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Burroughs C3260 Electronic Calculator

The Burroughs C3260 is the Burroughs version of Sharp's EL-160. At the time (early 1971), Burroughs sold Sharp-designed calculators with slight cosmetic and packaging changes. Burroughs didn't have their own calculator design and manufacturing facilities, so they leveraged Sharp's extensive expertise to design and build a line of machines that Burroughs could market in the U.S. and Europe.

Profile View of the Burroughs C3260

The C3260 (and Sharp EL-160) calculators were follow-on machines to the ground-breaking Sharp QT-8D, the machine that pioneered the use of a Large Scale Integration (LSI) chipset for implementation of a four-function calculator. In mid-1970, Sharp introduced the EL-8, which repackaged the QT-8D into a more portable (but marginally from hand-held), rechargeable battery-powered package. Both the QT-8D and the EL-8 were extremely popular machines, and the success of these models spurred Sharp to improve the basic architecture of the "ELSI" (Sharp's trademark name for their Extra-Large Scale Integration technology) chipset though 1972. The Burroughs C3260 exhibited here is a result of this on-going improvement of the basic ELSI design.

The Itron Vacuum Fluorescent Display Tubes in Operation

The C3260 is a basic four-function, 8 digit floating decimal calculator with memory (or double-precision, depending on a mode switch). As with the other related machines, it uses individual Itron vacuum fluorescent display tubes for the display. Itron tubes use a unique segmented pattern to create digits that look more handwritten than the traditional 7-segment digit rendition.

The Display Circuit Board Assembly (Black Rectangular Packages are Hybrid Display Driver Devices)

The Itron tubes in the C3260 are a bit larger than those used in the other Itron-display machines in the museum, with a digit height of 5/8-inch versus the 1/2-inch digits on the other machines. These larger digits make the display one of the more readable displays of all small desktop calculators.

The label on the back of one of the Iseden Itron Tubes

The Itron tubes in the C3260 are made by the Japanese component manufacturer Iseden, and have part number DG12B for the number tubes, and SP12A for the sign indicator tube. The digit tubes contain the standard Itron 9-segment digit rendition with right-hand decimal point, but add a 'tick' mark above and to the left of each digit. Calculators using these 'ticks' generally use them as a reading aid to the user, to break up numbers on the display into three digit groups (like a comma used when writing out a long number longhand). However, the 'ticks' are not used in the C3260 implementation. The right-most tube is a special tube used for indicating if the number on the display is negative. This tube also contains an "I" symbol, usually used on Sharp calculators for indicating non-zero content in a memory register, but on this machine, this indicator appears not to be used even though the machine does have a memory register.

The C3260 with Cabinet Opened Up

As with the other ELSI-based machines, the C3260 uses a four-chip LSI chipset to make up its brains. The IC's in this machine are made by Rockwell, but were designed by Sharp. The chips have part numbers 2256, 2271, 1152, and 1156. The first two chips are shared with the earlier EL-8 and Facit 1111, but the 1152 and 1156 chips are newer designs, that add the functionality of the memory register and double precision mode that the C3260 offers.

The Main Logic Board of the Burroughs C3260

Two other IC devices provide support for the calculator chipset; a can-packaged clock generator for the chipset (part number CG1121), and a DIP-packaged small-scale Hitachi HD3113. These IC devices are all mounted on a printed circuit board which plugs into an edge connector that connects the brains to the rest of the machine. The Itron display tubes and associated driver circuitry (Three 6205 hybrid display driver packages) reside on a separate board that plugs into another connector. A printed-circuit backplane board connects the main logic and display boards together, as well as providing a connection point for the keyboard cable. The power supply extends along the back section of the chassis, using a conventional transistor-regulated linear design.

Burroughs C3260 Keyboard Close-up

As with other calculators in the ELSI-family, a somewhat unusual arrangement is used to minimize keyboard real-estate, by combining the multiply and divide functions onto one key. To multiply, the first number is entered, followed by the [÷X]" key, then the second number is typed in, followed by the [+=] key to calculate the product. The same sequence is used for division, but rather than pressing the [+=] key, the red key (with Burroughs' unusual "dot = dot" nomenclature) is pressed to perform the division. The keyboard panel has two slide switches to control the operational mode of the machine. The upper-most switch, with positions labeled "MEM" and "16", is used to select the operating mode of the machine. As mentioned earlier, the C3260 has two operating modes. One mode (when the mode switch is in the "MEM" position) provides a memory accumulator, with a key to add the display to the memory [+], and one to recall the current content of the memory register to the display (diamond shape). The other mode (with the mode switch in the "16" position) redefines the use of the memory register as an double-precision register, allowing results of multiplication and division operations to return answers up to 16 significant digits in length. In this mode, when a result is greater than 8 significant digits, the display shows the upper eight significant digits of the result upon completion of the calculation. The [diamond] key can then be pressed to display the lower eight digits of the result. When the lowest digits are being displayed, a single press of the [C] key will return the upper-most digits of the answer to the display. In "16" mode, the [+] key has no function. This double-precision mode is good only for use on single calculations...chain calculations in this mode result in the low part of the answer being lost, and answers that are of limited use. The second slide switch is a momentary contact switch, used when the calculator is in "MEM" mode to clear the memory register. Sliding the switch downward to the "CM" position and releasing it causes the memory register to be set to zero.

Burroughs C3260 Overflow Indication (Note "half-size" zeroes)

The C3260 is a relatively quick machine, with all calculations providing results within a 100 to 150 milliseconds, even when operating in double- precision mode. The machine indicates overflow or division by zero error by clearing the display to all zeroes, and lighting all the decimal points. The error condition is cleared by pressing the [C] key.


Text and images Copyright ©1997-2012, Rick Bensene.