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Monroe 1610 Desktop Calculator


This exhibit is dedicated to the memory of Mr. Gerald B. Dunn, professional surveyor from the New Orleans, Louisana area, who used this calculator for surveying calculations for over 30 years. Mr. Dunn passed away in 2007.

The Monroe 1610 an entry-level calculator in Monroe's 1600-series of advanced electronic calculators targeting the scientific and engineering calculating marketplace. The 1600-series machines with Nixie tube displays included the 1610, 1650, 1651, 1652, 1655, and 1656. The 1610 provided a relatively low-cost entry point for people needing more advanced scientific and engineering calculating capabilities, but did not need the features of the higher-end machines, such as programmability. Selling for $1,495 (April '72 price), the 1610 provides logarithmic (base e and base 10), trig, square root, reciprocal, radians to degrees conversion, rectangular to polar coordinate conversion, along with the usual four math functions. The machine also provides ten memory registers (numbered 0 through 9) which can be stored into, added to, and recalled. The 1610 also provides one key recall of the constant π and a two-key recall of the constant e. 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).

The math functions that the 1610 provides are a great fit for surveying calculations, as trigonometric functions are critical in the math relating to surveying. Compucorp calculators (as well as those from licensed OEM customers like Monroe and Sumlock) were great sellers into the surveying markets due to their comprensive math functions and low price when compared to competitor's (Wang Laboratories, Hewlett Packard, Mathatronics) offerings in the early 1970's.

In 1970, Monroe Calculating Machine Co. entered into an OEM agreement with Computer Design Corporation (a.k.a. Compucorp) which allowed Monroe to package the calculator chassis manufactured by Computer Design Corporation in Monroe's own cabinetry, and market, sell, and support the machines under the Monroe brand. In the case of the Monroe 1610, the closest Compucorp equivalent is the Compucorp 110/X, with the only difference being that the Compucorp 110/x has a [ax] key in place of the key that recalls the constants π and e on the 1610.

Monroe 1610 Serial Number Tag (Located on Bottom of Cabinet

Like all of the machines that Monroe marketed that were based on Computer Design Corporation's customizable calculator architecture, the 1610'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 Monroe 1610, 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 Monroe 1655 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 chips used, and 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. To read an essay with more detailed information on Computer Design Corporation, click HERE.

Keyboard Layout of the Monroe 1610

The scientific functions available on the 1610 include base e and base 10 logarithm; 10x; ex; square root; reciprocal; sine and cosine (and inverse functions, which oddly return their results in Radians rather than degrees). Also included is a rectangular to polar coordinate conversion function [∡]. On keys that have two functions listed on them, both functions are actually calculated using the number on the display as the argument when the key is pressed. The result of the top-most function is displayed when the calculation completes. To bring up the result for the bottom-most function on the keycap, the [2ND FUNC] key is pressed after the calculation.

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), with 1/2" tall digits that made for a very clear and easy to read display. At the right end of the display panel, a neon indicator lights a legend reading "OVERFLOW" 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.

Nixie Tube Display Showing Blanked Digit as Cursor

The 1610 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 [↓[ ]] 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 [↑[ ]] key followed by the memory register number. The [+[ ]] 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.

The guts of the Monroe 1610 are identical to any of the models in the Monroe 1600-series (or machines in Compucorp's 100-series). The only difference between the different models is the keyboard layout, and the ROM microcode stored in the control ROMs of the machine.

Monroe 1610 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 1600-series.

Monroe 1610 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.

HTL-18 Chip Manufactured by General Instruments

This particular calculator is interesting in that it contains two HTL chips that are manufactured by General Instruments (HTL-18). Computer Design Corporation developed the logic design for what became the HTL chipset, but the company did not have integrated circuit design and fabrication facilities. The development of the actual chips was farmed out to chip maker American Microsystems (AMI). AMI took the logic design provided by Computer Design Corporation and translated it into the series of 19 different types of chips (using PMOS fabrication technology) that make up the HTL chipset. At the time these chips were developed, AMI stretched the state-of-the-art in Large Scale Integrated (LSI) chips, pushing the technology to include upwards of 300 logic elements on a single chip. Afer some time with AMI being the only source for the chips, Compucorp, in the interest of having a second source for their critical chips, engaged the services of General Instruments (GI), another chip manufacturer, licensing GI to also fabricate the HTL chipset.

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. The keyboard assembly connects to the "I/O Logic" board via a cable with edge connector termination.

Power Supply Circuit Board in Monroe 1610

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.

Chassis Serial Number Sticker

The date codes on the integrated circuits and other components within the machine range from the early to late part of 1971, indicating that the machine was likely manufactured sometime toward the end of 1971 or early 1972. At some point during the production of the series of calculators, Computer Design Corporation instituted a process by which the serial number of the calculator was noted by a tamper-resistant sticker on the chassis. This was done to help in theft recovery cases and warranty verification.

The basic math operations of the calculator operate in algebraic mode, with no notion of operator precedence. The basic math operations complete virtually instantly, but some of the advanced math functions (e.g., trig functions) take up to nearly two seconds to complete. During calculations, the Nixie display is left active, resulting in wonderful "dancing digits".

This particular calculator was owned by Mr. Gerald B. Dunn, who was a land surveyor in the greater New Orleans, Louisiana area. He worked as a surveyor for over forty years, and for a good many of those years, he used this calculator to churn through the complex math required in surveying calculations. Mr. Dunn referred to the calculator as his "computer", saying he didn't need one of those newfangled Personal Computers to do his work. Mr. Dunn was instrumental in developing subdivisions in New Orleans, New Orleans East, Metairie, Kenner, and on the north shore of Lake Pontchartrain. Mr. Dunn also performed surveying work on the Louisiana Superdome project. This exhibit is dedicated to the memory of Mr. Dunn, who passed away in 2007. The Old Calculator Museum was honored to receive this calculator as a donation from Gerald's sons, Craig and Steven, in July of 2014.


Sincere thanks to Craig and Steven Dunn for donation of Monroe 1610 Serial #G 934042 to the Old Calculator Museum

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