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Sharp Compet 363P Programmable Desktop Calculator

Updated 4/22/2004

The Sharp Compet 363P is virtually identical to another machine in the museum, the Burroughs C3660. The two machines are very similar, having only subtle differences in packaging and nomenclature. It is clear that Sharp was behind the design of the Burroughs machine, making a few minor changes for brand differentiation between its own 363P and the Burroughs C3660. This 363P, as well as the Burroughs C3660 in the museum were both built during the mid-1971 timeframe.

Compet 363P (Left) and Burroughs C3660 (Right) Internal Views

Removing the top half of the case (which includes the keyboard assembly), makes it clear that the Sharp 363P and Burroughs C3660 are virtually identical internally. The same compliment of six Rockwell-made LSI (Large-Scale Integration) integrated circuits make up the calculating brains of the machine. The board layouts are very similar, and the mechanical construction of the machines is identical. The main logic of the machine is spread across four plug-in circuit boards. The circuit boards plug into a hand-wired backplane that provides interconnection between the boards. The four boards seem to be fairly logically divided in their function. One board seems to be related to input processing, such as scanning and encoding of the keyboard, timing generation, and overall 'housekeeping' functions. Another board seems to provide all of the circuitry needed to operate the magnetic card reader in the machine. The main logic of the machine is housed on the next circuit board, which also includes the Nixie tube displays, as well as decoder/driver devices that run the Nixies. Lastly, a fourth board seems to provide the programming functions of the machine, including the memory for program steps and memory registers. Aside from the LSI devices, the boards are heavily populated with NEC uPD1xx and uPD3xx-series small- and medium-scale integrated circuits.

Display/Calculating Board(Top), and Programmer/Sequencing Card of the Sharp Compet 363P

The Sharp 363P uses a similar display subsystem to that of the Burroughs C3660, using sixteen NEC CA-80 Nixie tubes for the numeric portion of the display. Each of the CA-80's contains the digits zero through nine, a right-hand decimal point, and a 'tick' located to the left of each digit for grouping displayed numbers into three-digit groups for easier reading of large numbers. The machine performs leading-zero suppression, which also helps make reading numbers in the display easier on the operator.

The 363P Nixie Tube Display (Note "ticks" grouping digits into threes)

Along with the sixteen Nixies, three special gas-discharge tubes complete the display panel. The right-most tube in the display panel is a nine-segment tube, arranged in a narrow 7-segment rendition with two additional vertical segments bisecting the 7-segment pattern. This tube is used to display memory register numbers, branch destination codes, and certain program operation codes. The next tube to the left is a multi-segment display, in the form of the classic "starburst", allowing the tube to display stylized alphabetic characters. This tube displays symbols indicating the operation being performed in programs. Together, these two tubes are used only when the calculator is in any of the programming modes. The Burroughs version of the machine dispenses with the 'starburst' tube, opting for individual indicators on the keyboard panel of the machine for showing the program operation codes. The Burroughs machine has a slight modification to the 'main board' (the one with the Nixie tubes and calculator LSI's on it) that adds an edge connector that brings out the signals to drive the discrete indicators for program opcode display on the keyboard panel.

Program Function Display Tubes in Operation

The left-most tube of the group of three special tubes is used for sign and error indication. The tube displays a '-' sign when the number in the display is negative. Below the '-' sign, a symbol, which lights up in conjunction with the '-' sign to form an 'E', lights when the calculator detects an error condition. Along with the 'E' indication, the machine lights up all of the 'ticks' in each of the Nixie tubes to further reinforce the indication of an error condition. Error indications are triggered by overflow of the machine, illegal operations (division by zero), or overflowing of the program memory. Taking the square root of a negative number does not cause an error condition, and returns a negative square root, which is technically inaccurate.

Sharp 363P Error Indication

The Compet 363P, programming functions aside, provides the basic four math functions and square root. Square roots are calculated by entering the number to calculate the square root of, pressing the divide key to enter the number, then pressing the "+=" key to generate the result. The multiply and divide key contain indicators that light the function symbol when an operation is pending. For example, when performing 3 X 4, when the user hits the "X" key, the "X" nomenclature in the key will light up with a red glow to indicate that a multiplication operation is pending. Once the operation is completed, the indicator is extinguished.

The calculator provides ten accumulator-style memory registers. There are four keys to the right of the math function keys that control the memory registers. A given memory register is cleared (with the 'CM' key), recalled (with the 'MR' key), subtracted from (with the 'M-' key), or added to (with the 'M+' key) with these keys. When performing a memory function, the memory function key is pressed and is immediately followed by a single digit from zero through nine to indicate which memory register the operation is to be performed upon. A special keystroke sequence, 'CM' followed by '.', clears all of the memory registers to zero. Four incandescent indicators on the keyboard panel provide visual indication of the state of memory registers one through four (no status indication is provided for memory registers 0, 5, 6, 7, 8 and 9) by lighting up when the corresponding memory register has non-zero content.

Detail View of Compet 363P Keyboard (Note Illuminated "X" Key)

The other functions of the machine operate as they do on other Sharp calculators of the time. Addition and subtraction work adding machine-style, with the '+=' key used for addition, and the '-=' key for subtraction. Multiplication and division operate as expected, with the '+=' key generating the answer. A slide switch on the keyboard panel enables the constant function for multiplication and division. The 'CE' key clears the display register, and the 'RC' key recalls the last operand to the display. The '+/-' key toggles the sign of the number in the display.

The calculator can operate in fixed or floating decimal point modes, which is set by two nine-position slide switches at the left of the keyboard panel. The 363P provides separate decimal point settings for the display (R) and for the memory registers (M). Selections are provided for fixed decimal settings at zero through seven digits behind the decimal point, or for full floating (F) decimal point placement. The display and memory registers also have separate settings for roundoff. Two three-position slide switches select force up, 5/4 (5-up, 4-down), or truncate modes for rounding of numbers in the display or memory registers.

The operating mode of the machine is set with a slide switch on the keyboard panel that has five different settings available. "N" (Normal) mode is selected to use the machine as a normal calculator, with program execution disabled. The "A" (Auto) position of the mode switch puts the calculator in program execution mode. In program execution mode, the "+=" key serves as a 'RUN' key, which, when depressed, causes program execution to begin (or continue from a programmed HALT instruction). The "PRO" (Program) position of the mode switch is used for entering programs into memory. As keypresses are entered, they are stored in the program memory one step at a time. In program mode, the Nixie tubes display the current step number, and the special tubes display the operation code for each key as it is entered. The "DEB" (debug) position of the mode switch is used to step through programs step at a time (with a step executed with each press of the "+=" key). When in debug mode, the Nixies display math results as usual, and the special purpose tubes show the operation code for each instruction as it is executed. Lastly, there is "CHE" (Check) mode, which is used for program entry verification. In check mode, the Nixie display shows the step number, and the special function tubes show each program opcode as the user moves through the program with each press of the "+=" key.

The program memory on the 363P has a capacity of 144 program steps, the same as the Burroughs C3660. The machine has the capability of eight different conditional branches, and six unconditional branches within any given program. The calculator uses tags to mark locations in programs for branching operations to transfer to. Branching functions are designated by the "J->" (Jump To) and "->J" (End Jump) instructions. The tags consist of the Jump To or End Jump instruction, followed by a single keypress that indicates the 'tag'. The keys 0 through 7 and "H" are tags for conditional jumps, and the 8, 9, 0, decimal point, "+/-", and "C" keys are the tags for unconditional jumps. When a branch is needed in a program, the "J->" key is pressed, followed by the tag key identifying the type and destination of the branch. When executing a jump instruction, the calculator searches through memory for an End Jump instruction followed by the same tag key, and begins execution at that point. In the case of a conditional branch, the display is checked for zero content, and if it is zero, the branch is taken to the specified tag, else the tag key is skipped, and execution continues with the next step in sequence. While searching for the matching End Jump for a given Jump To, if the end of memory is reached, searching rolls over to the beginning of memory, making 'backwards' jumps possible.

Add to Memory Register 1 (M+) Subtract from Memory Register 1 (M-)
Recall Memory Register 1 (RM) Clear Memory Register 1 (CM)
End of Program Marker (END) Branch Tag Marker 1 (->J)
Branch to Tag 1 (J->) Halt (H)
Multiply (X) Divide
Add (+=) Subtract (-=)
Recall (RC) Clear (C)
Clear Entry (CE) Change Sign (+/-)
Decimal Point (.) Digit 9
Digit 8 Digit 7
Digit 6 Digit 5
Digit 4 Digit 3
Digit 2 Digit 1
Digit 0

Program Operation Code Displays

The "H" (HALT) instruction stops program execution at the end of a program, or when data input by the operator is required. Pressing the "+=" key when the machine is halted restarts program execution at the next step. The machine has the capability for two programs to be loaded into memory at once, with the "PII" key selecting a secondary program. When the calculator is in "automatic" mode, program execution begins when the "+=" key is pressed. If the "PII" key is pressed before starting a program with "+=", the machine searches through memory looking for two consecutive "END" instructions, and sets up for program execution to begin after the last of the two "END" instructions. This function is useful for programs that have inverse functions. For example, a program that calculates the logarithm of a given number could have the PII program set up to calculate the anti-log when invoked.

The content of program memory can be stored on magnetic cards for future use via the 363P's magnetic card reader. The program memory on the 363P is volatile, meaning that when the power is removed the content of the memory is lost. When the 363P is first powered up, program memory is automatically filled with "H" (Halt) instructions.

The Sharp 363P "Brag Tag"

The magnetic card reader, located at the top left of the cabinet, as a slot that accepts a four-inch long, by two-inch wide magnetic card that has two 'sides', A and B, for recording program steps. Each side holds 72 program steps, for a total of 144 program steps per card. Notches which can be cut out of each end of the card cause that side of the card to be write-protected. Each mag card comes with a protective paper envelope that holds the card and has areas on the outside to write information and instructions for use of the program contained on the card. The card itself has a writable surface on the front (non-oxide) side for writing information about the information stored on the card.

Mag Card & Storage Sleeve for Sharp Calculators

The card reader is controlled by three keyboard keys. The "ENTER" key is used for loading a program into memory, the "RECORD" key for writing program memory out to a card, and the "VERIFY" key for comparing the content of a card with memory for verification purposes. The card is placed in the slot on the top of the machine, and when one of the card reader control keys is pressed, and the card is drawn into the reader, then pushed back out by a small motor as the operation is completed.

The 363P is a reasonably fast calculator, with most operations completing almost instantly. The longest operation, calculating the square root of all-nines, takes less than 1/2 second to perform. All-nines divided by 1 takes perhaps 1/3 second to complete. However, as programmable machines go, the 363P isn't terribly fast. A simple program consisting of a 'jump tag', followed by "1" and "+=", followed by an unconditional jump back to the tag (making a tight loop that simply counts up by one on each loop) executes about 22 counts per second.

Text and images Copyright ©1997-2011, Rick Bensene.