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Monroe 990 Desktop Calculator

Updated 8/25/2000

The Monroe 990 is a good example of what would be considered a high-end office calculator of its time, which would be in the mid-1960's timeframe. The 990 is visually very similar to its somewhat less feature-laden stable-mate, the Monroe 950. The 990 is identical (other than cabinet and keyboard coloring) to the Canon 163. Given that Canon designed and manufactured the 990 for Monroe, there's no surprise that Canon would have a similar model marketed under the their own name.

The 990 is a 16-digit Nixie tube-display calculator. The machine performs the basic four math functions along with square root. It also has two memory registers, one that can serve as an accumulator, automatically accumulating results of calculations, and a second memory register that can serve as an item counter, sum of products accumulator, or a general purpose memory register with addition/subtraction function keys. The 990 is a fixed-decimal point machine, with a large rotary wheel at the right side of the keyboard with settings of 0, 2, 4, 6, 8, or 10 digits behind the decimal point. The display shares the comma indicators of the Monroe 950 that help make answers more readable by grouping digits in groups of three.

Monroe 990 with Top Cover Removed

The 990 shares the same mechanical chassis of the Monroe 950, with a circuit-board backplane with eight slots for plugging in circuit boards. As opposed to the 950, the 990 uses all eight slots of the backplane, whereas the 950 has two blank slots filled by filler cards.

The circuit cards that make up the Monroe 990.

The Monroe 990 uses the same types of Texas Instruments 14-pin dual-inline, plastic packaged small-scale integrated circuits used in the Monroe 950, albeit more of them. The machine uses a total of 170 IC's, 76 more than the 950. The extra complexity is no doubt a function of the 990's ability to perform square roots, as well as the extra memory functionality of the 990. All of the IC's are Texas Instruments SN39XX and SN45XX-series devices, a family of DTL (Diode-Transistor Logic) devices pre-dating the famous 7400-series TTL IC's.

Display driver details

The display consists of 16 Nixie tubes, each with a decimal point position to the right of the digit. There are also 16 discrete neon bulbs located above and between the Nixie digits, used for the comma indicators. There are also two discrete neon bulbs that indicate overflow and negative conditions, as well as two more that light if the content of either of the memory registers is non-zero. The Nixie tubes are driven by discrete transistor/transformer driver circuitry, and run from a 220 Volt DC supply.

Monroe 990 Keyboard Detail

The left-most group of keys on the calculator are control keys. The [AM] key, a 'push-on/push-off' key activates memory register one when depressed. Memory register one serves only as an accumulator, where the results of calculations are automatically accumulated into when the [AM] key is activated. The [RV] key swaps (ReVerses) the operands of any two argument math functions. The [back arrow] key deletes entered digits one at a time, allowing easier correction of incorrect inputs. The [RM1] key recalls the content of memory register one to the display. The remaining keys clear various sections of the calculator; with [CI] (clear indicator) clearing the display, [CM1] clearing memory register one, [CM2] clearing memory register two, and [C] clearing the entire machine (except the memory registers). To the right of the standard digit-entry keypad are the math functions. Addition and subtraction operate adding machine style, with sums and differences accumulating as each [=] key is pressed. The white [=] key adds, the red [=] key subtracts. The multiply and divide functions work as expected, with the white [=] key generating a positive result, and the red [=] key negating the resultant answer. The square root key causes the square root of the number on the display to be calculated. The [K] key, a push-on/push-off key, activates a constant operator for multiply and divide operations when on. Two [M2] keys, one white, one red, work to accumulate values in memory register two. The white [M2] key adds the content of the display to the memory register, and the red [M2] key subtracts the display number from the memory register. A three-position slide switch located above the right group of keys selects the functional mode of memory register two. The "M2" position selects normal operation of the memory register, with the [M2] keys adding/subtracting. The "SUM OPD." position disables the [M2] keys, and instead causes memory register two to automatically accumulate the sum of the results of all multiply and divide operations. The "COUNT" position of the memory mode switch causes memory register two to simply count the number of calculations performed, useful for things like calculating averages. To the left of the memory mode slide switch, another three position slide switch selects the rounding mode of the machine, with positions to round up/down, truncate, or force round-up of results on the display.

A profile view of the Monroe 990

The back-most circuit board in the machine is a controller for an magnetostrictive acoustic delay line that is used to store the working registers of the machine. The 990 uses a completely different delay line than the 950, and another similar machine, the Canon 141. The delay line in the 990 is significantly thicker than the delay line module used in the 950, likely because more bits of storage are required for the additional memory registers and functions of the 990 versus the less complex 950. The delay line, as with the one used in the 950, is made by NEC, and has a Quality Assurance stamp on it that is dated June 24, 1970. The delay line control board has a 1MHz crystal on it that is used to generate the master clock that orchestrates the timing of the logic functions of the machine.

Monroe 990 Power Supply

The 990 uses virtually the same power supply circuitry as the 950. A circuit board mounted at the back of the machine contains the various components used to filter and regulate the various supply voltages needed by the circuitry. The supply uses transistor regulation for most of the voltages, with large filter capacitors to keep ripple minimized. Most of the logic appears to operate from a 5V DC supply, with a few other voltages used for operating the delay line circuitry, as well as the high-voltage supply used to drive the Nixie tubes and Neon indicators.

The 990 is somewhat faster than the 950. Addition and subtraction produce virtually instantaneous results. It takes about 1/2 second to perform all nines divided by one, almost twice as fast as the 950. The 990 properly performs this operation, while the 950 errors out if you try to divide 16 nines by 1. The 990 behaves the same as the 950 when division by zero is attempted, going into an apparent endless loop trying to find an answer where one doesn't exist. Pressing the [C] key halts the futile search for an answer and returns the machine to normal operation.

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

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