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Compucorp 120 'Scientist' Desktop Calculator

Updated 8/9/2009

The Compucorp 120 "Scientist" is one of a number of machines in Compucorp's 100-series of specialized calculators targeting the engineering calculating marketplace. In 1970, Compucorp and Monroe Calculating Machine Co. created an OEM agreement which allowed Monroe to package the electronics for Compucorp calculators in their own cabinetry, and sell the machines under the Monroe brand. In the case of the Compucorp 120, the Monroe equivalent is the Monroe 1652.

The 120 is functionally identical to another 100-series calculator in the museum, the 122E with the exception of programmability. The 120 and 122E machines share the same keyboard and display assembly, power supply and chassis, and even the "ROM LOGIC" circuit board. The major difference between the 122 and the 120 is that the 120 uses a different "I/O LOGIC" circuit board, which is substantially less complicated than the I/O LOGIC board in the programmable 122E, and virtually identical to the "I/O LOGIC" board in the Compucorp 140 Statistician.

As with the other members of Compucorp's 100-series calculators, the 120 is based on the same generalized calculator chip set, designed by Computer Design Corporation, called the "HTL" chip set. The functional capability of a given model within the 100-series calculators is determined by the firmware coded into the machine's read-only memories. This notion of a generalized calculator processor that was designed specifically for use as a common core for a variety of calculators is an important point along the timeline of evolution of the microprocessor, and it appears that Compucorp was one of the first to use this idea in electronic calculators. The HTL chip set that makes up the core of Compucorp's 100-series calculators is essentially a multi-chip, general purpose CPU, designed in the late 1960's when microprocessors didn't exist. For more information on the formation and history of Computer Design Corporation, click HERE.

Keyboard of Compucorp 120

The scientific functions available on the 120 are the same as those of the 122E, including: base 10 Logarithm; base e Logarithm; 10x; ex; square root; 1/x; and ax; sine and cosine (and inverse functions); hyperbolic sine and cosine (along with inverse hyperbolic sine and cosine); factorial; and radian to degree conversion. The hyperbolic trigonometric functions are accessed using a special function key [Φ]. Pressing this key, followed by a single digit, selects the special function to be performed. The hyperbolic sine and cosine functions are accessed by pressing the special function key, followed by 1. The inverse hyperbolic sine is calculated by pressing the function key, followed by 2, and inverse hyperbolic cosine is accessed by pressing the special function key followed by 3. Other digits pressed after the special function key cause indeterminate results. Sometimes the machine hangs, requiring a press of the [RESET] key to restore the machine to operation. Other special function invocations result in no apparent operation being performed.

As with all of the 100-series Compucorp calculators, the display is made up of individual Burroughs-made Nixie display tubes. The display is made up of 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. At the right end of the display, a neon indicator lights a legend reading "OFLOW" to indicate when the range of the machine has been exceeded, and at the left end of the display a similar legend lights up "ERROR" when an invalid function has been attempted. When the and overflow or error condition exists, the machine locks, requiring a press of the [CLEAR x] key to clear the condition and unlock the machine. Pressing the [RESET] key will clear error/overflow conditions, but also disturbs the content of some memory registers. 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.

The calculator has a selection 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. Memory registers 7, 8 and 9 are used for some of the summing functions the machine has, 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 not retained when the machine is powered off.

Profile view of Compucorp 120

Internally, the Compucorp 120E is of the same general design as the other machines in the 100-series. All of the 100-series calculators are based on the same chassis, all sharing the same power supply, display subassembly, card cage, keyboard assembly, and cabinet base. The only differences between the various 100-series machines are the circuit boards that populate the card cage, the upper half of the cabinet, and the key caps on the keyboard. The functional difference between models within the 100-series is the circuitry and microcode contained on the two main circuit boards that make up the brains of the machines.

The Back Panel of the Compucorp 120 (note block-off where DB-25 expansion connector would be)

The functionality of the calculator is contained on two plug-in circuit boards. Each board measures approximately 8" x 10", and are populated mostly by the devices that make up the HTL chip set. The boards plug into a edge connectors that provide the 'backplane' for communication between the boards. The boards have nomenclature on them indicating their general function. The top board is called the "I/O LOGIC" board. This board has a number of edge connectors arranged across the back and sides of the board for plugging in the keyboard and display modules. This board appears to contain all of the necessary logic for scanning the keyboard, multiplexing the display, decoding and executing the microcode operations, the various mathematic logic functions such as the adder/subtracter, and perhaps some of the working registers. The I/O LOGIC board also contains the Learn Mode Programming (LEMP) functionality when used in programmable calculators, but in the 120, the seven chips that make up the LEMP and program memory are not populated on this board.. The bottom board is called the "ROM LOGIC" board. This board consists of the ROM (Read-Only Memory) that contains the microcode that makes up the personality of the machine, the RAM (Random Access Memory) that contains the general purpose working and memory registers, along with the necessary address decoding and timing logic to make the ROM and RAM accessible to the rest of the machine. The board is of a general design, such that varying configurations of RAM and ROM chips can be placed depending on the particular application.

This particular 120 is different from the Compucorp 122E and 140 in the museum by virtue of the fact that it does not have the DB-25 expansion connector on the back panel. The other two machines have this connector, which is used for connecting an external punched card reader (for programming) or printing device. This 120 simply has a block-out plate occupying the space on the back panel where the DB-25 connector exists on the 122E and 140 machines.

The keyboard and display subsystem are modular, being shared between the various 100-series calculators. The display circuit board contains the Nixie tubes themselves, along with discrete transistor drivers. The display subsystem connects to the main logic via a cable with an edge connector on the end which plugs into the "I/O Logic" circuit board. The keyboard uses high-quality contact-type switch modules, with removable key-caps. The key-caps have molded in nomenclature. The keyboard assembly connects to the "I/O Logic" board via a cable with edge connector termination. The power supply of the machine is a conventional transformer-based linear supply with transistor regulation. The power supply resides behind the display panel, taking up the rearmost area of the cabinet.

The basic math operations of the calculator operate in algebraic mode, with no notion of operator precedence. Most basic math operations complete virtually instantly, but some operations, such as the factorial function, can take up to three seconds to complete. During calculations, the Nixie display is left active, resulting in quite a light show in the display.

Text and images Copyright ©1997-2023, Rick Bensene.

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