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Monroe 920 Desktop Calculator

Updated 9/7/2017

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 nearly identical to the Canon 120 and 1200 models, with the main differences being subtle keyboard layout differences and color schemes for brand differentiation, and the Monroe 920 provides a constant function controlled by a [K] key on the keyboard, which isn't present on the Canon machines.

The 920 was the lower-end of a pair of machines introduced at the same time, sometime in the mid-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). Other than this difference in capacity, the functional aspects of the two machines are the same.

Inside the 920, the circuit boards have Canon logos in both etch and silkscreen on them, proving the calculator was designed and manufactured by Canon. Another example of this OEM relationship between Monroe and Canon is the Monroe 950.... 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 utilizes small-scale integration (SSI) integrated circuits for its logic, along with a small count of discrete components (mostly resistors). 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 early Texas Instruments 7400-series TTL (Transistor-Transistor Logic) Quad Two-Input NAND IC is also included in the logic. Why only one TTL device was used amongst all of the DTL logic is a mystery. The 920 was designed using a fairly small complement 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.

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 right-hand decimal point. The Nixie tube cathodes are switches by discrete transistor (Toshiba 2SC780) drivers in combination with an unusual anode anode drivers for each Nixie tube consists of 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 logic of the Monroe 920 is contained on two 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 bits being launched into the delay line, and their recovery when the bits reach the end of the delay line. Also included on the top circuit board are the read and write amplifiers for the magnetostrictive delay line. The top board contains 21 of the 66 IC's in the machine. The bottom board contains 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 a backplane with edge-connector sockets that are hand-wired together.

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

The Monroe 920 operates very typically for calculators of this vintage. It uses adding machine style logic for add and subtract, with a key with a white [=] legend for addition and one with a red [=] legend for subtraction. 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 clears only the display, and is used for correcting input errors. The machine also features a push-on/push-off [K] key for enabling a constant multiplier or divisor when depressed.

It is interesting to note that the design of the machine is such that other decimal point settings are possible by 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 the 920 and 925 calculators is that they do not handle negative numbers. A negative result is displayed as the 10's compliment of the negative number. For example, 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 complement 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 icon). 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 the machine is in this strange 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 (entering multiple digits for one keypress) 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.