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Compucorp 140 'Statistician' Desktop Calculator

Updated 8/9/2008

Compucorp Calculators, Inc., an American firm which was a division of Computer Design Corporation, made a fairly significant line of specialty and high-end calculators in the 1970's. Machines that Compucorp built ranged from calculators with specialized functions for bond trading, statistics, and scientific, to computer-like calculators which were programmable and could interface to a wide variety of peripheral devices. Compucorp also manufactured machines for Litton/Monroe, with an example being the Monroe 1860. Compucorp's machines had a definite style about them which was unique, and to some, could be considered eccentric. An example of this is a '2nd Function' [2ND FUNC] key, which on most calculators, pressed before a function key causes the alternate function on that key to be executed. On the Compucorp machines, it works the other way around. You press the function key with the desired alternate function on it, which causes the calculation of the primary function on the key to occur, then press the [2ND FUNC] key, and the result of the alternate function is then displayed. Another example of Compucorp doing things their own way was the unique 'left to right' entry of numbers on the display. When entering numbers, the numbers begin entry on the left end of the display, with the 'next digit to be entered' blanked, to serve as a cursor.

This particular example of the Compucorp 140 appears to have been built in the early part of 1972, based on date code information on the IC's in the machine. Amazingly, at introduction, the Compucorp 140 had a suggested retail price of $1,995, which, for the time was quite inexpensive for such a versatile calculator.

Compucorp 140 minus top cover

The Compucorp 140 is a rather unique machine, with special functions related to statistics calculations. The author does not have much experience with statistics, so some of these functions remain a bit of a mystery. Along with these statistics functions, there's a useful compliment of scientific functions, such as Base 10 Logarithm, Base e Logarithm, 10x, ex, square root, reciprocal (1/x), and ax. Also included are functions for calculating the mean(average) and standard deviation of a list of numbers, performing linear regression, and factorial calculations, and some other functions which I haven't figured out yet.

Detailed View of Left side of Keyboard

The 140 uses a nixie tube display, made of genuine Burroughs Nixies. The display is organized into two sections, one section with 10 digits plus sign for general number display and display of mantissa in numbers displayed in scientific notation, and a second section consisting of a sign plus two digits for display of the exponent in numbers shown in scientific notation. To the right of the number display an "OFLOW" neon indicator lights to indicate when the range of the machine has been exceeded. To the left of the number display an "ERROR" neon indicator lights when an invalid function has been attempted. When the machine overflows or has an error condition the machine electronically locks out the keyboard, requiring a press of the [CLEAR x] key to clear the condition and unlock the keyboard. This locking of the keyboard is not physical, e.g., the keys are not actually block from being pressed down. Instead, the machine simply ignores any keys being pressed until the [CLEAR x] key is used to reset the machine. Pressing the [RESET] key will clear error/overflow conditions, but also disturbs the content of some memory registers. The machine has a calculating range of -9.99999999x10-99 to 9.999999999x1099. A switch on the keyboard panel selects the display mode, with one selection causing numbers to always be displayed in scientific notation, and the other position displaying numbers in scientific notation only when necessary.

Detail view of Right side of Keyboard

The machine has a compliment of 10 memory registers, ranging from 0 through 9. A number on the display can be stored directly into a memory register by pressing the [STn] key, followed by a single digit on the numeric keyboard indicating which register should receive the value. The [EXCHn] key exchanges the content of the selected memory register with the content of the display. The content of any memory register may be recalled by pressing the [RCLn] key followed by the memory register number. The [+n] key adds the number in the display to the specified memory register without affecting the number in the display. The [LEFT] and [RIGHT] keys allow memory registers to be split into two smaller-capacity registers, the left and right halves. Pressing either the [LEFT] or [RIGHT] key after a memory function key, and before entering the memory register number, specifies the appropriate half-sized memory register for the selected function. For operations requiring a smaller dynamic range of numeric quantities, this functionality provides for additional data storage capabilities. Certain memory registers are used for intermediate storage for some of the statistical functions, accumulating things such as item counts, sums, and sums of squares of lists of numbers entered into the machine. The memory registers are volatile, meaning that the content of memories is lost when the machine is powered off.

Back View of Compucorp 140 with Cover Removed (showing power supply circuitry)

The [RESET] key acts as a master clear, clearing the machine except for memory registers 7 through 9. The [EXP] key is used for entering exponents on numbers which are entered in scientific notation. The [CLEAR x] key clears the display register. The [π/e] key puts the constant value of Pi into the display register, or e if the [2ND FUNC] key is pressed afterward. The [INT/FR] key returns the integer (or fractional) part of the number on the display. The [CHG SIGN] key is used for toggling the sign of the mantissa or exponent. The [Φ] key is used to invoke special functions, with the numeric key following the press of this key indicating which special function is to be performed. I have not been able to figure out what these functions do, but on other Compucorp and Monroe machines this key is used to invoke special math functions, however on the Compucorp 140 is isn't at all clear what functions the [Φ] key invokes. Some of functions cause immediate "ERROR" indications, some cause the machine to churn for a moment, then "ERROR", some apparently do nothing, and others cause the machine to lock up with no error or overflow condition being indicated, requiring a press of the [RESET] key to resume operation.

Side Provide View of Compucorp 140

The keys on the right side of the keyboard relate mostly to the math functions of the machine. Many of the functions are obvious from the keycap, and I won't go into detail about them, however, the ones which are not intuitive, I'll try to explain. The [XY] key appears to have something to do with entering coordinate pairs, but I'm not sure exactly how it works or is used. The [LR] key initiates a linear regression calculation, but I'm not sure how you enter the data for the calculation (maybe with the [XY] key?). The [Sigma2n] key is used for entering lists of numbers for use by the Standard Deviation and Mean functions. When this function is used, the number of items entered is counted up in memory register 1, a running total of all of the numbers entered is kept in memory register 2, and a running total of the squares of the numbers entered is kept in memory register 3. When the [Standard Deviation/MEAN] key is pressed, the content of these memory registers is used to calculate the result of the function. The [Sigma2n GRP] key is a mystery. It appears to do similar things as the [Sigma2n] key, but the first number entered seems to be ignored. The key with the curves and shaded areas is also a mystery. Ditto with the [Sigma0] key. If you know statistics, or have experience with use of these functions on the Compucorp calculators, or happen to have an operator's manual for the Compucorp 140, please let me know.

A partial view of the CPU board

The Compucorp 140's brains are contained on two approximately 8" X 10" circuit boards that plug into a backplane. The top-most board contains the CPU of the machine, which consists of seven Large-Scale Integration (LSI) chips. These chips make up the microcoded processor that executes the instructions contained in Read-Only Memory (ROM) that gives the calculator its function. There are spaces on the CPU board for eight more LSI chips, which would be populated if the CPU board was used in a programmable model of Compucorp 100-series calculator. The bottom circuit board contains the Read-Only Memories (ROM) that contain the code that the CPU executes to carry out the various operations the machine performs. This board also contains some vacant space on it, apparently being used with different sets of chips for differing calculator applications. A number of different types of IC's are used here, with parts made by AMI, General Instruments (who served as a second-source for AMI for some of the LSI's used in Compucorp's 100-Series calculators), and Texas Instruments. Three other circuit boards make up the rest of the machine, with one providing power supply functions, the other handling the display drive for the Nixie display (which uses discrete transistor drivers to run the tubes), and the last board is the base for the keyboard. The keyboard and display boards connect to the CPU board with cables which terminate with edge card connectors that plug onto the CPU board.

The "ROM" Board

The standard four-function operations of the calculator operate in pure algebraic mode, with no notion of operator precedence, e.g., calculations are performed as they are entered. The calculator can take quite some time to perform some operations, with an example being calculation of 99! (factorial). While this calculation exceeds the range of the machine and lights the "OFLOW" indicator partway through the calculation, apparently the pre-programmed functions are not interruptable, so the calculation continues until completed, even though the overflow condition exists. The calculation takes about five seconds to perform, giving a partly useful answer (though digits beyond the two in the exponent are lost). Arguments greater than 99 to the factorial function result in an immediate "ERROR" condition. 69!, the largest factorial the machine can calculate without overflow, takes about three seconds. The basic four functions complete quickly, with the slowest operations being performed in perhaps 1/4 second. During more complex calculations, the digits in the Nixie tubes dance about frantically, as the display is left active while the programs which carry out these functions are executed.

For more reading about Compucorp and its history and calculators, click HERE.

The Old Calculator Web Museum greatfully thanks Mr. Anders Classon for information relating to the function of the "LEFT" and "RIGHT" Keys on the Compucorp 140.
Text and images Copyright ©1997-2017, Rick Bensene.