The Sharp 364R is one of the newer (by date of manufacture) Nixie-tube display calculators that the museum has come across. 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 Burroughs Panaplex-style displays because Nixie tubes rather expensive compared to these other display technologies. On top of using Nixie tubes, the machine also has discrete 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 using large-scale integration calculator chip-sets.
The Sharp 364R Opened Up
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. There 364R features four memory-related registers. Two of them are regular memory registers, with individual keys to add to, subtract from, display, and clear each of the two registers. There is also an item counter memory that keeps track of the number of entered in addition and suntraction operations, useful for calculating averages and other types of statistical functions. The 364R also has an unusual "VM" (verify memory) register (more on the VM register and its function later in this exhibit). The memory functions of the machine also offer a number of advanced functions, selected by a bank of five push-on/push-off buttons at the upper right of the keyboard panel (see the Compet 364R manual for more information on these functions). The 364R also has a constant function that operates in all four basic math functions (as opposed to constant only working on multiply and divide on many machines). The 364R operates 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 Old Calculator Museum exhibit for the Burroughs C3350 for a similar vintage Sharp-designed machine with this feature) is the ability to separately define the decimal point setting for calculations as well as the memory functions of the machine. Rounding mode is also separately definable for calculation and memory registers.
Closer View of Internals of the Sharp Compet 364R
Note transistorized display driver circuitry near the Nixie Tube displays
The 364R is based on a four chip Large-Scale Integration(LSI) calculator chip-set made by Hitachi. The part numbers of the chips are HD3520R, HD3521, HD3528, and HD3522-1. All of the LSI devices are in 28-pin dual-inline plastic (DIP) packages. The chip-set 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 double-sided fiberglass circuit board with soldermask and silkscreen, utilizing plated-through holes for connection between the component and solder sides of the circuit board. Traces are routed on both sides of the circuit board. The Nixie display is particularly interesting, as the Nixies have the digits zero through nine, decimal point, and a special digit-grouping "tick" located low and to the left of each tube. This tick is lit up to group the integer portion of numbers into groups of three to make the numbers in the display more readable for the operator who may have to record results from the display. Some other Nixie-display calculators, like the Monroe 950 use separate neon or incandescent indicators for digit grouping, but the Nixie tubes used in the Sharp Compet 364R integrate this capacity into the tube. The right-most tube on the display is a specialized gas-discharge tube which has indicators for "I", "II", and "III" that 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 with double-shot molded key-caps for long life and reliability. The keyboard connects to the main board via an edge-connector. The calculator uses a conventional linear power supply with transistor regulation.
Closeup of the Hitachi-made Chip-set in the 364R
The 364R performs the standard four functions with addition and subtraction as an adding machine does. The red [=] key is used for subtraction, and seems like it should be labeled "-=", but Sharp opted only to label it with an "=", likely under the assumption that the red color of the key was sufficient indication that the key performs subtraction. Multiplication and division work as expected, with indicator lamps (incandescent bulbs) that light up under red-colored jewels in the multiply and divide keys that indicate when an operation is pending. The square root function is invoked by entering the radicand then pressing the [÷] key immediately followed by the [+=] key, a scheme used by Sharp to save adding a separate square root key to the keyboard. 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 unique function that continuously compares the current number in the display with the number in the VM (Verify Memory) register, and if the two are exactly equal, lights the "VM" indicator on the keyboard panel. The idea of the Verify Memory is to provide a means to verify the result of a calculation that is performed twice to assure that the calculation was entered properly. This function would be used in certain types of critical calculations to validate the result of a calculation. To use this feature, the user would perform a calculation, and with the result of the calculation in the display, toggles the VM key to the "VM IN" (up) position. The calculation would then be performed again, with the user looking to see if the VM indicator on the display is lit after the calculation is completed. If it isn't lit, the answers to the two operations do not match, indicating to the user that an entry error was made, and that the two calculations should be performed again to assure a correct result. To see what is in the VM register, the VM key is toggled to the "VM OUT" (down) position, with the number in the VM register replacing whatever is in 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 "13.000000" is entered then the VM switch toggled to "VM IN", then perform " [+=]  [+=], 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. If the match is not exact, even with equivalent numbers, the VM light won't come on. While this doesn't really affect the usefulness of the verify memory capability as repeated calculations will yield the same result if entered correctly, this oddity gives insight as to how the comparison operation functions.
The calculator indicates error or overflow conditions by blanking the display except for all of the digit separator 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 and the verify memory (VM) register. Taking the square root of a negative number does not result in an error condition, which is technically incorrect behavior. Taking the square root of a negative number should result in an error condition as such a calculation is impossible within the realm of real numbers.
Display Detail on Sharp Compet 364R
The 364R is a reasonably fast calculator, with the longest calculations taking less than 1/2 second. Sharp-provided specifications indicate that the square root operation takes the longest to complete, at 0.48 seconds. During the time calculations are occurring, the digit part of the Nixies is blanked, with all of the tick marks lighting while the machine is busy performing the calculation.