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.