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Compucorp 110/X Desktop Calculator

The Compucorp 110/X an entry-level calculator in Compucorp's 100-series of advanced electronic calculators targeting the scientific and engineering calculating marketplace. The 110/X provided a relatively low price point for users needing more advanced scientific and engineering calculating capabilities, but did not features such as programmability. At the time, this machine was significantly less expensive than similarly capable machines from other manufacturers such as Wang Laboratories, Hewlett Packard, and others. The 110/X provides a nice selection of advanced math functions suitable for use in engineering, surveying, and scientific applications, as well as a complement of ten memory registers useful for storing intermediate results and constants. The calculator has a capacity of ten signficiant digits, and a signed two digit power-of-ten exponent (for displaying numbers having more than 6 digits in front of the decimal point, or more than 6 consecutive zeroes behind the decimal point in scientific notation). A convenient switch on the front panel of the calculator selects the display format, allowing numbers to be automatically formatted with floating decimal, or forced to be displayed in scientific notation.

Compucorp 110/X Serial Number Tag (Located on Bottom of Cabinet

Like all of the machines that Compucorp marketed the 110/X's brains are provided by a calculator chipset designed by Computer Design Corporation called the "HTL" chipset. The HTL chipset consists of 19 different types of chips, but in the Compucorp 110/X, only 13 of the chips are used because the machine does not have the programmability that some of the other models within the line (see the exhibit on the Compucorp 122E for an example of a similar machine with the programming capabilities) have. With the HTL chipset, the functional capability of a given model is determined by the microcoded firmware stored within the machine's Read-Only Memory(ROM) chips, rather than complex arrangements of hard-wired logic. This made such a wide variety of calculators possible with only relatively minor microcode changes and keyboard configuration. To read an essay with more detailed information on Computer Design Corporation click HERE.

Keyboard Layout of the Compucorp 110/X

The scientific functions available on the 110/X include base e Logarithm; 10x; ex; square root; 1/x; raising a number to a power, sine and cosine (and inverse functions, which oddly return their results in Radians rather than degrees), and exponential functions both for base 10 and base e arguments. Also included is a rectangular to polar coordinate conversion function [TO POLAR]. On keys that have two functions listed on the keycap, both functions are calculated using the number on the display as the argument when the key is pressed. The result of the function listed first is displayed when the calculation completes. To bring up the result for the other function on the keycap, the [2ND FUNC] key is pressed after the calculation.

Replaced Nixie tube (Note longer tube with no brown spacer)

The display subsystem consists of individual Nixie display tubes. The display divided into two sections, one section with eleven Nixie tubes (ten digit tubes and one sign (+/-) tube) for display of general numbers as well as the display of the mantissa in numbers represented in scientific notation. The second section contains three Nixie tubes, consisting of a sign tube plus two digit tubes for display of the exponent in numbers displayed in scientific notation. The Nixie tubes used are geniune Burroughs-made tubes (Burroughs Corp. was the first company to manufacture Nixie tubes after purchasing the technology from the Haydu brothers), with 1/2" tall digits that made for a very clear and easy to read display. It appars that one of the Nixie tubes was replaced at some point in the machine's life, as one tube is different than all of the others. Nixie tubes in general are very reliable, but occasionally they can fail. Whether this was a factory authorized repair, or a repair performed by an end-user, the replacement tube appears to be indentical as far as the display goes.

Nixie Tube Display Showing Blanked Digit as Cursor

At the right end of the display panel, a neon indicator lights a legend reading "OFLOW" to indicate when the numeric 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 either or both of the overflow or error condition exist, the machine ignores keyboard entries, requiring a press of the [CLEAR x] key to clear the condition and unlock the machine except for the [CLEAR x] or [RESET] keys. Pressing [CLEAR x] will clear the display and the internal working registers, and also reset the error/overflow indications, allowing normal operation of the calculator to continue. Pressing the [RESET] key will also clear error and overflow conditions, but disturbs the content of some memory registers. A switch on the keyboard panel selects the display mode. This switch has two positions, "E" and "." The "E" position causes numbers to always be displayed in scientific notation, with the other position (".") displaying numbers in scientific notation only when necessary. When displaying numbers in scientified notation, the decimal point is always located to the right of the most-significant digit of the number. When displaying numbers in normal mode, the decimal point is fully floating, with the decimal point being placed to maximize the accuracy of the number being displayed. Insignificant leading zeroes are suppressed, while trailing zeroes behind the decimal point are displayed. An unusual aspect of these Computer Design Corporation-designed calculators is that digit entry begins at the left-most end of the display, advancing to the right for each digit entered. A blanked digit acts as a cursor, indicating the location of the next digit to be entered.

The 110/X has a selection of ten memory registers identified by a single digit from 0 to 9. A number on the display can be stored directly into a memory register by pressing the [STOn] key, followed by a single digit on the numeric keyboard indicating which register should receive the value. Any memory register can 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 sometimes used for some of the functions the machine performs, making these registers less-useful than the others, as their content may be changed during calculations.

Compucorp 110/X minus cabinetry

The insides of the Compucorp 110/X are identical to any of the models in the Compucorp 100-series of Nixie display calculators. The only difference between the different models is the keyboard layout and the ROM microcode stored in the control ROMs of the machine.

Compucorp 110/X "I/O LOGIC" Board

The logic of the calculator is contained on two plug-in circuit boards. Each board measures approximately 8" x 10", and are populated with the HTL chipset devices. 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 along the edges of the board for plugging in the keyboard and display modules. This board contains 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/subtractor, and perhaps some of the working registers. Unused IC locations on the "I/O LOGIC" board are where the Learn Mode Programming (LEMP, as called by Compucorp) chips which are populated on the programmable models in the 100-series.

Compucorp 110/X ROM Logic 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; and 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. The RAM and ROM chips in the HTL-chipset design are bit-serial devices, meaning that their content is read out a single-bit at a time, and the microcode word is assembled into a shift register that, after all of the bits of the addressed word have been read. In the case of writing to RAM, the word to be written is placed in a shift register, then the memory address is provided to the chips, and the selected chip receives the content of the shift register a bit at a time, until the entire word has been transmitted. While this is a rather slow way to transfer data between chips, it is a very practical design, since most calculator architectures operate on a single bit, or at most, four bits at a time. Bit serial architectures require less component count, and even in the early days of Large Scale Integrated (LSI) circuit technology, minimizing component count was still a concern.

Keyboard Key Function Jumpers

The keyboard and display subsystem are modular, being shared between the various 100-series calculators. The display circuit board contains the Nixie tubes along with discrete transistor driver circuitry. 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 keycaps. The keycaps have molded in nomenclature that are made from a very high quality plastic that withstand the test of time very well. There is a section on the keyboard circuit board that contains spots for a number of jumpers that are used to customize the keyboard to different applications. The configuration of the jumpers in this area of the circuit board encode the function of certain keys on the keyboard that vary depending on the model of the machine. For example, the Compucorp 110/X has the ax function, while the virtually identical Monroe 1610 has the same key assigned to a function that recalls both the π and e constants. The microcode in both machine's ROMs is the same, but the jumper configuration on the keyboard circuit board determines the keycode sent to the logic, which is interpreted by the microcode and the selected operation is carried out. The keyboard assembly connects to the calculator logic via a cable with an edge connector at the other end that plugs into the "I/O LOGIC" board.

The power supply of the machine is a conventional albeit complicated transformer-based linear supply with transistor regulation. The power supply resides behind the display circuit board, taking up the rearmost area of the chassis. The power supply board is removable, plugging into a set of connectors that provide filtered AC voltages, along with the regulated DC voltages used throughout the calculator. The power supply board also contains circuitry that generates the master clock pulses that orchestrate the operation of the calculator logic.

Date Stamp Inside Upper Cabinet

The date codes on the integrated circuits and other components within the machine range from the mid to late part of 1971, and some from early 1972, indicating that the machine was likely manufactured sometime around mid-1972. To confirm this, inside the upper part of the cabinet, a date is stamped, reading May 18, 1972. This could be a final Quality Assurance inspection date, a date of final assembly, or it could be the date of manufacture of the cabinet piece, but no matter the intent of the date, it does help confirm the general assertion that the machine was built sometime in the mid-1972 timeframe.

The 110/X calculator operates in algebraic mode, but has no notion of operator precedence. The basic math operations complete virtually instantly. Some of the advanced math functions (e.g., trig functions) can take up to nearly two seconds to complete. During calculations, the Nixie display is left active, providing the user with an entertaining "shuffle" of digits as the machine churns through the more time-consuming operations.

User-Added "Slowdown" Switch

At some point during its life prior to arrival in the museum, the exhibited calculator had a modification performed to it; a two-position switch was installed to allow the master clock rate of the calculator to be slowed down. The switch was installed in the block-off plate for the DB-25 connector that exists on some other models of the 100-series calculators. One of the positions of the switch adds additional capacitance to the clock generation circuitry, slowing the master clock down to about 1/5th of its normal operating speed. With the switch in the other position, the calculator runs at its normal speed. When the calculator is running in "slow mode", it is fascinating to observe the algorithmic processes carried out by the calculator as it performs calculations. It slows down the machine enough that the Nixie tubes flicker as their multiplexing refresh rate is also slowed by the reduced clock rate. By watching the display as the machine performs math operation while slowed down, it is clear that it uses algorithms that leverage the built-in math operations that the machine has coded directly in microcode (square root and logarithmic functions) to carry out some of the other math operations as sequences of keypresses that are played back quickly, such as ax (which uses addition of logarithms, multiplication, then an anti-logarithm), and trig (using Taylor-series approximation), as well as other math operations that aren't directly coded in the machine's microcode.

Text and images Copyright ©1997-2015, Rick Bensene.