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**Monroe/Litton Model 1920 Scientific Desktop Calculator **

*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.

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 ModelPhoto 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 supresses 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, e^{x} (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 abritrary power (a^{x}). 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 keycaps with moulded in nomenclature. The keyboard is made up of magnetically-activated microswitches, 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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