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Monroe 920 Desktop Calculator

Updated 9/12/2015

The Monroe 920 is another example of Monroe leveraging the electronic calculator expertise of the Japanese company Canon to provide calculators for Monroe to market and sell. The 920 is almost identical to the Canon 120 and 1200, with the only small differences between the Canon and Monroe models. The only major difference, other than subtle keyboard layout differences and color schemes for brand differentiation, is that the Canon machine does not provide a constant [K] key.

The 920 was the lower-end of a pair of machines introduced at the same time, sometime in the 1969 timeframe. The 920 was the little-brother of the 925, which upstaged the 920 by virtue of providing an extra digit of capacity (13 digits for the 925, versus 12 for the 920).

Inside the 920, the circuit boards have Canon logos in both etch and silkscreen on them, proving the origin of the machine being Canon. Another example of this OEM relationship between Monroe and Canon, although somewhat more subtle, is the Monroe 950, also in this museum. This particular example of the 920 was made in the late part of 1969, and has a maintenance sticker on the back that lists "12/12/69" as the date of installation. Date codes on the IC's in the machine range from 6846 (late 1968) through 6930 (Q3 '69), which seem to correlate with the installation date.

Monroe 920 with the case opened up

The Monroe 920 is a small-scale integration IC-based calculator. All but one of the 66 IC's in the machine are Texas Instruments-made DTL (Diode-Transistor Logic) devices in the SN39xx and SN45xx-series. A single very early 7400 TTL (Transistor-Transistor Logic) Quad Two-Input NAND IC was likely a field replacement for a failed chip, as DTL and TTL logic are mostly compatible. The 920 was designed with a fairly small compliment of different chips. The devices (with the device type in brackets, and the number of devices in parenthesis following the device part number) used in the machine are: SN3920(6), SN3925[Quad 2-Input NAND](32), SN3931[Triple 3-Input NAND](11), SN4553[Dual 4-Input NAND w/expander](4), SN4554[Quad 2-Input NAND](12), and the lone SN7400 Quad 2-Input NAND TTL device.

Close-up of Monroe 920 Circuit Board

The 920 uses a magnetostrictive delay line device for working storage. The delay line appears to be very similar to the delay line used in the Monroe 950. The delay line has a total delay of 539.5 microsoconds, with one read tap, and two write taps, with one write tap pushing pulses in that will arrive at the end of the line in 507.5 microseconds, and the other, adding an additional delay of 32 microseconds. Providing injection of pulses into the line with different amounts of delay allows shifting operations to occur by simply gating pulses into the delay line via the different taps. For more information on the use of delay line technology in early electronic calculators, I recommend reading an interesting article written by Nicholas Bodley on the internal workings of the ground-breaking Friden 130 calculator.

Like most electronic calculators of its time, the 920 uses Nixie tubes as the display technology. Each Nixie tube contains the digits zero through nine, along with a decimal point. The Nixie tubes are driven by discrete transistor (Toshiba 2SC780) drivers. It is interesting to note that the Nixies are multiplexed on this machine, which makes the 920 one of the earliest examples of a multiplexed non-CRT-based display that I've seen. Another oddity of this machine is that the anode drivers for each Nixie tube consists of a transistor and a small signal transformer. The transformer design simplifies the decoding circuitry necessary for driving each of the twelve Nixie tubes as they are multiplexed.

The Arithmetic (Bottom) Circuit Board

The calculating brains of the machine are made up of two fairly good-sized circuit boards stacked one atop the other across the bottom of the machine. The top circuit board contains the Nixie tube displays, driver circuitry, and the master clock oscillator which uses a 2.0MHz crystal for generating the timing for the machine, as well as the divider chains that divide the 2MHz clock down to the fundamental clock rate of 250KHz. A crystal-controlled oscillator is used because it allows for accurate and stable timing of the signals being launched into the delay line. Also included on the top circuit board are the read and write amplifiers for the delay line. The top board contains 21 of the 66 IC's in the machine. The bottom board seems to contain most of the calculating and state logic, with the remaining 45 IC's on it. The keyboard connects to the bottom board via an edge-card connector, and the two boards are connected together by edge-connectors which are hand-wired together.

The Indication (Top) Circuit Board (with Power Supply/Delay Line Module Set Aside)

The Monroe 920 operates very typically for machines from this vintage. It uses adding machine style logic for add and subtract, with a key with a white [=] legend for adding and one with a red [=] legend for subtracting. For multiplication and division, the problem is entered an algebraic form, with either of the [=] keys used to calculate the result. The calculator uses a fixed-point decimal system, with a three position switch on the keyboard panel selecting 2, 3, or 4 digits behind the decimal point. As with just about all Canon-made calculators, the [C] (clear) key clears the entire machine, and the [CI] (clear indicator) key just clears the display, and is used for correcting input errors. The machine also features a push-on/push-off [K] key for allowing a constant multiplier or divisor.

It is interesting to note that the design of the machine is such that other decimal point settings are possible, through changing feed-through jumper wires in a specific area of the Arithmetic circuit board. A service bulletin outlines the procedure for changing the decimal point selections, along with ordering information for adhesive labels which are used to cover over the default decimal point selection legend on the front panel of the calculator.

A View of the Power Supply and Delay Line Module

One weakness of this 92x-series of machines is that it does not do negative numbers (much the same as the Brother Calther 412 which is of similar vintage and technology). A negative result shows up as the 10's compliment of the negative number. e.g., 10 - 15 results in 9999999995.00 (with the decimal point set at two digits behind the decimal point). Pressing the [=] key with such a number in the display will immediately compliment the number (with the example returning 5.00). The machine does detect overflow, and indicates an overflow or input error (such as two keys pressed at once) by lighting a neon indicator positioned at the left end of the display in the shape of a left-facing arrow (another Canon trademark). When the machine overflows, it logically locks up, and requires a press of the [C] key to continue. The 920 doesn't know that dividing by zero is impossible, and computes in futility trying to find an answer to an insoluable problem without lighting the overflow indicator. Pressing the [C] key in such a situation will return the machine to normal. Interestingly, pressing the [CI] key when the machine is churning trying to divide by zero results in the machine unlocking, but doing strange things ranging from lighting up all of the decimal points but doing nothing, to accepting numeric input, but not performing any math operations. Again, pressing [C] when in this weird mode will restore the machine to normal.

Detail of Magnetic Reed Switch Keyboard

The 920 uses a magnetic reed-switch keyboard that is very reliable, and was the predominant keyboard technology of the time. The picture above clearly shows how this technology works. The fact that there is no physical connection between the key and the switch itself makes these keyboards last a very long time. This is why many of these very old calculators still have keyboards that work as they did when they were new, whereas later calculators that used other forms of 'direct contact' keyboards have gotten 'bouncy' or have quit working altogether.

The 920 is quite fast for a machine of its vintage. With a main clock rate of 250KHz, it is substantially faster than earlier all-transistor designs. Additional chains of counters are clocked by the 250KHz master clock to generate the various operational states of the machine. 9999999999.99 divided by 1 takes less than 1/3 second to give a result, and addition and subtraction give virtually instantaneous results. The machine does not blank the display during calculation, so some action of the Nixies can be seen on operations that take longer to perform. The machine also lights up all decimal points when it is busy calculating.


Text and images Copyright ©1997-2017 Rick Bensene.