Web Museum to acquire this artifact.

Text and images Copyright ©1997-2011, Rick Bensene.