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Commodore C112 Calculator

Updated 10/8/2023

The Commodore C112 is a Vacuum- Fluorescent (VF) display electronic calculator using a two-chip MOS Large Scale Integration (LSI) chip set made by American Micro-systems, Inc. (AMI) for its calculating logic. The exhibited example of the C112 was manufactured in the mid-to-late 1971 time-frame as indicated by date codes on various components in the machine. The C112 uses rather unusual individual VF display tubes, manufactured by Futaba in Japan, each of which have ten segments to form the digits, along with a right-hand decimal point, and a ',' symbol, which isn't used in the C112. The fluorescent segments light up a pleasant light blue, but a green-tinted lens makes the digits appear more green than blue. The most notable difference in the digit rendition of this display and traditional seven-segment displays is that the digit "4" has the horizontal line cross the vertical centerline using a small segment to the right of the centerline. The digit "1" is also centered within the tube rather than being rendered using the right-most vertical segments in the array of segments. The resulting digits have a more pleasing rendition than traditional seven segment displays, though still having a segmented appearance. The most unusually-segmented VF display tubes in a calculator in the museum are the calculators using Iseden-manufactured Itron display elements in them, with an example being the Sharp EL-160.

This calculator wasn't manufactured by Commodore. As with many calculators from the earlier days of Commodore's calculator business, the C112 was manufactured by another company and sold under the Commodore brand name. The C112 was designed and manufactured by Unicom, a business unit of integrated circuit manufacturer American Micro-systems, Inc.(AMI), which explains the use of the AMI-made two-chip LSI chip set that makes up the logic of the calculator.

Originally, Unicom was a relatively small calculator distributor based in Southern California that imported low-cost Japanese-made calculators and sold them under the Unicom brand name. In the fall of 1971, Unicom was acquired by AMI, and Unicom began manufacturing calculators using MOS Large Scale Integration calculator chip sets designed and manufactured by AMI. As well as selling Unicom-branded calculators, the Unicom division also sold machines to OEM customers, including Commodore in Canada, Ricoh in Japan, "Digitron" in Yugoslavia, Sears & Roebuck in the US, and Precisa in Switzerland, and possibly others.

After just over a year, Unicom was sold by AMI to Rockwell International. For a short time, Rockwell continued to sell calculators under the Unicom brand-name, mainly to eliminate backlog of Unicom inventory, but later abandoned the Unicom trademark and began selling their calculators under the Rockwell brand name. When Unicom was sold to Rockwell, the remaining stock of Unicom-made calculators with AMI chips in them were sold off, and after they were gone, AMI chips were no longer used in the calculators made by Rockwell. instead, calculator chip-sets (and eventually, single-chip calculator ICs) designed and manufactured by Rockwell's Microelectronics division were used. When Unicom was sold to Rockwell, Commodore negotiated with Rockwell to continue their OEM relationship, and for some time, sold Rockwell-made calculators under the Commodore badge.

It wasn't long before Commodore acquired chip-maker MOS Technology, a chip-maker famous for developing the efficient and very popular 6502 microprocessor used in the original Apple I and Apple II, Atari 400/800, MOS KIM-1, and of course, the Commodore PET, VIC-20, and the famous Commodore 64. Once Commodore had its own MOS LSI chip manufacturing capabilities, they began designing their own calculator chips for use in Commodore-branded calculators, and for the first time, Commodore calculators (by this time, they were all handheld calculators) actually produced its own internally-made calculators rather than selling other manufacturer's calculators under the Commodore brand name.

Close-Up of Futaba Ten Segment Display Tube

The C112 is a twelve-digit, four function desktop calculator, with switch-selectable constant mode. It follows in general design aesthetic of the other Unicom-made calculators including the Unicom 1200, Unicom 1212, as well as the versions sold to OEM customers, including the Commodore 412F, Ricoh Ricomac 1200, Ricoh Ricomac 1212, Precisa 1200, Precisa 1212, Digitron db1200, Digitron db1212, and the Sears & Roebuck 444.58200. The C112 operates in floating decimal mode. Two neon-tube indicators behind red jewels at the right end of the display panel indicate overflow and negative sign conditions. The keyboard uses magnet-activated reed switches, making for reliable and bounce-free operation.

It appears that two different versions of the C112 were manufactured during its market lifetime. The unit exhibited here is one of the earlier machines. Sometime during 1973, a design change was made to reduce cost, which consolidated the two circuit boards with the logic and display electronics of the calculator onto one circuit board. The earlier machines, such as exhibited here, utilize one circuit board for the main logic of the calculator and a separate circuit board which contained the clocking circuitry and display drive electronics. It appears that the consolidation was made possible through the use of hybrid circuit modules (black rectangular modules in the photo below) which miniaturized the display driving electronics such that everything could fit on one board. This change surely reduced the manufacturing cost of the machine, allowing it to remain cost- competitive in the highly volatile calculator market of the 1972 through 1974 time-frame. Both versions utilized the same Large-Scale Integration (LSI) two-chip calculator chip set manufactured by American Micro-systems, Inc. It appears that the C112 remained on the market well through 1974, as models have been found with date coded components dated through the latter half of 1974.

Commodore C112 Internals

Since the C112 exhibited here is one of the early machines, the brains of are contained on two circuit boards that plug into a card cage with backplane connections (partly hard-wired, partly via a small bridge-type printed circuit board). An interesting note about the machine is that it clearly seems to have been designed for serviceability. The case is made up of three parts. The first is a panel at the rear of the machine which can be removed after taking out two screws to gain access to the card cage without having to take the rest of the case apart. The second part, by removal of four screws, allows the hood over the display assembly to be removed for easy access to the display subsystem, and last, the main part of the case making up the base of the calculator can also be removed, leaving the entire calculator chassis bare.

One of the AMI-manufactured chips in the C112

One circuit board contains the calculating logic, with the AMI-made two chip set (AMI Part numbers 0566 and 0567) that run the show. A few smaller small-scale IC's on this board provide support functions. The other circuit board, containing a couple more small-scale IC's and a lot of discrete components, provides the decoding and drive functions for the display as well as the clock generator circuitry that creates the master timing for the logic of the calculator. The C112 is powered by a linear, transistor-regulated power supply that is located across the back of the machine.

The two circuit boards of the early C112

The later C112 single-board implementation
Image Courtesy of Mark Wyman

The C112 performs the standard four math functions. A push-on/push-off keyboard switch labeled [CON] enables or disables the constant function, which works only for multiplication or division only. When the [CON] key is depressed, the number entered after the [X] or [÷] key is pressed is saved for further operations after the [+=] key is pressed to complete the calculation. Once a constant is saved, entering a number and pressing [+=] will multiply or divide the entered number by the saved constant, and display the result. For example, entering [2] [X] [3] [+=] with the [CON] key depressed will save the 3 in the calculation as the constant, and display the result of 2 X 3, 6. Then, entering [5] [+=] will perform 5 X 3(the saved constant), displaying 15. The constant value and the operation that preceded it are retained for as long as the [CON] key remains in the depressed condition, or until the calculator is cleared using the [C] key, which clears the constant along with the rest of the machine. Addition and subtraction operate adding machine-style, with multiplication and division using the [+=] key to calculate the result. A slider located below the display panels controls four plastic arrows that the user can position to mark comma locations. The C112 does not provide any leading/trailing zero suppression.

The Display in Operation

The C112 is a rather fast calculator, generating results almost instantly. The C112 seems to have trouble generating the correct result with twelve nines divided by one (it gives and answer of 0.99999999999). Eleven nines divided by one works fine, and generates a result almost instantly (less than 1/10th of a second). This is likely due to one of the digits of a working register being used as a counter during division operations that precludes the use of that digit in a division operation. This was a common way of saving additional circuitry for a temporary counter register during multiplication and division operations. Division by zero causes all the decimal points to light up with no error indication, except if the dividend is zero, which gives an answer of zero. Pressing digit keys during the "all decimal points on" state causes the digits to be entered into the display. Pressing a function key at this point seems to carry out the function as if the dividend was entered as the first number in whatever function was entered. Of course, pressing the [C] key during the "all decimal points on" state clears everything, and the machine returns to normal. Another abnormality observed is that this machine (which may have been corrected in the second version of the calculator, but the museum has not yet acquired one of these) seems not to properly handle negative overflow. For example, entering twelve 9's, followed by the [-] key results in 999999999999.- in the display. Then, pressing 10, followed by the [-] key, results in a display if 000000000009.- . In this case, it appears that the overflow detection circuitry doesn't kick in, and the result simply "rolls over". This is a rather serious bug, as this could cause incorrect results in certain types of calculations if not noted by the operator.

Rear View of C112 Interior

Performing 999999 X 999999 gives the correct answer almost instantly, but incorrectly causes an overflow indication. In experimenting with the machine, it seems that it has a tendency to give overflow indications when the result of a calculation should be within the range of the machine. It is believed that this has something to do with the calculator having problems with its implementation of floating decimal. If a calculation results in a number which has many digits behind the decimal point, e.g.: .12345678987, and you multiply this by a number with a few digits in front of the decimal point, for example, 157, the answer comes up as 9.38271601116, which is incorrect. To add insult to the injury, the overflow indicator is lit. This calculation should be within the range of the machine, but it appears that the decimal point positioning logic gets confused in such calculations and ends up mis-positioning the decimal in the calculation, causing an errant overflow. It is most likely that this is not a malfunction of the exhibited calculator, but is more likely a design defect that exists in all calculators that use this particular AMI chip set. As with most electronic calculators, an overflow logically locks out the keyboard until the [C] key is pressed to reset the calculator.

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

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