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Denon DEC-61A4 Electronic Calculator

Updated 10/23/2014

I have a particular affection for Denon electronic calculators, though, sadly, they are now forgotten relics in the history of electronic calculators.

Years ago (perhaps 1970), a neighbor-kid's father worked for an accounting firm. One day back then, my neighborhood friend came over to my house saying that his Dad had brought home a cool machine that could do math and had 'tubes'. I had no idea what he was talking about, but went over to his house to see what the hubbub was about. There on the dining room table was a Denon Nixie tube display calculator (don't remember the model number, but the machine was quite similar if not identical to the DEC-61A4 exhibited here). My friend's father had just been assigned this machine to be used for taking work home with him (a somewhat sad precedent set by the advent of portable computing devices). His Dad was rather concerned about us playing with the machine, for fear of us "breaking" it, but he did let us play with it for a while under close supervision. I swear this kid's Dad nearly had a heart attack when I commanded the machine to divide by zero, which caused the display to go blank with no response to the keyboard. He felt much better when I pressed [CLEAR] and the machine returned to normal, however, at that point, he'd had enough, and told us play-time was over. Years later, when a machine like the one I played with became available, I was quite happy to be able to add it to the museum..both from the point of view that it is a very interesting hybrid IC/discrete component machine, and also simply from the memories a machine like it made in my younger days.

Protective Dust Cover

Denon is known today for high quality audiophile equipment. The respected brand name for Tokyo-based Nippon Columbia Co., Ltd., has been around for quite a long time, and as with many electronics-oriented companies, Denon had a brief foray into the electronic calculator business beginning in the fall of 1968 through sometime in 1972 when it exited the highly competitive market.

The DEC-61A4 was Denon's first entry into the calculator market, introduced in September of 1968. A number of other machines ended up following the 61A4, including the DEC-621, DEC-521, DEC-411, and the bargain basement DEC-311, Denon's rather strange attempt to capture the very low-end of the desktop calculator market.

Denon 61A4 with Top Cover Removed

The Denon 61A4 was built at a time when integrated circuits were just starting to become available for use in anything but military equipment or large-scale computers. The cost of integrated circuits was still quite high in comparison to discrete component logic. However, the integration of a number of logic gates, or a few flip flops into a single small package made it possible to make the machines smaller, a real consideration when desktop space always seems to be a limited commodity. So, as IC technology became available, calculator manufacturers started to integrate (excuse the pun) the technology into their designs. The 61A4 is a hybrid machine, using some integrated circuit logic elements, and some discrete component logic. This machine sits pretty much in the middle of the range of hybrid machines, with an example of the low-integration hybrid machine being the Burroughs C3350, and a high-integration example being the Brother Calther 412.

A Closer View of the Circuit Boards

The majority of the logic of the DEC-61A4 is contained on a total of seven plug-in circuit boards. The boards plug into a backplane board located across the bottom of the chassis. Each board is approximately 11x3 inches, and packed at quite a high density with components. The boards use a lot of diode-resistor gate elements, with the gates built up vertically, with a common bus-bar across the top ends of the components. Each board has a plastic shield on the back side of the circuit board to prevent components on the board behind from shorting to adjacent circuit boards.

Two of the DEC 61A4 Circuit Boards

The circuit boards are made of phenolic, and have traces on both sides of the board, with plated-thru holes for interconnections between the sides of the board. The sprinkling of integrated circuits in the machine are all DIP (Dual-inline Package)-style plastic-packaged devices made by Mitsubishi. Most of the ICs are of the M53xx/M59xx-family, which are small scale bipolar devices, of DTL (Diode-Transistor Logic) construction. Each circuit board plugs into the backplane via two edge connectors.

Keyboard/Display Detail

The 61A4 uses fourteen Nixie tubes for its display, mounted on a separate circuit board that connects via jumper wires to the main board (backplane) of the machine. Each Nixie tube has the digits zero through nine and a right-hand decimal point. The calculator provides leading zero suppression. During calculations, the displays are blanked, making the machine less entertaining than other machines which leave the displays active during calculation. A neon lamp situated at the left end of the display panel lights through a minus-shaped cutout to indicate negative numbers. This same indicator seems to be used as a busy indicator, but only when calculating square roots. Other calculations do not seem to use the '-' indicator in this way, which could indicate that perhaps the behavior is simply an artifact of the way that the square root operation is carried out. Two neon indicators at the right end of the display indicate the status of the memory register, and an error(overflow) condition.

Another Internal View

The 61A4 operates quite conventionally, with arithmetic logic. There are some inconsistencies in the way the machine operates, which I've not yet quite figured out. Unfortunately, at this time, the museum does not have posession of an operator's manual for the machine, so some aspects of the machine's operation are inferred by simply tinkering around with the machine. Operation of the constant function (engaged by a push-on/push-off red pushbutton marked [K]) seems rather unique. The [RC] key swaps operands for multiplication and division operations. The memory function also operates quite conventionally, with separate keys for adding and subtracting the content of the display from the memory register. The memory register can also be used for sum of products operations through the red [T] key on the keyboard. This push-on/push-off key, when activated, automatically causes the results of multiplication or division operations to accumulate in the memory register. When the [T] key is de-activated, the content of the memory register is automatically recalled to the display. The memory register may be cleared independently of the rest of the calculator via a [CM] key, and the content of the memory register can be recalled to the display with a press of the [RM] key. An indicator on the right end of the display panel lights up (via a Neon lamp) "MEMORY" to indicate non-zero content of the memory register. The [CD] key clears the display in case of entry errors, and the [CLEAR] key clears the entire machine except for the memory register. The [CLEAR] key also clears any error condition. Error (actually, overflow) conditions result in the ERROR indicator lighting up at the right end of the display panel. The error indication does not cause the keyboard to lock or otherwise inhibit further operation of the machine. Pressing ANY key when the ERROR indicator is lit immediately extinguishes the indication. The notation ERROR for this indicator is a bit of a misnomer, as the indicator truly only indicates an overflow condition. Overflows of the machine cause the error indicator to turn on, and any overflow digits to be discarded. Because the overflow (ERROR) indicator is cleared by pressing any key, the overflow handling on this machine is a bit dangerous, as a skilled operator who can run the machine without having to look at the keyboard may not notice a posted ERROR condition, which could cause undetected truncation of results that could really throw the result of a series of calculations out of whack.

The Denon Logo

The DEC-61A4 can operate with either fully floating or fixed decimal point. The operating mode of the machine is controlled by a thumbwheel switch located at the right end of the keyboard panel. The switch contains selections for Floating Decimal (F), Fixed Decimal (D) with no rounding function, Fixed Decimal with Round Up/Down (5/4), and Fixed Decimal with Always Round Up (R). A second thumbwheel switch provides the setting for fixed decimal point location, with positions for 0, 2, 3, or 6 digits behind the decimal point.

As with many calculators from this era that offer floating decimal point modes, the 61A4 has some "bugs" in its implementation. It seems that the machine can not deal with any calculations that have the 14th digit in non-zero state when operating in floating point mode. Any such calculations usually result in an incorrect answer and the ERROR light being lit. For example, 99,999,999,999,999 divided by 1 results in 1,999,999,999,999.0 with the ERROR condition indicated. In fixed decimal point mode (with decimal point position set at 0), this calculation returns the correct result. The square root algorithm on the machine also has some problems with large numbers, returning flatly incorrect results. Another oddity is that performing operations such as taking the square root of 999.000000 (with fixed decimal at 6), which returns the correct result, then multiplying the answer by itself results in an incorrect answer and an overflow! Other oddities occur with square root calculations. For example, taking the square root of the constant Pi expressed to 14 significant digits takes the machine almost 20 seconds to return a result, and the resulting answer has one digit with both a '2' and '8' on at the same time. Taking the square root of a negative number causes no error condition -- the calculation occurs as if the argument is positive. Division by zero definitely confuses the machine, with the display blanking and no sign of response to any keyboard keys except [CLEAR], which will return the machine to consciousness.

A Profile View of the Denon DEC-61A4

The 61A4 interesting with regard to its operational speed. The machine completes addition and subtraction virtually instantaneously. However, multiplication and division take a little longer than similar machines, with all nines divided by 1 taking around 3/4 second to complete. Square root operations can take even longer, with larger arguments seeming to take the longest. Calculating the square root of 99,999,999.999999 takes nearly five seconds to complete. Published specifications in old literature for the 61A4 document addition/subtraction as completing in 1/100th of a second, multiplication and division as 1/4 second, and square root in 1/2 second. These specifications seem to be average times, not worst-case timings.

Text and images Copyright ©1997-2014, Rick Bensene.