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Monroe/Litton Model 1920 Scientific Desktop Calculator

This exhibit is dedicated to the memory of Mr. Al Swing, who worked for Fluor, a global engineering firm for many years. Mr. Swing's daily workplace calculator was a Monroe 1920. On Mr. Swing's passing, his family donated his calculator to the Old Calculator Museum in honor of his life.

Updated 7/26/2012

The Monroe/Litton 1920 is an interesting and rather unique scientific calculator. It has a myriad of scientific functions, and uses a unique way of providing access to its many unctions with minimal keyboard real-estate, with no use of the "2nd-function" keys so common on other scientific calculators. On top of this uniqueness, the machine seems to be a member of a very small set of desktop, scientific, non-programmable machines from the mid 1970's. This was a small market segment at the time, because most makers were concentrating on handheld, battery-powered scientific calculators, or very complex desktop programmable systems. The 1920 seems to be one of very few calculators that filled the niche need for a desktop, non-programmable scientific calculator in the mid 1970's.

This 1920, based on date codes on the Rockwell IC's used to make up the machine, is from the mid-to-late 1974 timeframe. By this time, IC's were commonplace in calculators, with basic four-function with memory calculators all condensed down to one or two chips. However, with the complexity of operations this machine performs, quite a complement of chips are required to provide the brains for this machine.

Internals of Monroe 1920

The machine uses seven LSI (Large Scale Integration) devices to make up its brains. An empty socket for an eighth LSI indicates that the 1920 was just one in a series of machines with different functionality available via additional or optional chips.

The Monroe 1930 Statistical Model
Photo Courtesy Bernadette Fase

Monroe definitely had another machine in the 19x0 series, the 1930, that looks virtually identical to this machine, but substitutes statistics calculations for the scientific functions of the 1920. Another hint that there were various versions of this same general machine comes from the serial number plate on the bottom of the case. The Model Number designation has the "19" of 1920 pre-printed, and the "20" part of the model number is stamped in. All of the LSI IC's in the machine are made by Rockwell, and have date codes on them that range from early to mid-1974. The machine is quite highly integrated, with very few discrete components, with the majority in the power supply, and a few resistors and capacitors sprinkled here and there amongst the IC's. The 1920 uses a Burroughs Panaplex II display, which integrates all of the digits of the display into one display panel. The display module contains 15 seven-segment digit positions; the left-most digit position showing a '-' if the number on the display (or mantissa if in scientific notation) is negative. The next 13 digit positions are used to display the number (or 10 digits of mantissa if in scientific notation), which includes commas grouping digits in front of the decimal point into groups of three. The remaining digits in the display module are used to indicate the sign and exponent when the display is operating in scientific notation, with two digits of exponent ranging from -99 to 99. The calculator always suppresses leading and trailing zeros. If the number is too large to fit in the display using the decimal point position specified, the machine automatically switches the display to scientific notation. The calculator displays "Error" using the seven-segment display elements to form the letters on the display when invalid operations are attempted, and "OFLO" when an operation results in a number which exceeds the capacity of the machine. Either error condition results in the keyboard locking, requiring a press of the [C] key to clear the machine and unlock the keyboard. The [CD] key clears the display only, useful for correcting entry errors. The content of memory registers is not affected by [C] or [CD] keys, however, memory content is lost when power is removed.

Close up on one of the LSI IC's

The Monroe 1920 has an extensive selection of scientific functions. The calculator has 10 memory storage registers (0 through 9). The number in the display can be stored into any of the memory registers by pressing the [STR n] key, followed by a single digit indicating which memory register the number is to be stored in. The number in the display can be added to a memory register by using the [ACC n] key. To recall one of the memory registers, the [RCL n] key is used, and the number in the specified memory register is recalled to the display. The machine uses algebraic logic, and includes parenthesis keys which do allow nesting to up to three levels. The machine performs sine, cosine, and tangent trig functions, with [INV] key allowing the arc-versions of each of them; with a three-position slide switch on the control panel selecting degrees, radians, or grads for the argument/result of the trig functions. The machine also can perform natural logarithm, ex (by pressing [INV] followed by the [LN] key), base 10 logarithm (using the special [II] key with the [II] key definition switch set to [LOG]), reciprocal (1/x), as well as raising a number to an arbitrary power (ax). Two special keys in the left-most group of keys, labelled [I] and [II], are 'definable' keys, whose function at any given time is determined by the position of two corresponding slide-switches on the control panel labeled "I" and "II". Functions selectable by these slide switches include recalling the constant Pi, performing rectangular to polar conversions (and back), performing degrees/minutes/seconds to decimal notation conversions (and back), base 10 logarithm, special memory clearing options, and various memory summing options. The exponent of numbers being entered in scientific notation is entered by pressing the [EXP] key, and the sign of the number in the display (or exponent) can be toggled using the [CHG SIGN] key. The machine has an automatic constant in all four basic math functions.

Control switches

The machine can operate in fully-floating decimal point mode(F), scientific notation(S), an accounting mode using fixed two-digits behind the decimal point(with cent-sign nomenclature), and some rather unconventional modes for display of numbers in alternate forms of scientific notation. The display formatting mode is selected by a single slide-switch at the center of the switch panel. The [D] key, located above the display format slide switch, forces the number on the display into the setting on the display mode switch.

Power Supply & Display construction details

The 1920 uses a regulated linear power supply. The machine is very solidly constructed, and has a very high-quality feel to it. The machine is styled purposefully, but is also quite pleasing to the eye, with a rather stylish swoopy look to it. The keyboard uses high-quality key caps with molded in nomenclature. The keyboard is made up of magnetically-activated micro-switches, and has a nice feel to it. This machine is quite fast, with no operation consuming more than one second to complete, and the basic four functions giving apparently instantaneous answers. During calculation the display is blanked.

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