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Master Model H-1 Desktop Calculator

Updated 4/27/2023

This old calculator appears to be rather unique and quite unusual. It was found by my aunt at a Seattle-area flea market. She paid $0.50 for it! The calculator was manufactured the mid-1972 time-frame. It was made by Master Calculator Co., a division of 6/C Inc, of Grand Prairie, TX. Master Calculator Co. was registered by the Texas Secretary of State on June 9, 1972, which is an indication of when the company formally began operating as a legal business entity in Texas, meaning that this calculator was certainly an early-production unit. Interestingly, documentation has been found indicating that the calculator was actually introduced by the company in the Summer of 1971, nearly a year before the company was registered.

This is actually not that unusual, as many calculator companies in the early 1970's were started by a relatively small group of folks who got together and designed a calculator using off-the-shelf Large Scale Integrated Circuits, which were becoming available on the open market, building a prototype or perhaps a very small pre-production run. Once there was something demonstrable as a working product, it was announced in a press release, usually considerably before the the company was at a point of being able to sell their creation. Right after the press release was made, the calculator would make a few appearances at business machine trade shows to drum up potential investment and/or distributors for the machine. Depending on the situation, start-up cash would come through investment, large pre-orders from interested and cash-rich distributors, OEM agreements, or combinations thereof, to raise the funding to tool up for volume production of the machine, as well as creating documentation and a support infrastructure. Once enough funding was appropriated (likely in this case due to 6/C Inc. buying a controlling stake in the company in return for a nice chunk of the profits), the company was registered with the state, production ramped up, and the distributor network and OEM customers primed with the first batches of machines off the line to sell off to to office machine retailers eager to under-cut the pricing of their competitors.

If anyone reading this has information about Master Calculator Company, 6/C Inc., or the apparent follow-on to the H-1 which was called the Master II calculator, please contact the museum by visiting the EMail page. It is hoped that someone associated with either Master Calculator Co. or 6/C Inc. will find this exhibit and contact the museum and be able to help fill in details about the history of Master Calculator Co.

The SCM F-16

It appears that Master Calculator Co. forged an OEM relationship with Smith Corona Marchant (SCM) fairly early in its history. The Master H-1 was marketed by Smith Corona Marchant(SCM) as the SCM F-16. Under the agreement, Master Calculator Co. built the calculators and shipped completed calculators to SCM complete and ready to sell under the SCM/Marchant brand names. It appears that at least the Master II follow-on to the Master H-1 was also marketed by SCM with the only differences being badging and model numbers.

Inside the Master H-1

The Master H-1 has some rather unusual features for a four-function desk calculator. It has a full 16-digit capacity, but only has an 8-digit display. A special key on the keyboard (a double-ended arrow, e.g. [↔]) toggles the display back and forth between the most significant and least significant eight digits of the 16-digit number on each key press. It is possible to enter numbers larger than the eight digits on the display, however, when entering numbers from the keyboard, the [↔] key doesn't work...you end up entering numbers larger than 8 digits blindly. The [↔] key only works when results of calculations are in the display. However, any numbers greater than eight digits entered will be retained and used within calculations.

The H-1 uses fixed decimal point logic. However, on inspection, it is not at all obvious how to set the decimal point location. It was obvious when powered it that it sets itself to zero digits behind the decimal point, which is rather limiting, but with no dial, switch, or other obvious selector to set the decimal point to a more useful position. It took a bit of time fiddling around with the machine to figure out how to change the decimal point position setting. I found that pressing and holding the [CE] key, while at the same time, pressing a digit from 0 to 7 on the keyboard would set the decimal point to the location specified by the held-down digit. This method is reminiscent of the fixed decimal location setting as used by the earlier Marchant Cogito 412 and Cogito 414 calculators, but at least with these calculators there are markings on the keyboard to indicate the decimal point setting method.

The Master H-1 Logic Board

The Master H-1 is based on a four-chip LSI chip set made by Electronic Arrays, Inc.(EA), of Mt. View, California. EA no longer exists, having been purchased by Japanese electronics giant NEC after falling upon hard times in the late 1970's due to extreme competition in the memory chip marketplace. The chips used in the H-1 are numbered "190B-7010", "280B-7008", "310B-7014" (Input Chip), and "150B-5005" (Output Chip). The Input IC takes input from the keyboard, conditions it (de-bouncing switch contacts), and encodes it such that the control logic can carry out the sequence of operations required to carry out the function of each key. The Output IC is responsible for driving the display, which involves translating the internal representation of the number to be displayed into seven-segment digit form, and multiplexing that information out to the display at a speed fast enough that even though only one digit is lit up at any given time, it appears to be a constant eight-digit display to the human eye. The other two chips combine forces to contain the working registers of the calculator, a simple arithmetic unit that can add two single-digit numbers together, the control sequencing logic that steps the machine through its various micro-operations (such as shift digits right or left, add two digits, process carries/borrows, etc.) that combine to create the means by which the machine carries out the functions commanded by the keyboard, and lastly, orchestrating the flow of data between the working registers, the keyboard and display, and the arithmetic unit.

The Electronic Arrays chip-set has date codes ranging from the earliest being 7148(48th week of 1971), to the latest of 7201 (1st week of 1972). This chip-set appears to be an updated version of Electronic Arrays' first calculator chip set, designated the S-100 chip-set, used in another calculator in the museum, the ICM 816. The Electronic Arrays S-100 chip-set was the first set of MOS Large Scale Integration (LSI) chips that were for general sale to anyone. Prior to the S-100 chip-set, calculator chip-sets were proprietary items manufactured specifically and exclusively for the use of large calculator manufacturers such as Canon, Sharp, Casio, Busicom(Nippon Calculating Machine Co.), Monroe, and Smith Corona Marchant(SCM). The S-100 chip-set was fully documented with data sheets and applications notes that just about anyone with a modicum of electronics skills could use to build their own calculator, and in fact, a variant of the S-100 chip-set was used by a company called MITS to develop a kit electronic calculator called the MITS 816 that was written up as a construction article in the November, 1971 edition of the electronics hobbyist magazine Popular Electronics, sparking the beginning of a revolution in "Personal Computing" that led to the eventual development of the earliest of personal computers. But, that's a completely different story. Suffice it to say that Electronic Arrays desire to make a low-cost, accessible electronic calculator chip-set generally available truly changed the electronic calculator marketplace from one that was dominated by the long-established players mentioned above to one where the entrepreneurial spirit and a bit of electronics know-how would create a whole new breed of electronic calculator manufacturers, with Master Calculator Co. being one of many.

The EA chips are all in 24-pin plastic packages. Putting the chips in plastic packages was considerably less expensive than the ceramic packages of earlier calculator chip-sets, but it did take some time before plastic packaging could be made such that the delicate chip inside was completely protected from the external environment. The tiny slivers of Silicon that made up the chips were quite sensitive to humidity, electrostatic discharge, and any form of dust or dirt, meaning that initially chips were contained inside elaborate packaging using ceramic packages, gold leads and bond-out wires, and hermetically-sealed packaging. The use of plastic packaging, along with conservative chip layouts and clever management of the interconnects between the chips to allow them to fit in 24-pin rather than 40-pin packages, allowed Electronic Arrays to make the chip-sets priced well within the realm of affordability of a dedicated electronics hobbyist, with the original S-100 six-chip set retailing for $126.10 for a single set of the chips. As time went on, Electronic Arrays was able to reduce the number of chips in the chip-set to five, then to four, which is the chip-set used in the Master H-1. Eventually Electronic Arrays was able to combine all of the logic onto a single chip, but by this time, a number of other IC manufacturers, including MOSTEK, Texas Instruments, and General Instrument(Pico Electronics) already had single-chip calculator ICs on the open market, with Electronic Arrays a late arrival on the single-chip calculator IC scene, partially contributing to their eventual financial difficulties. A National Semiconductor DM8880 chip provides segment drive for the gas-discharge display panel. The circuitry of the machine (except the power supply transformer) is mounted on a double-sided fiberglass circuit board, which uses plated-through holes for connectivity between the top and bottom of the circuit board. The power supply is a basic transistor-regulated linear supply. The keyboard uses individual key modules that contain magnet-activated micro-switches. The key-caps are molded plastic with printed-on (rather than more-expensive molded in) key-cap legends. Screen-printed legends on key-caps was less expensive than the double-shot injection molding required to embed the key-cap legends in the key. Printed-on legends tend to wear off over time, while molded-in legends held up much better. Given the high quality of the key-switch modules used, it is surprising that Master Calculator did not use molded-in key-cap legends.

The Display Module

The Master H-1 uses a planar gas-discharge display, similar to a Burroughs Panaplex, but is definitely not a Panaplex design. The Burroughs Panaplex gas-discharge displays have transparent electrodes deposited on glass plates that are bonded and sealed together with a narrow area between the glass where the Neon gas mixture resides, making for a very simple display panel. The display module used in the Master H-1 uses a glass panel in front, with a metal mesh screen in front of each digit as the common cathode. The segments are made of segment-shaped metal electrodes, with common segments in each digit wired together with metal busses deposited on a ceramic back plate that is sealed to the front glass. In between the mesh screen and the segment electrodes is a metal mask with cutouts for the lit segments and decimal points to shine through, providing isolation so that light from each segment is separated from the other segments. It is possible that the display used in the Master H-1 is a Burroughs-made device called a Segmatron, which was a precursor to the Panaplex display panel, however, no identifying markings or labels can be found on the display to positively identify it as such. Once the Panaplex display was introduced, the significantly more complex (and expensive) Segmatron display panels likely became obsolete rather quickly. The Segmatron was announced by Burroughs in the early Summer of 1970, not long before the Master H-1 calculator was introduced.

The display panel is arranged as eight digits, each with seven segments, and a right hand decimal point. The display is driven by a combination of the National Semiconductor DM8880 bipolar IC for driving the common seven segments and decimal points, and discrete transistor drivers for each digit screen. The calculator does not provide leading or trailing zero suppression. Negative results are indicated by a discrete Light-Emitting Diode (LED) situated to the left of the display panel. Though the display digits glow orange, the cabinet has a red filter situated in front of the display module, making the digits look very much like large red LED digits rather than orange-red gas-discharge digits.

Display showing most significant and least significant digits of result of 99999999 X 99999999

The Master H-1 is quite fast for its day. The Electronic Arrays chip-set is designed to operate at clock frequencies of up to 200KHz, which compared to most small and medium-scale MOS IC-based calculators is considerably faster, with earlier calculators clocking in at around 50-60KHz. Considering that the Master H-1 calculates to a full 16-digits of capacity it makes its speed even more apparent. Sixteen nines divided by one takes less than 1/3 second to complete. Eight nines times itself results in an almost instantaneous answer, with the most significant digits displayed by default (with no decimal point present), and the least significant digits being displayed by pressing the [↔] key, as demonstrated in the images above.

This exhibit is dedicated to the memory of my Auntie Dyann, who found this calculator for me. She was an amazing woman, and a wonderful aunt who spoiled me rotten. I miss you Auntie Dyann!

Sincere thanks to Mr. LaFarr Stuart for information on Electronic Arrays

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

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