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Casio "Mini" Handheld Calculator

Updated 5/3/2023

The Casio "Mini" was Casio's first attempt at a handheld calculator, and, even though the machine was rather a compromise, the development of the machine signalled the beginning of events that shaped the future of the calculator business. First, the Mini was one of the first handheld form-factor calculators to use a single-chip LSI calculator chip. Next, the Casio Mini was introduced at a price that shocked the whole marketplace, significantly lower in price than anything else on the market at the time. Casio caught the world by surprise in August of 1972 with the introduction of this low-price machine, which spurred competitors to have to very quickly come up with their own inexpensive, handheld, battery-powered calculators or lose precious market-share to Casio. The Mini was inexpensive enough that, for the first time, electronic calculators began showing up in the hands of school children - an event that forever changed the way the school system dealt with mathematics. Today, a calculator is an indispensable part of the mathematics curriculum in most American schools.

At the time the Mini was being developed, just prior to Hewlett Packard revolutionizing the calculator world with the feature packed (and very hand-held) HP-35, it was quite a challenge to cram all that was needed to make a useful calculator into a hand-held package. Another challenge that the designers had to face was making a handheld device that could get sufficient battery life given the power requirements of display technology such as the Vacuum Fluorescent displays used back in the early 1970's. At the time, LED displays were still early in development, and were power-hungry and expensive. All of these constraints put together made the Casio Mini quite a compromise. Despite the constraints this little calculator was a true marvel of technology. This particular Casio Mini was built sometime in the mid-to-late 1972 time-frame, based on date codes on devices within the machine This makes this particular machine an early example of the Mini, given it's August, 1972 introduction date in Japan. This machine was originally purchased in Japan as evidenced by the label on the machine from "United Office Machine, Inc." in Tokyo, and somehow made its way into the US. I found it at an antique store in Sisters, Oregon. The machine is in meticulous condition, with virtually no signs of usage. It would certainly be interesting if this machine could tell of its travels.

Later versions of this basic design, such as the Sperry-Remington 661D improved upon the functionality of the early Casio-designed handhelds by adding support for zero or 2 digits behind the decimal point, and added a decimal point key to the keyboard to allow entry of numbers with digits behind the decimal point.

The compromises in the Casio Mini are quite evident. Straight away, one notices that the machine has only a 6-digit display. Six tiny 8-segment (the 8th segment is never used) Vacuum Fluorescent display tubes made up the display of the machine. The reason for the small number of digits is simple -- cost. The display is generally one of the more expensive parts of a calculator, and reducing the number of display elements cuts the cost to the end-user. Another factor that contributed to the six-digit display is power requirements. Vacuum Fluorescent (VF) tubes, while much less demanding than gas-discharge type displays, still require a higher voltage (around 45V) to operate on. Fewer tubes in the display means less power is required. Since the Mini only has the power of four AA batteries to operate from, minimizing the number of higher-power devices was a requirement. Another quick observation when looking at the machine is the lack of a decimal point key on the keyboard. However, there is a [→] key where one would assume a decimal point key should go (more on this key later). Another observation would be that, for a handheld machine, the Mini is somewhat heavy, and although packaged nicely, is a bit large to be considered a true handheld machine. When powered up, the compromises become more evident. The machine has no concept whatsoever of a decimal point. It works purely on integer math, with no provision for entering a decimal point, nor displaying one (though the VF tubes used do have a decimal point, they are not used). The [→] key provides the portion of an answer behind the decimal point, but it's up to the user to figure out the actual location of the decimal point. For example, performing 1/3, the display will show '0' Pressing and holding the [→] key will then show "333333". This [&rarr'] key also provides the least significant digits of results that exceed the six-digit display, for example, performing 123456 X 123456 results in "152413" being displayed. Pressing and holding the [→] key will show "83936", so the total result is "15241283936". This gives the machine a capacity of 12 digit results, but only for a single calculation, and only for multiplication and division operations. The twelve digit result capacity for multiply and divide is only good for one operation, as any operation key pressed after a result is generated causes the register displayed by the [→] key to be cleared. Operations that overflow/underflow the machine when performing addition or subtraction simply cause any overflow/underflow digits in excess of the 6-digit capacity of the machine to be discarded. The Mini has no provision for detecting or indicating overflow conditions. Division by zero causes the machine to to display all zeroes on the display, and to act strangely until cleared. An interesting effect can be achieved by pressing the [÷] key followed by the [=+] key twice, followed by any digit, then the [=+] key one final time, then holding down the [→] key. The display will show that the calculator is counting upwards from zero at approximately a 30 count-per-second rate. Pressing [C] stops the counting (and clears the machine). Pressing any single digit while this counting is going on will result in a flickering zero followed by the digit pressed showing up on the display. Pressing and holding the [→] key at this point will show a mad dance of digits flitting about the display. Why all of this occurs is a mystery, but it all seems to be related to a strange state that the logic gets into when division by zero occurs. The Mini does process negative numbers, but uses a digit position for indicating a negative number, so if a negative number exceeds five digits, the negative indication has no room to be displayed. This could result in negative answers being interpreted as positive results, making it all the more important for the user to take an active role in interpreting the answers from the machine. For example, performing "99999", [-], [1], [-], results in the display showing "100000". After this, entering [1], followed by [=+], will result with the display showing "-99999". The sign is retained, but not displayed when the negative number is greater than five digits in length. Addition and subtraction operate adding machine accumulator style. Multiplication and division operate in algebraic fashion, with the [=+] key generating the result.

The Inside of the Casio Mini

The Mini uses a very early Calculator-on-a-Chip' Large Scale Integration device made by Hitachi, part number HD32127, and another NEC-made display driver chip to run the display (µPD129C). The display, keyboard, calculator chip, and display driver chip are mounted on the main board of the machine that runs the width of the machine. A smaller (about half the size of the main board) board is mounted underneath the main board, connected by individual wire jumpers, contains the small switch-mode power supply to generate the necessary operating voltages for the MOS calculator chip and the display from the 6V DC battery or AC adapter supply. Along with the power supply circuitry, there is also the clock-generation circuitry for generating the master timing signals for the calculator IC. This board is populated mainly with discrete components. The Mini is powered by four AA-cell batteries, and also has a jack for an external 6V DC power supply. The Mini is not a very fast machine, with 99999 / 1 taking about 1/2 second to perform. The clock runs at only 15KHz, which explains why the machine isn't very fast. In spite of its compromises, the Casio Mini was truly a landmark in the history of calculator technology, setting a new low-price record for a four-function (albeit limited) calculator that created a price war in the marketplace that led to a great shakeout in the calculator industry within a year of the Mini being introduced.

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

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