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Heathkit IC-2008A Desktop Calculator

Updated 12/29/2017

Heath Company, famous for their high-quality, reasonably-priced electronic kit products, hopped onto the electronic calculator bandwagon of the early 1970's in their own unique way. By offering electronic calculator kits that could be assembled from parts in a matter of hours, Heath enabled many hobbyists for whom the dream of owning their own electronic calculator was out of reach due to the cost of these machines in the early '70's. With Heath's kit products, hobbyists had the opportunity to 'build their own', learning how these marvels of technology operate, as well as saving money over the price of a 'fully assembled' calculator from makers like HP, Texas Instruments, Canon, or Monroe. This particular IC-2008A was purchased by a hobbyist in Wisconsin, in April of 1972.

View of Heathkit IC-2008A with Top Cover Removed

The Heathkit IC-2008A is a revised version of the original Heathkit IC-2008. The IC-2008 utilized a very obscure member of TI's first "calculator- on-a-chip" ICs, the TMS1875. The TMS1875 device appears to be a slightly different version of TI's original calculator-on-a-chip, the TMS1802 (introduced in September of 1971), with one segment output omitted. Although not officially confirmed, it appears that the TMS1875 may be been been a lower-cost version of the TMS1802, omitting the "extra" H-segment output. It is suspected that the actual chip inside the package of the TMS1875 is identical to that of the TMS1802, with the difference being that the H-segment output is not bonded out to Pin #23 on the TMS1875 device. The H-segment adds a tail to the display of the '4' digit, which, unless the display devices have the added tail, is of little use on standard seven-segment display devices. It appears that once the TMS0100 series of chips was introduced, the TMS1802 and TMS1875 devices were discontinued. As a result, Heathkit adopted the use of the TMS0101 chip (which was a pin-for-pin replacement for the TMS1875, but required slightly different external interface circuitry) and introduced the IC-2008A. The TMS1875 chip is a rarity, and at the time this exhibit was written, the only calculator known with this chip is the Heathkit IC-2008, although it is possible that other calculators were produced using this chip. If you know of any other calculators that use the TMS1875 chip, please contact us by clicking the "EMail" button in the menu-bar at the top of this page.

The TMS0101 IC in this particular IC-2008A has a date code of "7225", placing the production of the chip sometime in mid-June of 1972. With this chip as the brains, the machine provides a basic, four function calculator with constant. Given that the calculator was designed to be built as a kit, by people of varying skill levels, the layout of the circuit boards is very conservative. The machine is made up of two circuit boards, both made of fiberglass, with etch on both sides. The main board contains the power supply, keyboard, calculator chip, and clock generation circuitry. The second board is the display subassembly, and contains the transistorized display drivers, along with sockets for three Sperry SP-733 three-digit gas-discharge display modules. The display board connects to the main board with individual wires. Only one other IC is used in the machine, a TTL 7472 single flip-flop, used in the display multiplexing circuitry. The TMS 0101 chip uses a multiplexing scheme designed to drive LED displays, and the Sperry gas-discharge tubes tend to 'ghost' if adjacent digits within each display module are scanned one after another. To alleviate this problem, external circuitry is used to alternate the multiplexing of the displays to avoid this characteristic of the Sperry displays.

Another Internal View of the IC-2008A

The IC-2008A calculator has a capacity of eight digits, and can operate with a thumb-wheel-selectable decimal point mode of full-floating, or fixed decimal between 0 and 7 digits behind the decimal point. The calculator uses algebraic logic, where problems are entered as they would be on paper, with the '=' key generating the result. There is no notion of operator precedence, so calculations are performed in the order they are entered. The calculator provides a constant function, activated by a push-on/push-off keyboard switch labeled "K". The constant function operates in multiplication (with the first number entered being the constant) or division (where the second number entered serves as the constant). The display of the machine is made up of bright orange, 1/2-inch tall digits that are very easy to read. The digits are rendered using standard seven-segment form. The display performs a type of leading-zero suppression, with insignificant leading zeros displayed in half-height form. The left-most display element is used to indicate the sign of the number on the display (lighting a '-' when negative, and 'off' when positive), and also lights up as a "C" when overflow or error conditions occur. When error conditions occur, the keyboard is locked out until the "C" key is pressed to reset the machine. A "CD" key clears the display, and is used to correct mistaken entries. The "+/-" key toggles the sign of the number currently in the display.

Front Cover of Assembly/Operation Manual

It is clear that Heath engineers designed the IC-2008A kit to be relatively easy to assemble. The assembly manual that comes with the kit is extremely well laid out, with very clear, step-by-step instructions that lead the assembler through every detail of building the machine. High-quality components are used throughout. The integrated circuits are socketed using Molex pin strips, making it possible to leave the insertion of the static-sensitive MOS calculator chip to one of the last steps, minimizing the chance for damage either due to static discharge, or soldering temperatures.

In operation, the IC-2008A is a fast machine. The displays are left active during calculation, but the machine operates quickly enough that the displays merely flicker slightly during any calculation. The 'all-nines divided by 1' benchmark takes a barely detectable period of time. The machine properly catches division by zero, and displays a "C" in the left-most display position, locking the keyboard. One peculiarity is that the machine always must display a single digit in front of the decimal point, meaning that, even in floating decimal point mode, a maximum of seven digits can exist behind the decimal point. Also, in fixed-decimal modes, if a number can be displayed with more precision than indicated by the decimal point position selector, the machine will automatically override the decimal point selection and display the answer as accurately as possible.

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

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