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Commodore C112 Calculator

Updated 7/28/2008

The Commodore C112 is an early Vacuum- Fluorescent (VF) display electronic calculator, made in the mid-to-late 1971 timeframe. It uses rather unusual individual VF display tubes, made by Futaba, each of which have ten segments to form the digits, along with a decimal point, and a 'comma' (which isn't used in this application). The resulting digits have more fully-formed looking digits than more conventional seven segment displays, but aren't quite as unique as the Itron display elements in the Sharp EL-160.

This calculator wasn't really made by Commodore. In fact, it was actually manufactured by Unicom, a business unit of integrated circuit manufacturer American Microsystems, Inc. (AMI). AMI formed Unicom as an outlet to market calculators based on AMI's early MOS/LSI calculator chipsets. AMI sold a number of different calculators under the Unicom brand name, as well as selling to a number of OEM customers, including Commodore in Canada, and Ricoh in Japan. Unicom was sold to Rockwell International in 1972. For a time, Rockwell continued to sell calculators under the Unicom brand-name, but later abandoned the Unicom trademark, and began selling their calculators under the Rockwell brand name.

Close-Up of Futaba Ten Segment Display Tube

The C112 is a twelve-digit, four function machine, with switch-selectable constant. It 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 all of the logic and display electronics of the calculator onto one plug-in circuit board. The earlier machines, such as the one exhibited here, utilized one circuit board for the main logic of the calculator, and a separate circuit board which contained the 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 timeframe. Both versions utilized the same Large-Scale Integration (LSI) two-chip calculator chipset made by AMI. 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 which 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 appears 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, with 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 'bottom' of the machine can also be removed.

One of the AMI-manufactured chips in the C112

The C112 is an LSI integrated circuit-based calculator. One board containing most of the logic has the AMI-made 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 drive functions for the display. The machine is powered by a linear, transistor-regulated power supply which is located across the back of the machine. The C112 is a slightly later version of an earlier Unicom-produced machine marketed by Commodore, the Commodore 412F.

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 functions. A push-on/push-off keyboard switch labeled "CON" enables or disables the constant function, which works for multiplication or division only. The "C" key clears everything, as expected. Addition and subtraction operate 'adding machine' style, with multiplication and division using the "+=" key to calculate the result. A small slider located to the left and below the display controls four plastic 'arrows' which the user can position to mark comma locations. The C112 does not provide leading/trailing zero suppression.

The Display in Operation

The C112 is a rather fast calculator, which seems to be a common trait of Commodore machines. While it seems to have trouble generating the correct result with 12-nines divided by 1 (it gives 0.99999999999), 11-nines divided by one works fine, and generates a result virtually instantly (less than 1/10th of a second). Division by zero causes all the decimal points to light up with no error indication, except if the dividend was zero, which gives an answer of 0. 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 is back 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 apperas 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 result in certain types of calculations.

Rear View of C112 Interior

Performing 999999 X 999999 gives the correct answer instantly, but incorrectly causes an overflow indication. In playing around 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. I believe that this has something to do with the fact that the machine appears to have some difficulty with its implementation of floating decimal. If a calculation results in a number which has a lot of digits behind the decimal point, IE: .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, and to add insult to injury, the overflow indicator is lit. This calculation should be within the range of the machine, but it appears that whatever algorithm is used to position the decimal point gets confused in such calculations and ends up mispositioning the decimal in the calculation, which causes an overflow. I don't believe that this is a problem with this particular machine, but a design defect which exists in all machines which use this particular AMI chipset. As with most electronic calculators, an overflow locks up the machine until the "C" key is pressed.

Text and images Copyright ©1997-2011, Rick Bensene.