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Monroe 950 Desktop Calculator

Updated 9/26/2017

The Monroe 950 isn't particularly technically sophisticated, nor does it perform any special functions. What's intriguing is that this machine is made by Canon, even though it has "Monroe" on its name tag. At the time this machine was built, Monroe had a strategic agreement with Canon such that Canon would design and build machines for Monroe that would actually compete with Canon's machines in the US marketplace for the calculator buyer's dollars! The machines that Canon built for Monroe always had subtle differences from the equivalent Canon machines, but in terms of design and implementation, both the lines of machines shared very similar components. A prime example of this, though not exactly the same model in the line, is the Canon 141, a Canon machine of very similar design, features, and age.

The Monroe 950 is a 16-digit Nixie tube-display calculator. It performs the four basic math functions, and has a memory register. This calculator appears to have been built in early 1970, based on date codes on IC's within the machine which range from mid 1969 through early '70.

Monroe 950 with Top Cover Removed

The calculator is built in a modular fashion, with an etched circuit board backplane located across the bottom of the machine which contains a small number of components, with the rest of the backplane containing edge-connector slots for up to eight circuit boards. The 950 was a the lower-end machine in a pair of machines, with the Monroe 990 providing additional capability. Interestingly, there are two slots in the card cage which contain dummy cards, which are populated by real circuit boards in the 990.

One of the Circuit Boards from the Monroe 950

All of the IC's used in the Monroe 950 are Texas Instruments SN39XX and SN45XX-series DTL (Diode-Transistor Logic) small-scale integration devices. The 950 contains a total of 94 IC's, all of which are in 14 pin plastic dual-inline packages.

Detail of TI 39XX-series IC's

The 950 is a fixed-decimal point machine. The decimal point location is set via a large thumb-wheel-style rotary switch located at the upper-right of the keyboard panel. The decimal point location can be set to 0, 2, 4, 6, 8, or 10 digits behind the decimal point. A slide switch located near the decimal location wheel selects roundoff/truncate mode.

Monroe 950 Keyboard/Display Detail

The display on the Monroe 950 is rather unique, with a special row of indicators located above and between the Nixie digits. These displays light to show the location where commas would be placed when writing the number out on paper. The indicators are small neon tubes behind opaque white plastic windows. These indicators light when a result is displayed, and are extinguished during number entry. The Nixie tubes themselves are pretty generic, with the standard 0-9 and decimal point inside the glass envelope. Small neon tubes are situated behind cutouts in the display panel which light up to indicate overflow and negative sign of the number in the display. Another small annunciator located below the display panel at the left edge of the machine indicates when the memory register contains non-zero content.

Rear View of Monroe 950 with Cover Off

From an operator's point of view, the Monroe 950 would feel very familiar to someone who was familiar with Canon calculators. The keyboard layout and key functions are clearly patterned after the similar Canon machine, with the only real difference being the color scheme. The left-most group of keys on the calculator are general control keys. The [AM] key, a push-on/push-off key, puts the memory into accumulate mode, where the results of multiply and divide operations are automatically accumulated in the memory register. The [RV] key swaps the operands of any two argument math function. The [←] key works just like the backspace key on your PC keyboard, allowing the user to back out incorrectly entered digits without having the clear the display and start over. The [RM] key recalls the content of the memory register to the display. The remaining keys clear various sections of the calculator; with [CI] clearing the display, [CM] clearing the memory register, and [C] clearing the entire machine (except the memory register). To the right of the standard digit-entry keypad are the math functions. Addition and subtraction operator 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 when the red [=] key is used, the resultant answer is negated. The [K] key, a push-on/push-off key, selects a constant operator for multiply and divide operations when actuated. Two [M] keys, one white, one red, control accumulation of numbers in the memory register. The white [M] key adds the content of the display to the memory register, and the red [M] key subtracts the display number from the memory register.

The NEC-made Magnetostrictive Delay Line

Toward the rear of the machine, there is a metal enclosure that contains a magnetostrictive delay line. (like that used in the ground-breaking Friden EC-130). The delay line is used to store the working registers of the machine. At the time the 950 was built, IC technology was still fairly expensive, and the level of integration was still rather low. The number of logic elements required to store all of the bits required for representing the numbers inside the machine would have been prohibitive. Using a refined version of delay line technology used in the Friden 130, the Monroe 950 packs all of the bits necessary to hold the working registers of the calculator. The delay line used in this series of Monroe/Canon calculators was made by NEC, and is quite a marvel of electromechanical technology. The delay line consists of a spiral coil of specialized wire with transducers connected to each end. Driver circuitry takes a serial bit stream (with the bits representing the registers of the calculator) and launches the bits into the coil of wire as a series of physical twists (no electricity travels through the coil of wire). The torque twists travel along the wire until the other end of the wire is reached, where a transducer picks up each twist and translates the physical energy back to electrical energy. The characteristics of the wire loop are such that the torque pulse travels at a specific rate through the wire.

A look inside the Delay Line

Depending on the length and physical characteristics of the wire, a given number of pulses are maintained in the wire for the time it takes them to move from one end of the wire to the other. Circuitry that drives the delay line simply picks up the bits at the far end, re-times and conditions them (and perhaps gates them into the working areas of the calculator as needed, and, depending on the operation, modifies the bits as the result of a calculation), and launches them back into the delay line wire again. The math logic of the calculator operates on a bit by bit basis, so that when calculations occur, the bits that stream out of the delay line are directed bit at a time into the calculating circuits, that perform the necessary math operations, and the resultant answer bits are launched back into the delay line for future reference.

Another view of the workings of the Delay Line

When the calculator is idle, the bits just circulate around and around in the delay line. This technology, though a bit arcane, was very successfully used in early electronic computers, and with refinement and miniaturization, found a home in early electronic calculators. After a short time, delay lines were no longer needed, as the price of magnetic core memory (another popular storage medium for calculators) came down, and shortly thereafter, the levels of integration available in integrated circuits rose to the point where it was possible to keep all of the working registers of a calculator inside of the chips rather than having to use technologies such as delay lines and magnetic core.

The calculator operates from 110 Volt AC power only, and uses a conventional transformer-based linear power supply to generate all of the working voltages needed to operate the machine. A circuit board mounted at the back of the machine (see the back view photo above) contains the various components used to filter and regulate the various supply voltages.

The machine is not blazingly fast, with even addition operations taking a noticeable period of time to complete. I'd estimate that the 950 takes perhaps 0.1 second to perform an add or subtract operation, and a good part of 1 second to do an all nines times all nines multiply, and a little over one second to do 999999999999999 (note, 15 9's, not 16!) divided by 1. The machine errors out if you try all 9's (16 of 'em) divided by one, which is due to the calculator logic using one of the digits as a counter for the multiplication operation. Division by zero results in the calculator going into a very strange mode. The display shows all zeroes, with one decimal point lit dimly. Pressing a digit key results in the dim decimal point moving around, but no digits are entered. Pressing other function keys results in nothing. Pressing the [C] key resets the machine and returns it to normal operation.

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

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