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Commodore US*1 Calculator

The Commodore US*1 is another example of the fairly large line of US* machines marketed by Commodore in the 1972 through 1975 timeframe. Other examples of the US* line of calculators in the museum are the Commodore US*8 and US*10 calculators. The US* line of machines distinguished themselves by low-cost, simple construction, and basic functionality. The US*1 is an entry-level, eight digit floating decimal calculator with constant. It was clear that the US*1 was designed to be a 'price point' calculator, with the primary goal of making the machine inexpensive. Even though it was an inexpensive calculator (with a list price of $59.95) for the time, it is built with a nice level of quality and obvious attention was paid to the design and aesthetics of the calculator.

The US*1 provides the basic four math functions, and provides a constant all four functions, which is an unusual feature, even today. Most electronic calculators provide a constant only for multiplication and division. The US*1 uses algebraic logic, which is also rather uncommon in calculators of this time, with a separate "=" key used to calculate the result of an operation. The calculator provides a fully-floating decimal, with the decimal point always positioned to provide the maximum amount of accuracy within the eight-digit display capacity of the machine.

Inside the Commodore US*1

Internally, the machine is quite simple, utilizing a single-chip calculator LSI (Large Scale Integration) IC for the brains of the machine, a General Instrument C-500. General Instrument was not known as a major manufacturer of calculator IC's, making this machine rather unique. Most of the other Commodore US* line of calculators used Texas Instruments' single chip calculator devices. The C-500 chip is plugged into a Molex-style strip socket, which seems unusual for a low-cost calculator, as normally the LSI's were simply soldered into the board. The remainder of the circuitry makes up power supply and display drive circuitry. The planar display is driven with discrete transistor drivers. All of the circuitry of the machine, with exception of the keyboard module itself, is contained on a single printed circuit board. The circuit board is made of a phenolic material, with circuit traces only on the back side of the board.

Odd Display Behavior When Calculator is Told to Divide by Zero

The US*1 uses a planar gas-discharge display, similar in design to a Burroughs Panaplex panel, but the display is not manufactured by Burroughs. The display uses the standard seven-segment digit arrangement, with a right-hand decimal point included with each digit.

The US*1 exhibited here was built sometime in the 2nd quarter of 1973, based on the date code on the LSI. The calculator is typical in speed for single-chip calculators of this time, with all nine's divided by one taking about 1/3 of a second to complete. The display is not blanked during calculation, and flickers about a bit while calculations are occurring.

As mentioned above, the US*1 offers a constant function on all four math functions. The "K" key is used to set the constant and the type of operation to be performed. For example, to set a constant addition of 1 (turning the calculator into a counter), the user simply presses "1" followed by the "+" key, followed by the "K" key. Then, each successive press of the "=" key will cause the number in the display to be incremented by 1. Similarly, constant multiplication by two would be done by pressing "2", then "X", followed by the "K" key. Once constant operation is set, the only way to clear constant mode is to press the "C" key, clearing the machine and the constant.

Keyboard Detail

The keyboard of the US*1 uses spring-type contacts. The keyboard is a separate module, wired to the main circuit board with individual wires. The keycaps are made of plastic, with molded in color and nomenclature. The US*1 opts for a rather patriotic (given the US designation) color scheme for the keyboard, with blue, red, and black background on white keycaps, and white nomenclature.

The US*1 forgoes any form of overflow or error detection -- any results in excess of the capacity of the machine are simply discarded. The machine does seem to keep track of decimal point locations up to another 8 digits beyond the end of the display. For example, performing 99999999 X 99999999 results in 99999998, with no decimal point lit. Following this calculation with a division by 10000000 results in 999999998. The machine has no input overflow detection; it simply ignores any input in excess of eight digits.

Closer View of General Instrument C-500 LSI and Display Driver Circuitry

The US*1 displays negative numbers by lighting a "-" on the digit preceding the most significant digit in the result. A side-effect of this choice of display method for negative numbers is that the largest negative number that the machine can accurately represent is -9999999, losing a digit of display to the '-' sign.

Performing division by zero on this machine causes an interesting 'counting' effect, with the least-significant digit counting very quickly, and the third digit from the right counting somewhat more slowly. All of the decimal points light up, and the other digits that aren't counting display digits that somewhat relate to the divisor. The only way to exit this state is to press the "C" key to clear the calculator. A static example of this behavior is shown in the image of the display above.

Text and images Copyright ©1997-2021, Rick Bensene.