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Denon DEC-311 Electronic Calculator

Updated 8/12/2019

This machine, at first glance, looks like a typical desktop electronic calculator from the late '60's or early '70's. However, upon a little closer inspection, it becomes clear that this is rather an unusual calculator. A couple of things may jump out at more than a casual observation, but the most obvious question is: "Where's the divide key?" There is no divide function key to be found on this machine...and, the fact is -- the machine can't perform division. The Denon DEC-311 is a three function calculator! A more discerning eye may notice another omission...there is no decimal point key! Sacrificing division led to a byproduct that further reduced the cost of this calculator -- without division, who needs decimal points? The calculator is an integer-only machine, with no provision for decimal point on input or display. In fact, to save money, the Nixie display tubes don't even have decimal points in them. Some of Casio's early electronic calculators, such as the Casio 121-A/AS-A reduced cost and package size by eliminating decimal point input, but Casio never went as far as omitting the division function.

It seems like the DEC-311 hit the market with some significant handicaps. First, the machine can't have been very practical for use in business calculations, as the user would have to manually keep track of the decimal point position for dollars and cents types of calculations, as well as making sure to enter trailing zeroes as needed. Next, the absence of division also seems like it would have fairly seriously limited the usability of the machine for all but the most simple applications. Given that competitors such as Sharp, Casio, and Canon had relatively low-cost, yet full four-function calculators already on the market, the DEC-311 seemed destined to relegated to the dustbin of history. That makes the calculator all the more interesting from the standpoint of its unusual nature.

DEC-311 Keyboard (Note no decimal point or divide key)

It is possible that this chip set was was intended to provide full calculator functionality, but somewhere in the logic there was a fault that led to division not working properly in all cases, and the fault wasn't detected until a large initial run of chips had already been manufactured. To recover from this catastrophe, the chips were marketed by Mitsubishi as a three-function, integer only chip set for making low-priced calculators. In such a case, the division key was simply not provided on the keyboard, and no driver electronics provided for decimal point display. To the author, this device doesn't truly qualify to be in the museum, because part of the author's definition of a calculator (versus an adding machine) is a machine that can fully-automatically add, subtract, multiply and divide. Without division, the DEC-311 doesn't meet the definition. From a market standpoint, this machine had to sit on the shelves at retail outlets for a long time before selling, and were probably deeply discounted from their initial retail price to get rid of them. For all but the most simplistic usage cases, the machine was inadequate, even if it was relatively inexpensive compared to true four-function calculators that were its contemporaries. There were calculators on the market at the time that sold for slightly less, and though generally provided fewer digits (eight versus ten) of capacity, the machines provided all four math functions, as well as support for fractional numbers. As a student of the history of electronic calculators, the author truly struggles to imagine how anyone in Denon's marketing department would have felt that this machine would have been a successful product, nor can the author imagine how a poor salesperson would try to convince a potential customer to purchase one of these severely hobbled machines. The only reason that the author can think of to explain why this machine was ever built was that Mitsubishi must have virtually given away the three-chip chip set that is the brains of the machine, knowing full-well that a calculator chip set that can't divide or represent fractional numbers has very limited usefulness.

Denon 311 with Top Cover Removed

The exhibited calculator is likely an early example of Denon's last electronic calculator. The DEC-311 was introduced sometime in the fall of 1971, and there are no records of any other electronic calculators produced by Denon after this time. Denon left the electronic calculator market in the 1973 time-frame due to the highly competitive nature of the marketplace. There is no obvious dating anywhere inside the calculator. The IC packages have cryptic date codes which defy deciphering, and the other components in the machine seem to be devoid of any recognizable date codes. A guess is that the exhibited machine was manufactured sometime in late 1971 to early 1972. Denon's first generation of electronic calculators (such as the Denon 61A4 ) used few small-scale integration (SSI) integrated circuits, along with a large number of discrete component logic. It appears Denon's second and third electronic calculators, the DEC-411 and DEC-521 calculators utilized fewer discrete components in favor of more advanced integrated circuit technology. Denon's move into the use of Large Scale integrated circuits certainly didn't play on the benefits of LSI chip-sets, which could offer more functionality in less space. It's the author's belief that this poor introduction of LSI technology, hobbled by it's lack of a full calculator implementation, marked the beginning of the end for Denon in the electronic calculator marketplace. It appears that Denon did not market any more calculators after the DEC-311, making it the last (and least) of Denon's play on the electronic calculator business.

The DEC-311's Main Circuit Board

The DEC-311 leverages Large-Scale Integration (LSI) IC technology to allow all of its electronics to fit on a single circuit board. Earlier Denon calculators relied on a mother board, into which would plug numerous circuit boards containing the logic of the machine. A trio of Mitsubishi-made Large Scale Integrated Circuits (Part numbers MA8111, MA8112, and MA8113) combine to take up the majority of the real estate on the DEC-311's circuit board, providing the calculating brains of the machine. The remaining circuitry consists of a number of hybrid-circuit modules (which appear to be related to keyboard encoding), and discrete transistor-based Nixie tube display driver circuitry. Across the back of the machine lies the power supply circuitry, nicely enclosed in a metal cage. The keyboard assembly plugs into the main board via an unusual pin-style connector.

This particular Mitsubishi chip set was not uniquely used by Denon for the DEC-311. Brother Industries, Ltd. marketed a calculator badged as the Procal 310 (also marketed in the UK as the Jones 310, and North America as the Remington Rand E-3) that used the same chip set, and those machines provide no division function. The main difference between the DEC-311 and the calculators manufactured by Brother (who also manufactured the Jones and Remington machines) is that the DEC-311 uses classic Nixie tubes for its display, while the Brother-manufactured calculators use segmented vacuum fluorescent display tubes.

A Close-up View of one of the Mitsubishi LSI's

The IC's in the DEC-311 are mounted in a most unusual fashion. The IC packages are pretty much standard ceramic flat-pack style packages, with gold leads that extend horizontally from the long edge of the package. The unusual part is the way that the IC's are connected to the circuit board. The IC packages sit above the circuit board, suspended by two plastic connector blocks that have gold fingers in them which contact the leads of the ICs. These fingers connect to pins that extend down through the plastic block and are soldered to pads on the main circuit board, providing connections for each lead on the IC to the circuit board. Another plastic block with ridges that correspond to the leads of the ICs sits above the bottom block, so that the leads of the ICs are pressed into the contacts on the bottom plastic block. A rather substantial metal frame surrounds the package. This frame is screwed down, providing pressure to keep the leads of the IC package in contact with the fingers in the plastic connector. Other calculators using similar IC packaging technology simply solder the leads of the IC's directly to the circuit board -- a seemingly much easier and less expensive method. Why Denon opted for this unusual interconnect scheme isn't clear, although this scheme did allow for relatively easy service replacement of a chip if a failure occurred.

Unusual IC "Socketing"

The DEC-311 has a capacity of ten digits, with ten small Nixie tubes making up the display. Each Nixie tube contains only the digits zero through nine. At the left end of the display panel, two neon indicators behind red-colored legends light up to indicate a negative number, or an "ERROR" condition (overflow). The display circuitry is made up of individual transistor drivers. The Nixies are multiplexed to reduce the component count. The display subsystem performs leading zero suppression, blanking insignificant leading zeroes to make the display easier to read.

Detail of the Denon DEC-311's Display Circuitry

Operation of the DEC-311 is pretty straightforward. The machine provides addition, subtraction, and multiplication, along with a memory accumulator. Addition and subtraction operate arithmetically, with the [+=] key adding, and the [-=] key subtracting the operand from the number on the display. Multiplication operates straightforwardly, but the display during multiplication operations is very unusual. The DEC-311 keeps a record of the multiplicand on the display while the multiplier is entered, by placing a number of zeroes to the left of the multiplier that matches the number of digits in the multiplicand. It's kind of hard to explain in text, so an example of the display will clarify. Suppose the operation 1234 X 567 is to be performed. Shown below are the individual key strokes, and the resultant display after each key-press:

1            1
2           12
3          123
4         1234
X         1234
5        00005
6       000056
7      0000567
+=      699678

As can be seen, after the [X] key is pressed and the first digit of the multiplier is entered, four zeros are displayed to the left of the multiplier, to hold the place of the four digits entered for the multiplicand. This method of putting both the multiplicand and multiplier on the display at once is similar to that used on other machines in the museum such as the Brother Calther 412, and Canon 161.

The DEC-311 doesn't allow more than 10 significant digits of input, and when multiplying, the zero place-holders are treated as significant, so it's impossible to enter a multiplication problem that would result in overflow. However, doing a little tinkering with the machine, it was found that the [►] key, used for correcting input, can be used to erase the placeholder zeroes, allowing larger multipliers to be entered, and, provided the resulting calculation doesn't overflow the machine, correct results are provided. For example, without use of this trick, doing 4444444444 X 2 would be impossible, even though the result is still within the capacity of the machine. However, by entering 4444444444, pressing the [X] key, then pressing the [→] key such that all of the place holder zeroes are removed, then pressing 2 followed by the [+=] key, the proper answer is displayed. Using this trick to fool the machine into doing multiplications that exceed its capacity can result in some odd and interesting effects. Performing 999999999 X 9999999999 using this method results in a strange counting display, counting backwards at quite a high rate, but with strange digit decoding, with some numbers represented by Nixies with multiple digits lit at the same time. Pressing [C] or power cycling the machine is the only way to halt this counting behavior.

The memory functionality of the machine is controlled by a push-on/push-off switch labeled [T]. When the [T] key is activated, the normal function of the [+=] and [-=] keys change from performing addition and subtraction directly, to that of adding or subtracting the number on the display to/from the memory accumulator. Also, the results of multiplication operations are automatically accumulated in the memory register. The [RM] key recalls the current content of the memory accumulator to the display, and the [CM] key clears the memory accumulator. The [►] key, located on the numeric keypad area where one might expect the decimal point key to reside, provides a backspace function, allowing for easy correction of erroneously entered digits. The DEC-311 has no "clear entry" function to allow complete removal of an incorrectly entered number, expecting the user to either use the [╠] key to back up to the point of the error, or to use the [C] key to clear the machine and start the entire operation over again.

Magnetic Reed Switch Keyboard Construction

The DEC-311 properly handles negative numbers, lighting the "-" indicator at the left end of the display when a number is negative. Overflow conditions cause the "ERROR" indicator to light, which also electronically locks the keyboard preventing any further entry until the [C] key is pressed to clear the machine.

The 311 performs all operations very quickly. Multiplying 99999 by 99999 gives the proper result within a few tens of milliseconds, with very little fanfare on the display while the calculation is occurring.

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

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