Sharp Compet 364R Calculator
The Sharp 364R is one of the latest Nixie-tube display calculators that I've seen. It dates from April of 1974 based on the serial number information. Date codes on parts in the machine range from late 1973 to early 1974. By this time, many manufacturers had abandoned Nixie tube displays in favor of Vacuum Fluorescent or Panaplex-style displays because Nixies were rather expensive compared to these other display technologies. On top of using Nixie tubes, the machine also has discrete component, transistor-based driver circuitry for the Nixie tubes. All in all, the Compet 364R is somewhat of a throwback to the earlier days of electronic calculators in terms of display technology, but has all the bells and whistles associated with later model calculators.
The Sharp 364R Split Open
The Compet 364R is a 16-digit, 6 function calculator. The machine performs the standard four functions, along with square root and percent functions. It has quite a complement of memory functions, some of which I've not figured out yet. There are at least four memory-related registers in the machine. Two appear to regular memory registers, with keys to add to, subtract from, display, and clear the memory registers. There also appears to be an item counter memory, and also the peculiar "VM" memory register (more on the VM register and its function later in this article). The memory functions of the machine also offer a number of advanced functions, selected by a bank of 5 push-on/push-off buttons at the upper right of the keyboard panel. The 364R also has a constant function which operates in all four basic math functions (as opposed to constant only working on multiply and divide on many machines). The machine can operate in fixed decimal point (with decimal point located at 0, 1, 2, 3, 4, 6, or 8th digit), or full floating point mode. An interesting, and characteristic feature of this vintage of Sharp-designed calculators (see the page on the Burroughs C3350 for a similar vintage Sharp-designed machine) is the ability to separately define the decimal point setting for calculations versus the memory functions of the machine. Rounding mode is also separately definable for calculation and memory registers.
Closer View of Internals of Sharp Compet 364R
The 364R is based on a four chip calculator chipset made by Hitachi. The part numbers of the chipset are: HD3520, HD3521, HD3528, and HD3522-1. The chipset is supported by a number of small-scale MOS devices, also Hitachi-made. All of the calculating brains of the machine reside on a single board. The Nixie display is particularly interesting, as the Nixies have the digits zero through nine, decimal point, and a special 'tick' located low and to the left of each tube. This tick is lit up to indicate the location of commas in the number to help make the numbers in the display more readable to humans who have to record results from the display. I've never seen other Nixie like this..other Nixie calculators, like the Monroe 950 use separate neon or incandescent indicators to show where commas are located in numbers. The right-most tube on the display is a specialized gas-discharge tube which has indicators for "I", "II", and "III" which light up when there is non-zero content in the three memory registers, and a "-" for indicating a negative number in the display. The keyboard in the machine uses the standard arrangement of magnetically activated reed-switches for long life and reliability. The keyboard connects to the main board of the machine via an edge-connector. The calculator uses a linear power supply with transistor regulation.
Closeup of the Hitachi-made Chipset that runs the 364R
The calculator performs the standard four functions, with addition and subtraction operating in 'adding machine' mode. The red [=] key is used for subtraction, and seems like it should be labeled "-=", but Sharp opted only to label it with an "=", probably assuming that the red color of the key was a good indicator that the key subtracts. Multiplication and division work as expected, with indicator lamps (incandescent) that light up under red-colored jewels in both the multiply and divide keys that indicate when an operation is pending. The square root function is invoked by pressing the [÷] key immediately followed by the [+=] key. A push-on/push-off key labeled [K] turns on the constant function, with a "K"-labeled incandescent lamp lighting when the constant is enabled. The [RC] key swaps the content of the hidden operator register with the display, useful in cases where the divisor and dividend in a math operation need to be interchanged. The unusual "VM IN/VM OUT" switch, which toggles up (VM IN) or down (VM OUT), is used for a special function which continuously compares the current result of calculations with the number in the VM register, and if the two are exactly equal, lights the "VM" indicator on the keyboard panel. To place a number into the VM register, the user keys the desired comparison number into the display, then toggles the VM key to the "VM IN" (up) position. The VM register initially contains zero on power-up. To see what is in the VM register, the VM key is toggled to the "VM OUT" (down) position and the number in the register will appear on the display. It is interesting to note that the comparison between the result of a calculation and the number in the VM register is "as displayed". For example, if you type in "13.000000", then toggle "VM IN", then do a " [+=]  [+=], resulting in "13." (with the decimal point position at 0), the "VM" indicator will not light, as the number in the VM register is literally compared to the display digit for digit, and if there isn't a match exactly, even with equivalent numbers, the VM light won't come on.
The calculator indicates error or overflow conditions by blanking the display except for all of the ticks and decimal points being lit at once. Pressing the [CE] or [C] keys clears the error condition. The [C] key clears everything except the memory registers. Taking the square root of a negative number does not result in an error condition, which is not technically incorrect behavior. Taking the square root of a negative number should result in an error condition.
Display Detail on Sharp Compet 364R
The 364R is a relatively fast machine, with the longest calculations taking less than 1 second. During the time calculations are occurring, the digit part of the Nixies are blanked, with all of the tick marks lighting to indicate the busy state.