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Denon DEC-311 Electronic Calculator

Updated 10/23/2014

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 machine. 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 doesn't know how to divide! The Denon DEC-311 is a three function calculator! 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 decimal point on input or display. In fact, the Nixie tube display elements 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. My guess is that this machine probably found very little success in the marketplace. 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. The absense of division also seems like it would have fairly seriously limited the usability of the machine for all but the simplest of applications.

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

One may wonder why this machine ended up so stripped down in terms of function. One likely answer was to reduce cost. The reason why the DEC-311 came into existence are somewhat obscured by the mists of time, but it's certain that the machine is quite unusual, and the Old Calculator Museum is happy to have found an operating example for inclusion in the museum's collection.

Denon 311 with Top Cover Removed

This machine 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 timeframe due to the highly competitive nature of the marketplace. Denon's first generation of electronic calculators (such as the Denon 61A4) were early "hybrid" calculators, using a few small-scale integration (SSI) integrated circuits, along with a large number of discrete component logic. Denon's first-generation machines were made in the late 1960's when small-scale IC's were just beginning to be used in consumer electronic equipment. It appears Denon's second and third electronic calculators, the DEC-411 and DEC-521 calculators utilized fewer discrete components in favor of more complex small-scale integrated circuit technology. Finally, Denon moved into early Large Scale Integration (LSI) IC's for the brains of their last calculator, the DEC-311. Sadly, I can't find any date information anywhere in the machine. The IC packages have cryptic date codes which I cannot decipher, and all of the other components in the machine seem to be devoid of any recognizable date codes. My guess at this point is that the exhibited machine was made sometime in late 1971 to early 1972.

The DEC-311's Main Circuit Board

The DEC-311 leverages 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 combine to take up the majority of the real estate on the circuit board. 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.

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. I've not seen an arrangement like this before. The IC packages are pretty much standard ceramic flat-pack style packages, with gold leads that extend out horizontally from the package. The unusual part is the way that the IC's are connected to the circuit board. The IC packages 'float' above the circuit board, suspended by two plastic connector blocks which have fingers in them which contact the leads of the IC's. A 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.

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, 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 keypress:

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

As you can see, 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 Sharp Compet 20, Brother Calther 412, and Canon 161.

The 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, I found that the 'backarrow' key, used for correcting input, can be used to 'back out' the placeholder zeroes, allowing larger multipliers to be entered, and, the result is valid. 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 "back arrow" 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. A right-facing 'arrow' 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.

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 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 ©1997-2019, Rick Bensene.